Book Author: Bridget Stutchbury, PhD Bird Detective
Walker Publishing, New York, 2010
Why Females Cheat
What Makes a Male Attractive to The Finnicky Female?
Songs of the Best Males
Why Birds Divorce?
Dynamics of Nesting and Incubation
Finding a First Home
Aggression and Defense
Will Birds Succeed in a Rapidly Changing Environment?
This is a book about bird behaviors. In the introduction, we gain some insight into the scientific method as practiced by biologists: observe behavior, run experiments, formulate a hypothesis about the purpose behind behavior, collect data to prove or disprove the hypothesis.
As we will learn throughout the book, our avian friends are complex, and their innate behavioral motivations are not always straightforward and easy to deduce. We also learn that birds are able to adapt innate behavior to changing environments, within limits. These are the dynamic pillars of biological existence: an innate, evolutionarily successful behavior that has genetically adapted to environmental and internal change (slow change), as modified through learned behavior modifications (fast change).
Birds are tough subjects. Bird detectives spend untold hours searching and waiting to find subjects, then untold more hours planning for and performing data collection on these subjects. It is painstaking work, requiring years of study and training of eyes and ears to enable productive observation.
Early on, avian biologists were motivated to understand the evolutionary bases for bird behavior. More recently, a principal motivation for some biologists is to apply what is known about bird behavior to predict the birds chances of long-term success in our rapidly-changing environment. Perhaps there are things humans can do to triage the inevitable loss of species as our planet enters a period of expected rapid climate change. The author speculates that 1,000 bird species could be lost in this century, a 15% extinction rate.
The book is addressed to a general audience. The author suggests to us why we will find her book of interest: “Birds are linked together, and to us, by the challenges of living in the drastically remodeled environment that has come with explosive human growth. Birds connect people with nature because they are beautiful, have fascinating lives, and in many ways remind us of ourselves.”
Why Females Cheat
Before inexpensive DNA testing, the cheating habits of birds had unknown scope. Purely from observation, it had long appeared that bird pairings were largely monogamous. Post-DNA, the author can now cite that 41% of Acadian fly catcher nestlings are not the offspring of the nesting female’s ‘social’ mate. Even monogamously paired birds experience intense sexual competition.
In the case of flycatchers, the female remains on territory and males from other territories visit her briefly to attempt to mate. Big news: males sneak around. They come calling as soon as their breeding plumage arrives each year. They spend several visits impressing neighboring females, who maintain a good memory of which males came courting. The females indicate their fertility by increasing their calls fourfold. Since extra-pair males are visitors to the territory, they are challenged by the social mate who defends it; may the best man win. Those that can meet the challenge may gain access to the female, for they have demonstrated superiority.
Unlike the flycatchers, in the case of superb fairy-wrens, it is eventually the female who goes after dark to seek out the males that have most impressed her during the courtship period.
European Robin females beg for food during courtship time. Her begging calls, every few seconds, can be heard on other territories as well. If her social mate does not satisfy her hunger, her call rate increases as a signal to neighboring males to come and trade food for sex. Thus she blackmails her social mate to perform or be cuckolded.
When razorbills mate, the female must be prone; thus she controls the mating process. All the male birds hang out in a local ‘singles bar’ (rock ledge), although most all are socially mated as well. They jostle each other, trying to knock each other off the ledge. Females come to the ‘bar’ and mate with whomever they choose. Mostly these females choose a few of the most studly males for these extra-social encounters, frequently in the presence of their social mates who stand by helplessly.
Having established that avian fooling around is the rule, one wonders why such behavior is successful. It’s all about evolution: winners and losers. A female wants the best genes for her chicks. Since she ovulates multiple times during mating, having multiple fathers for her chicks is her method to avoid ‘putting all her eggs in one basket’. And this genetic diversity is the main benefit to her, for the extra-territorial males provide no support in nurturing the female during the breeding term. They are merely sperm donors. There is also some evidence that extra-pair young are healthier than their step-nestlings, as was observed in studies of blue tits and collared flycatchers.
The genetic basis of bird behavior has been repeatedly demonstrated. Genes matter. An experiment with great tits showed that risk taking and risk averseness are strongly genetically determined. By breeding for these behaviors and controlling for environment and parenting influences, after only four generations one group became 130% faster at exploring new territory than the control, while the other group became 60% more reticent. Other traits are similarly shown to be controlled by genes: e.g. migration behavior, song learning, and fear of humans. Even without captive breeding experiments, natural selection can be observed in action by noting which phenotypes provide the most mating/reproductive success and survival advantage.
Female choice is the driver for species success. By giving more sexual access to the top-quality males, the species is constantly improved. The males thus are in a constant arms race, competing for copulations. The female is a canny judge of health and energy. Her ‘good genes’ detector is powerful, operating on observed traits that innately suggest male quality.
Sometimes it is an abstract genetic quality that she favors, but often it is a self-complementary genetic outcome that drives the decision. Thus a female may seemingly overlook a grand male specimen, providing access to a lesser male who specifically enhances her own genes in the sense of being most dissimilar to her. It is not known how females judge genetic dissimilarity. Some non-avian species use odor, but perhaps in birds, the compatibility choice is made at the sperm-ovum level. Offspring of dissimilars often have enhanced immune systems, as has been tested in Savannah sparrows, half of whose males are bigamists; their territory hosts two resident females. Two-thirds of the females have illegitimate young.
Environment change will likely change standard mating behavior, as was demonstrated in Vermont with the Savannah sparrows. In June, 40% of hay fields are mowed early, wiping out nests. The birds begin again, but in a different environment where all territories are equalized in quality. In the mowed fields, the old bigamists were no longer in possession of prized territory, so no longer had two females. Cuckoldry lessened, lessening access to genetically compatible sperm. Early mowing has changed the delicate interplay of male competition, lowering nesting success and causing population declines.
Some non-tropical birds are genetically monogamous: barnacle goose, chinstrap penguin, common loon, Wilson’s storm petrel, New Zealand Robin, silvereye, Carolina wren. However, the author delves into a different aspect of monogamy, that related to asynchrony of fertile periods in tropical birds.
Where the climate’s seasons switch only between rain season and dry season as they do in equatorial areas, the avian breeding season is lengthened accordingly, meaning that females need not and hence do not become fertile all at the same time. It is typical in tropical birds, such as the dusky antbird, that fewer than 10% of females are fertile at any time. This unsynchronized fertility lessens the mating frenzy typically observed in songbirds of the northern temperate zones, where females become receptive to males all at once.
The author compares the northern tree swallow and its sibling species, the tropical mangrove swallow. A first clue is the relative size of the testes; although the birds of each species are similarly sized, the testes of the tree swallow are 15x the size of those of the mangrove swallow. The reason is the male tree swallow’s need to service many neighboring females all in a short time span. The male mangrove swallow has much less demand and hence much less need for a large semen supply.
The longer tropical unsynchronized breeding season also results in far fewer sneaky couplings and resulting extra-pair fertilizations. Since neighboring pairs are usually operating on a different nesting cycle, a wandering male will likely be rebuffeed by neighboring females, so that a male bird usually only copulates with his social mate. Field studies bear this out; in a study of the tropical buff-breasted wren, only 1 in 31 nests contained an extra-pair offspring. The comparison with northern mating is striking; 75% of female tree swallows tend eggs fertilized by more than one male. 86% of passerine (songbird) species practice similarly frequent extra-pair mating, where the degree of infidelity is proportional to the degree of fertility synchrony.
Many tropical avian species have ‘ant’ in the name to show they follow army ant hoardes through the forest. The approaching ants flush out forest insects, giving the birds that follow them some easy pickings. The dusky antbird, contrarily, does not follow the ants, but instead defends a modest territory to ensure a good year-round supply of insect food.
Dusky antbirds breed during the season of most plentiful food, the rainy season from May to December. The male and female do everything together, including building a bag-like nest in which the female lays the two-egg clutch typical of tropical passerines. The long breeding season permits several nesting opportunities per year, which is necessary because 90% of nests are eaten by predators. Such a high mortality means that even after several nesting attempts, many pairs end up with no offspring in a year. The one or two offspring that survive a nest live with the parents through the following dry season. DNA testing demonstrates that dusy antbirds are genetically monogamous.
Clay-colored robins are a tropical bird that may have transitioned from a temperate bird at some distant time. They mate during the dry season in February, because the adults survive on fruit that is still plentiful when insects are not. Their breeding is however synchronous and the males have large testes, so according to the author’s hypothesis, cuckoldry should be demonstrably rampant. Ineed, field tests confirmed 50% of nests contained extra-pair fertilizations.
The association of breeding synchrony and cuckoldry is not universally accepted, partly because avain behavior is complex and there is seldom a clear-cut answer to any observed behavior. The author cites a study of the wood thrush of temperate northern forests. Breeding is synchronous, but only 6% of nests contained extra-pair fertilizations. This is attributed to the male following the female on her hourly off-territory wanderings, robbing her of many extra copulation opportunities.
What Makes a Male Attractive to The Finnicky Female?
Females want good health in their mates. They also want good providers. Both traits are representative of good genes. How does the female determine good genes in a male?
Coloration, energy, singing competence, and male-male aggression are all evidenced when males display in front of a female. Females have an innate ability to choose well, so that sexual selection improves the species.
Coloration clues females unfailingly. Color can indicate age (survival competence), diet (provider skills), and aggression (defense skills). Birds’ vision extends into the UV range of the spectrum, so that they perceive colors differently than we do. Their enhanced color perception makes them able to detect more subtle differences than we can see.
In purple martin colonies, females will choose older males who advertise their age by their steel-blue iridescent plumage. Parasites are a leading cause of death in migratory song birds. Also, half of these song birds perish during migration. Birds that survive two years are likely genetically superior individuals. Females mated to older males will remain monogamous, while those mated to younger males (with female-like plumage) will try to cheat with older males, so the younger males must follow their mates around. Older males will normally control more territory (more nest boxes).
Females look for traits that males cannot fake. Bright red, yellow, and orange coloration of beak and feathers are due to carotenoids, only available through diet. Carotenoids are used by the immune system. Only in the presence of excess will a male have the luxury of storing carotenoids in his beak and feathers. Red coloration in house finches has been shown to indicate increased disease resistance.
Color is further an indicator of being a good provider. A male with access to carotenoid-rich food in excess will likely control better territory and be more efficient in foraging. European blackbirds with orange beaks are seen to be better providers than those with paler yellow beaks.
Blues with high UV reflectance are also not easy to fake and hence reliably indicate quality, as seen in the purple martin above, the bluebird, and the blue-footed booby. Bluebirds with the most intense plumage color are observed to control more territory, better provide for their mates and to produce more and healthier offspring. The same is observed in boobies with the brightest blue feet.
Black color comes from melanin, easily manufactured by a bird. In some cases, such as the common yellow throat, the size of the black mask indicates good genes to the female. Why don’t males cheat then and grow ever-larger black patches? The answer: they cannot back it up. The bigger the black patch, the more challengers want to defeat you; those with a faked black patch will be easily determined by watching the contests.
Energy level difference is demonstrable in many male song and performance displays. Males with parasite problems or other infections, or who have a lesser diet, cannot perform at top levels. These differences are invariably noticed by the female. An experiment with blue-black grassquits demonstrated the degree to which freedom from infection benefitted energy levels during courtship displays.
Avian habitat is constantly being modified by human intervention. Much of a bird’s mating display and performance evolves to be highly tuned to a specific environment, and when habitat suddenly changes, birds quickly adapt with different sexual selection characteristics. The small greenbul is native of pristine tropical rain forests. As these are chopped down, the greenbuls adapt to more sunlight and more space.
In the brighter light, the advantages of high UV reflectance diminished and male color changed accordingly. Males in the open evolve both shorter wings, to maneuver in the thicker understories of the plantation habitat, and shorter calls with higher pitch to remain effective in an environment of greater wind turbulence. These changes result in rapid speciation within a species, for example the forest greenbul and the plantation greenbul. In a hundred years, the deep forest variants may disappear.
The Songs of the Best Males
Male singing requires energy and thus is a form of energy display to females. Birds that can sing the earliest, the longest, and the most complex songs will win the girl. Since early morning singing is done in a fasting state, it is further demonstration of energy reserves. Like many birds, female hooded warblers find singing performance even more informative than coloration in their selection process.
Silvereye males may have up to a sixty syllable repertoire. Some can sing for 40 mminutes, while others can only go for 15. Experiments show that males who ate the most the day before did the best in their song display.
Male prairie cickens put on booming displays each morning on their tiny square meter territories of bluestem prairie, maintained by the Nature Conservancy. Such grassland once covered half of Minnesota. Prairie chickens have declined 91% in the last 40 years, reflecting a total 95% loss of habitat. Those remaining will use retired pasture and hayfields, but only those having vegetation a few years old. Many retired fields have been returned to production of subsidized biofuels, causing continued pressure on grassland bird populations.
First, the male raises his neck fethers erect to resemble rabbit ears, revealing bright yellow patches of skin on either side of his neck. Then stamping his feet rapidly, he lowers his head and inflates his yellow throat sacs like balloons, next releasing the air to make a sound like blowing on a conch shell. Every few minutes, dozens of times per morning, the display is repeated.
Female prairie chickens will sometimes come by to view the display, but may take days to make a choice of mate. Males who control territory near the middle of the display area are most preferred. Males on adjoining territories will try to face each other down, and if no one blinks, a kicking, pecking, wrestling brawl will ensue.
Birds, especially oscine passerines (songbirds), produce more detail in their songs than we can hear. Deep in their throats the airway splits into two tubes called the syrinx, analogous to our larynx but completely different. The syninx is controlled by a large number of tiny muscles to control pitch and volume. In some species, the two air tubes can be controlled independently, producing two different songs simultaneously.
While birds have a wider range of color perception, extending into the UV band, their hearing responds to the same set of frequencies that humans hear. But the time resolution of hearing and sound production exceeds our hearing ability; playing back bird song at half speed shows us the amount of detail we miss. Thus, for humans, looking at bird song waveforms is more instructive than simple listening. Blue-headed vireos have up to eight complex syllables in their repertoire, strung together about two seconds apart to make song. Single syllables can be selected for communication with the female.
Songbirds learn their songs during their first few months. The first egg is bestowed with more nutrients, so that brain development can be compromised by the last egg, affecting the size of vocal control centers and thus complexity of song ability. Parasites during development also compromise singing competence.
In European starlings, it has been shown that aggression and song output are inherited from the father, where polygynous males are more aggressive, control more nest boxes, and have higher song complexity than monogamous males.
When humans interfere with an avian species’ historical environment, the innate behavior patterns above can be interrupted, altering sexual selection balance sheets forever. Starlings that feed on worms in human sewage plants benefit from enhanced nutrition and also from estrogen-like chemicals that enlarge the vocal control centers of the brain. Hence these birds will sing very energetically, but also have compromised immune systems because estrogen is an immunosuppressant. With such birds, the singing competence belies a physically compromised male and the female cannot correctly detect the poor quality males; her historical and automatic selection strategy now fails her.
A study of great tits, one population near a metal smelter, showed that lead and cadmium-loaded males from the pollution zone sang with muted voices, their songs were a third shorter, and they knew only half as many songs. Robins were long ago studied for DDT pollution. DDT reduced steroid levels in the egg, leading to males that had 15% diminished brain volume and a 30% loss of vocal control area. The really bad news is that this damage results from DDT use 25 years prior to the study.
Birds in urban areas have long been modifying their songs to avoid being masked by human background noise. Bird vocal control admits a degree of plasticity. In natural environments, they can change their songs depending whether they need to sing in open of forrested areas; for example, short, high-energy songs do better in open windy sites, while long-wavelength, low-frequency song is more effective in avoiding interference by tree trunks.
Urban birds opportunistically use this plasticity to adjust to noisy environments. Robins may sing at night before noise ramps up. City nightingales sing louder. Birds recognize that their open terrain song versions are also more suitable in high noise environments. Great tits in urban environments across Europe have abandoned the low-frequency syllables in their repertoires.
These changes are attributed both to natural song plasticity and to learning by juvenile birds. Great tits are fortunate that their pre-evolved singing capability enabled them to adjust to a noisy environment. But we don’t know how the new songs affect the female and her sexual selection strategy. Separate species may result when rural birds of a species will no longer accept urban birds as mates.
Why Birds Divorce
Divorce rates among bird species vary from 0% for the wandering albatross to 99% for greater flamingos. Biologists attempt to deduce the consequences of divorce, thus to better understand who are the winners and losers, and thus to tease out the selection advantages to monogamy vs. non-monogamy. Divorce may happen because of incompatible mates (behavior or genetic mismatch), who will benefit evolutionarily from forming new pairs. But mostly, avian divorce is resource-driven.
Long-line commercial fishing is dooming the albatross. Before this fishing practice, the albatross was king of the oceans, flying effortlessly for any required distance to obtain food. Now, 19 of 22 species are threatened. Choosing a mate is a long affair that begins after they turn eight. The female visits the lek, dances a stylized dance with the male who ends the dance with his sky call. Then they bow to each other and the female walks away. They repeat this dozens of times over two years before the bonding is complete. She lays one egg a year. They take turns tending the nest and foraging. One partner cannot raise the nestling alone. The investment in selecting a mate and then caring for the young means monogamy is essential. Widowed birds must again complete the two year mate selection ritual prior to mating again. The albatross will require our help to remain viable.
The Eurasian oystercatcher lives over 10 years and has an annual divorce rate of 8%. Switching partners to improve territory seems the main reason; ocean front is preferred over inland, because the chicks can follow the parents to the mud flats, rather than waiting by the nest for the parents to fetch the food. The benefit of living by the mud flats is a 20% increased chance of reproductive success. This particularly works after bad winters, when there are widows and widowers needing new partners.
In good years, there is still benefit to try to invade a better territory and drive the same-sex bird off, a forced divorce, usually instigated by the female. It takes many days of perserverence to accomplish a forced divorce, and it is not possible if the mate offers defensive help. But mostly, the other-sex bird just watches passively to see who will win. Along with the dumped mate, the eventual divorced bird suffers by half in future reproductive success rate, while the new pair prosper on their better territory.
Divorce always has costs, even to the winners. New oystercatcher pairs take longer to begin nesting, causing a ten to twenty day delay in egg laying, lowering reproductive success. Pairs with history exhibit a finely-tuned coordination needed to jointly defend their territory, find food, and defend their nest from predators.
Common guillemots, cliff-nesting shorebirds, live over 30 years and have an annual divorce rate of 10%. They are fishers feeding off-shore and living in dense colonies of tens of thousands. The female lays a single egg on bare ground, likely a ledge of the cliff. The egg, being quite pointed on one end, will roll in a circle, but many still fall off the ledge during bad weather or parent handoff errors.
Hotly-contested territories are only 10 centimeters across, not much larger than the egg. Pairs return annually to within a hands-width of the same site. Observations at the Isle of May in Scotland show the favorable sites have been 100% occupied since the 1930s. All guillemot dvorces are caused by aggressive interlopers. The bird forced out always experienced lower reproductive success afterward, the result of having to relocate to lower quality territory.
Tropical songbirds who remain in situ on permanent territories form more stable pair bonds than do migrating songbirds, principally because of the high death rate of the migrating birds. Stable examples are checker-throated antwrens and buff-breasted wrens in Panama. Pairs of buff-breasted wrens do everything together, including defending territory. In one study, the briefest pair-bond was 4 days, when the female was evicted by an interloper. The longest was still going after 4.5 years. Mate switching only happened among inexperienced pairs during non-nesting season. Once going through a breeding season, nothing broke the pair bond except death.
Divorce occurs infrequently in dusky antbirds. They have an observed 10% mortality rate annually and longevity of at least 12 years is reported. Mated pairs remain together for five years or more, but each year a few show with new mates. An empty slot in any territory does not last more than a day; the birds are opportunists. Each such switch creates a cascade of switches as birds seek oppoetunity in better territory. If the original bird that went missing returns again and successfully claims the old mate and territory, the entire cascade is then undone. Dusky antbirds are an example of birds exhibiting permanent pair bonds that result from lack of opportunity rather than loyalty.
Snail kites have up to a 60% divorce rate. The male builds the nest, helps incubate the eggs, and then feeds the young. One-third of females quit the breeding job early to hook-up with another male and rebreed the same season. But it’s not one-sided; 20% of males beat the females to the punch. The deserters come mainly from territories where snails are plentiful, so there is small threat to the offspring they abandon. Further evidence is that divorce rates of 90% are observed when only one chick is being raised; the rate plunges to 0% when three chicks need care.
In Italy, rock sparrow mates are also in a race to see who divorces first. 25% of males desert part-way through breeding season, while 10% of females beat the mate to the punch. The female doubles her foraging when left alone, fledging all her offspring. The lone male that gets left cannot increase his feeding efforts to compensate for loss of mate, and fledges fewer young. A test determined that when the female was fertile again, rather than anticipating her leaving and divorcing her first, he paid more attention to her, enticing her to stay.
The penduline tit has a 100% divorce rate and hence their pairings cannot be called a bond. The pair builds a hanging, bag-like nest together, but as soon as the female lays the eggs, one or the other leaves. Most often, the female is left to care for the young, since insemination occurs before egg laying, the male gets a head start. The female tries to disguise the process of egg laying, burying the eggs at the bottom of the nest bag and not allowing the male to enter. The rush to divorce is so strong that 30% of the time, both male and female leave, dooming the eggs. The strategy pays off, because eggs are cheap, and the ability to re-nest up to six times a year makes foregoing parental care productively profitable.
Usually, humans handling nestlings during such bird studies does no harm. But the blue-headed vireo female has been seen to abandon the nest. In such cases, the female must be removed from the area first. Apparently, females divorce as soon as fledging is near, leaving the male to finish up. If a female returns to see the nest empty, she assumes the young have already fledged and she leaves abruptly. If the young are returned to the nest after that, there is no one to keep them warm at night; that’s not the male’s job.
Dynamics of Nesting and Incubation
Cooperation is essential in caring for young, but the familiar tug-of-war regarding who does the most work is frequently part of the dynamic that leads to tension. Divorce while raising young is sometimes the abrupt result, but there are subtler ways that self-interest manifests during breeding season.
Even in monogamous species, conflict exists when mates do not stand to gain equally, as in a cuckolded male. The hooded warbler makes a thousand trips to feed young, 30% of which are probably not his. But he has no choice because he cannot tell if they are his. To compensate, the male will sneak copulations with neighboring females while foraging for his nestlings.
Males frequently must do double duty, caring for young vs. defending territory. When both male and female take turns with incubation, the eggs are seldom exposed to predators. In the blue-headed vireo, the male responds instantly to an intruder when it is not his turn to incubate. But while on the nest, the male does not leave the eggs to challenge the intruder. In half the situations, he sings a challenge song from the nest until his mate returned. Then quickly the male would be perched next to the intruder, challenging him. Frequently, the female would return early to allow the male to defend their territory when under challenge.
Highly promiscuous vasa parrot females have many males with whom they will copulate. When a female is incubating, she sings loudly and can be heard for a kilometer away. This attracts all males within hearing distance to visit her and share their food with her. She benefits because the dietary fruit comes from widely scattered trees that fruit at different times. She could not tend the nest and feed herself. The males benefit because the most dependable ones will be allowed to copulate with the female in the next breeding cycle. The birds are intelligent and have good memories.
Siblicide is common practice among herons, eagles, cranes, pelicans, and boobys. Two eggs are laid, and the first-born is programmed to kill the last-born while the parents look the other way. The mother lays an insurance egg, but if both hatch, she cannot easily feed both. The parents and first-born are each better off if the second-born meets a sacrificial death. In some species, the ul;timate decision on the second egg depends on available local food supply. In a good food year, the parents stand to double their reproductive success by rearing both offspring.
Male offspring are larger and require more food. Murres make 10% more feeding trips for sons than daughters, and the parents of sons lose more weight. Rhinoceros auklets depend for sustinence on sea upwellings that vary greatly in quality from year to year. Auklet pairs tend to have daughters when food is scarce and sons in times of plenty. It is thought that stress hormones influence the sex of the offspring.
The kakapo, a large, nocturnal, flightless parrot, is battling extinction. They breed only in years when food is plentiful, every 3-4 years, and hence produce more male than female offspring. The imbalance among the sexes is dangerous for their survival. They nest on the ground and are susceptible to predation by introduced mammals. In the 1980s, their population dropped to 82, only a quarter females. There were only a few nests each year, so predation was threatening survival of the species. All the remainder were moved to predator-free islands. But their decline continued. Many adults were several years old and had not bred since being moved. Making food more plentiful succeeded only in producing more males by a 13-5 ratio. As a final attempt, food was witheld until the eggs were laid. Then two-thirds of the offspring were female and they were successfully reared.
Selfish behavior is so predominant among birds that instances of apparent altruism demand explanation. Helpers exhibit such behavior, forsaking nesting themselves and then volunteering to help other nesting birds by sharing the burden of parenting. Because helpers forego breeding, they will be under represented in the next generaton and eventually should disappear altogether. Parents gain from helpers because their offspring will more likely survive, and the lessened stress on themselves may add a year to their breeding life.
Why is helping behavior so extensive, particularly in the tropics and among crows and jays? Australian fairy wrens have up to four adult helpers on their territory, foraging for food and helping to defend territory. In Venezuelan stripe-backed wrens, helpers are always female. South African sociable weavers breed in colonies of up to 200, building a large communal nest up to 30m across. The use of helpers depends on food and water availability; between 30% (abundance) and 80% (scarcity) of breeding pairs have helpers, some up to five helpers.
Male helpers would clearly benefit genetically if some of the offspring are theirs via surreptitious copulations. Also, helpers that are close relatives of the parents could benefit genetically. This is the case in socialable weavers, where genetic testing reveals most helpers are former offspring of the breeding adults.
Sometimes, offspring move away, only to return in subsequent years to be helpers. For this to be successful, they must recognize their parents. Long-tailed tits in Britiain recognize their parents via a special call they learn in the nest.
Why don’t helpers find their own mates? There may be a shortage of breeding territories, in which case, staying close and waiting for parents to die may be a good strategy. Also, spending a year or two learning how to parent may be advantageous. Food scarcity also appears to determine if young birds decide to help or to mate themselves.
Superb fairy wren nestlings received 20% more food via helpers. This benefited the mother more than the chicks, because when helpers are available, she under-invests in her eggs. The helpers compensate with more food, so the chicks end up a normal size, while the mother has more reserves to invest in the next breeding session. Particularly with long-lived birds, overall breeding success increases with the number of years a bird can breed.
Seychelles warblers completely occupy Cousin Island, where there is a large difference between quality of territory. Here the ability of parents to adjust their breeding to their circumstances has been well-documented. Pairs on good territory have daughters, because daughters will remain and help the next season. With food abundance, helpers are valued for extending the breeding capacity of the parents. Pairs on poor territory produce sons, because sons leave the territory. Poor-territory pairs have low access to insect food, so helpers are too big a drain on resources.
A large part of bird selfishness is driven by the need to conserve resources. In poor years parents may alter the gender of the offspring, allow siblicide, forego breeding and help another pair, or abandon a nest entirely. As human population growth continues to put stress on bird environments, their plasticity in accommodating down cycles may be their greatest evolutionary strength.
Finding a First Home
Young adults must find their own territory, although territories are scarce. Until they do, they may be floaters who wander widely, hoping to find a vacancy. Or they may cooperate with breeding pairs and live on that territory openly. Regardless of which style is adopted, first-time home ownership is difficult to achieve. In migratory species, the older birds return first and sieze all the good territory, so young adults have little opportunity even if migratory.
In cavity-nesting tree swallows, only 15% of first-year females find a tree cavity. If a breeding female leaves her cavity for more than ten minutes, a floater likely has discovered it and entered. Such interlopers quickly retreat if the rightful owner returns.
Male tree swallow floaters can take over a nest cavity if the owning male goes missing. If the interloper arrives after the year’s single mating is completed, the floater male would need to help raise offspring that are not his own. Further, with only a 50% chance of surviving to the next season, waiting until then to breed himself is risky. So the new male has litle biological choice except to kill the nestlings, forcing his new mate to re-nest.
In some species such as purple martins, rather than floating, non-breeders remain and harass owners of surplus territory and resources. Others species’ non-breeders will remain and wait patiently in line for an opening. Yet others steal others’ territories. Floaters and non-breeders form a ‘secret society’ that can be hard to observe. Since floaters do not defend territory or sing, counting them is difficult.
Chickadees form flocks in winter. Flocks have defined territories, and each flock has a pecking order. Many young birds join a flock their first winter and wait to move up the pecking order, hoping eventually to breed. But other more aggressive birds float between flocks waiting for a high-status vacancy. There is often no serious competition for such a vacancy, because the pair just below this slot in status has no real incentive to move up in rank just one notch. The floater takes advantage of this social inertia to jump the line. Such floaters ignore low-ranking vacancies which would not offer breeding opportunity.
In some species, for instance rufous-collared sparrows, a young adult has a large, defined wandering territory that overlaps various breeding territories, allowing it to stand in several lines at once. So long as the non-breeder does not pose a conflict to the breeding pairs, it is allowed to remain. This allows the non-breeder first access if a breeding vacancy occurs in its territory.
Floating is a risky strategy that often fails. Most Seychelles warblers stay at home the first year as helpers. These frequently end up with a nearby breeding opportunity by the next year. Those that leave home prematurely mostly are still non-breeders a year later. But staying home is not an option if one parent dies and is replaced by another mate. 15% of adults die annually.
Breeding territories of Siberian jays contain a mated-pair and up to four non-breeders, former offspring or wanderers who choose to remain. These older offspring, who can remain on home territory for up to three years, will aggressively expel any of the current year’s offspring. Being able to remain at home increases their breeding opportunity and future reproductive success.
Ousted young jays will travel up to 20km to find another territory that will accept them, usually one with a non-successful breeding pair that hence has no queue waiting on-territory to challenge newcomers. Then they may wait another three years to inherit a nearby breeding position. These non-breeders are not helpers. They are just forming resident queues for a possible breeding opportunity.
Jays are not inclined to create new territory. They always settle on an existing territory that will accept them. Thus jays among others will not recover easily when modern logging and forestry practice fragments forest and destroys available territory.
In southeast pine forest where fire used to be a regular occurrence, dead trees would burn but live ones would largely survive. Thus the red-cockaded woodpecker became the only North American woodpecker to drill holes into living trees for nesting cavities, old-growth trees at least 80 years old so the cavity can exist entirely in heartwood and so avoid sapwood that would foul the bird’s feathers. Such cavities are used for decades by several generations.
Males stay at home, so in any season, there may be the parents and several brothers and step-brothers to help with feeding, incubating, and cleaning the cavity. The birds ensure lots of sap flows around the cavity to discourage black rat snake predation.
Since less than 3% of long-leaf pine forest remains, the red-cockaded woodpecker is endangered. There are now only six thousand breeding groups spread from Texas to New Jersey. Further, the small remaining forests that have not been logged are managed, meaning fire prevention measures that encourage the growth of dense understory growth, preventing woodpecker access.
Young females must leave home and 70% die during their search for a new home, which is made more difficult every year as good environment diminishes. Biologists now intercede, drilling new cavities in the older available trees, and moving pairs of birds to such new territory, hoping they will persist there.
Aggression and Defense
Birds contest territory, food resources, and mate access. Intruders have less to gain than the resident, so usually fly away at the first sign of trouble. If there is a physical contest, the bird with the greatest genetically-determined need to win will prevail.
Male-male territory squabbles in early spring are usually about the exact location of the boundary, and are usually settled by negotiation after a day or two of posturing, threat calls, singing, and chasing. The bird that has motivation and skill will prevail; the one with less taste for the contest or less stamina will lose territory to the stronger bird.
Food resources are usually managed by the social hierarchy determined on any territory. Sneaky copulations among hooded warblers are successful four out of five times, so the intruder will leave if challenged, waiting for a later opportunity.
Battles for an entire territory can be the fiercest, as both sides have equal incentive to win. This has been seen when a male is removed from his territory and a floater moves in. If the rightful owner returns within an hour or two, the floater leaves without incident. But if the new owner takes uncontested possession for several days and then the original bird re-appears, a fierce ownership contest can ensue. And with tropical species such as the dusky antbird which lives for years on the same territory, the owner’s incentive to fight is greater than in the case of migratory birds who ‘own’ territory for only a few weeks.
Migratory birds get a rush of testosterone at the start of mating season, when contests are frequent and stakes are high. Later, when it is time to feed the offspring, the testosterone goes back low, because it inhibits the necessary parental behavior, and also inhibits the immune system. During spring, songbirds will sing over a hundred songs per hour, up to 40% of their day.
Tropical birds have low testosterone levels even during breeding; it isn’t needed because they fight infrequently. But they can still fight fiercely, so testosterone is not the entire trigger. They sing only about 12 times per hour, just 5% of their day.
Birds build social rather than physical fences. When a fence line is crossed, posturing and threat calls are frequently all that is needed to enforce the boundary, avoiding physical combat. Singing is a form of aggression at a distance, sparring without blows, most conspicuously at dawn or just before. We hear pretty singing, but they hear statements used to intimidate rivals and forestall theft of resources.
Threats are most effective if they are backed by real aggression potential. Color and song complexity are two natural clues to aggression potential, allowing birds to size each other up and to avoid any lopsided contests. An equalizer in any aggression calculation is which bird has the most to lose. That bird may be able to overcome a natural genetic or nutrition deficit and drive away a stronger combatant.
Singing before dawn in the cool of night is an effective demonstration of strength, because complex song at this hour of physiological low is a true test of energy reserves. While singing is frequently a response to territorial intrusion, it is also used for preventive maintenance, a pre-emptive display of strength.
Following an intrusion on their territory, male wrens in winter are seen to increase the energy of their song the following morning, anticipating the possibility of further insults. Some songs can suggest extreme aggression and are reserved until the need arises. Songs are distinctive, serving to identify the bird so that a response can be gauged based on prior interactions with that bird.
If a rival’s song overlaps one’s own song, it suggests a strong threat that must be responded to. A nightingale study shows that when a bird has received a threat from a strong rival, the next dawn’s song output is appropriately more elaborate and threatening. A nightingale can interpret certain parts of song as a measure of the real threat he faces. Rapid notes spread over a wide frequency range suggest high male quality and hence a serious threat.
Presented with simultaneous songs by two simulated challengers, the territorial male always responds by going to face-off against the more serious-sounding threat, aggression met with aggression. Males learn to know the rivals in their immediate vicinity. If a challenger is a higher-quality bird, the territory owner may need to negotiate a boundary and back off. An unfamiliar pretender, if a lower quality male and hence likely unmated, will be quickly identified in these exchanges; it will abandon the exchange and move away.
While song is used to communicate aggression at a distance, feather color and posture are used as a close-range display of quality and intent. Color reveals a bird’s condition when the feathers grew earlier in that year, a historical record to complement the present physical condition evidence of song quality. Feather posture is an instantaneous indiction of intent. Feathers and wings held close tend to make a bird look smaller, a submissive gesture. Ruffled feathers and extended wings indicate a willingness to engage physically.
Testosterone is the enabler of aggression, with feather coloring being a visible indication of testosterone level. Different colors in a species may serve different purposes. Lark bunting males are black with white wing patches. The larger the wing patch and the darker the black, the greater the aggressive tendencies. Yet only the size of the wing patches discourages intruders at a distance. Lesser wing patches allowed intruders to come closer. Only at close range could the shade of black be assessed. Darker black in the territorial bird would then discourage the intruder. The colors are effecteve at different distances.
In the Australian golden whistler, there are multiple contrasting yellow, black and white colorings, but only the the size of the white throat patch indicates aggressiveness. Intruders with the largest patches invariably attract the most attention from the territorial bird.
Males are not the only aggressors. Particularly in cavity nesters, control of a usable nest cavity is essential for reproduction, so females will defend them with great determination. Testosterone may still be the enabler of aggression, but it operates at much lower levels in the female body. The hormone estradiol has been shown to be higher in more aggressive females.
In migratory songbirds, sexual roles are highly specialized. In bluethroats, females will challenge intruders with similar singing displays as males use. Female singing is less complex than in males, frequently consisting of simple call notes to keep intruders at bay. The females do not receive help from their mates when defending nesting resources. Should a male come upon a female intruder while the territorial female is away, the male bird will perform a mating display, an invitation for the intruder to move in.
Acadian flycatcher females use one call constantly to let non-territorial females know the territory is occupied. Should an intruder show up anyway, the territorial female approaches and utilizes a more aggressive challenge call repeated every five seconds.
The tropical passerine sexes are more equal in sex roles and in aggressivity, offering shared response to threats. Both sexes sing, both participate in nest building, cuckoldry is infrequent, and mates live together year-round. Normally, they sing only 10 times an hour, but in the presence of a threat, the song rate goes up to 200 times an hour. Males take the lead, both by starting sooner, singing longer, and taking the lead in duets. Cooperation is evidenced by the female singing to counter a male intruder and the male singing to counter an intruding female. Removal of one bird, a forced divorce, was not followed immediately by acceptance of a new mate; vacancies remained open for a while.
For migratory songbirds such as the hooded warbler, the sexes split up in their wintering grounds and defend separate territories. In this unfamiliar environment, the migratory birds assume a background profile and sing little, communicating via innocuous calls. Usually, females and males share boundaries. Older females have more male-like coloring, which may help them defend their boundaries against younger floaters, even males. But older females gain no preemptive advantage against an aggressive male, making it an open question how male-like coloring benefits such females.
Some species, often cavity nesters, experience sexual role reversal. The Australian eclectus parrot female has the prominent markings and the male just blends in. Territorial contests are waged between females. Males hang around the tree cavities and offer food for access. Here, male aggression is limited to attempting to knock each other off the nearest branch to the nest cavity. In contrast, female contests for a good cavity are sometimes to the death.
Bird aggression is not usually noticed by humans since birds do not threaten us. Alfred Hitchcock attempted to dramatize bird aggression in his over-the-top movie The Birds. But from a bird perspective, the movie is right on.
Bird colonies can number in the hundreds of thousands of residents, or as few as dozens. The larger they are, the more noisy and chaotic life can be there, just as with human cities. In dense colonies, each resident may control only the space in which its body fits. Large colonies have all the negatives we would expect: non-stop competition and aggression, food depletion near the colony, attraction of predators to an easy feast, and spread of disease and parasites. Studies show that cuckoldry increases in larger colonies. Availability is the great facilitator, and the larger the colony, the more the testosterone and the larger the testes of the males that live there.
Obviously, there must be positive compensations, a subject of ongoing study. Safety in numbers is likely part of the success formula; thousands of eyes are on the watch for predators, plus better odds that if the colony is predated, one’s own nest might be spared. Seabirds often build colonies offshore in places unreachable by predators.
Birds are social and nosy. They know a lot about who their neighbors are, what is their quality, and how well they are doing. When young, they float around and observe. The most useful information is the location of a food source nearby. Whether insect or fish, food moves around. Birds observe where others successfully hunt, to know where to find food without searching themselves. Such inadvertent absorption of information causes a group of birds to become a dynamic information catalog, lessening dependence on individually wasteful trial and error.
It is easy to theorize that if a bird returns empty handed and a neighbor returns with food, the hungry bird will inevitably follow his neighbor next time. The author was unable to confirm such behavior either in bank or cliff swallows, however. It appears that the costs and benefits of coloniality, and the workings of the colonies, varies among species, and between colonies within species.
Colonies appear to be formed by independent birds following the usual rules for their species. If a good nesting place is found, any bird in the area will recognize it and set up house. Communal defense and group foraging may be by-products of a large group, but they are not what influences birds to live there. These dynamics operate in the background, enabling the colony to thrive. A single metric, colony size, represents the net result of all such inherent successful group strategies.
Observation confirms that larger appears to be better. When a bird switches colonies, it is always to a larger one. Lesser kestrels in Spain colonize buildings like abandoned farmhouses, with some colonies reaching 40 pairs. They feed mostly on large insects: grasshoppers, beetles. Nests in smaller colonies were three to five times more likely to be predated than in larger colonies. Survival of breeding adults were 10% higher in larger colonies, where predators were unable to reach the nests.
If all the advantages accrue to residents of large colonies, why would birds choose to occupy smaller colonies? Perhaps a bird would choose a small colony if the costs of living in a large colony would fall more heavily on it than other birds. For instance, young birds might better start out in small colonies. And some small colonies enjoy good nesting success. There is a further threat that larger colonies could be destroyed by rogue predators. Sometimes, attracting less attention is safer.
Crowded colonies have one very significant cost: increased disease transmission. In purple martin colonies, nest parasites such as feather lice, mites, and fly maggots are ubiquitous. Cliff swallows suffer swallow bugs, similar to bedbugs, that plague nestlings. Two hundred to seven hundred bugs feed on a single family.
Nestlings will sometimes abandon the nest for a certain death to escape the torment. Those that stick it out end up smaller and weaker than normal. Such parasites survive harsh winters and are waiting when migrating birds return. Studies show that reducing the number parasites in the nests doubles the number of nestlings that survive, and these will then be 10% larger.
A further threat in large colonies are viruses transmitted by the nest parasites such as swallow bugs. In larger colonies, 50% of nests will offer exposure to virus. Yet, young birds continue to return to the larger colonies each year. They can see by the number of nest cavities how large the colony is. But they are undeterred. Big is instinctively better. Large colonies are evidence of food availability and of safety from predators. Otherwise, they would not be large.
Black-legged kittiwakes nest in cliff colonies. Reproductive success is seen to be similar in consecutive years for a colony, but may over time diminish due to outbreaks of ticks. Young kittiwakes don’t breed until they are at least three. In those early years, they gather information. When they reach breeding age, they join a colony that they observed did well the prior year. Thus colonies that had a successful year will grow rapidly the next year.
Black kites are raptors that nest in cliff colonies and feed on patchy fish distributions in nearby lakes. Nesting success depends on nearby fish, and on protection from eagle owl predators. Neither quality of a nest site can be discerned from the physical appearance of a cliff. Young birds settle near a pair that was observed to do well the prior year. They also learn where to hunt from the successful neighbors. The established pair are not welcoming of the newcomers and their added competition, but the new pair has much more to gain than lose by remaining and fighting off efforts to expel them.
Starting a new colony is unusual because of the uncertainty of a blank slate, the absence of an information catalog. This intuitive adherence to established colonies troubles wildlife managers when they want to move all or part of a colony to another site, one that might have been previously wiped out. Unless birds are already at a place, it is nearly impossible to get colonial birds to nest there. So they must be tricked using decoys and recorded bird calls in the hope that any socially attracted ‘volunteers’ will remain long enough to attract others. Once a good nesting year is experienced at a newly seeded site, the colony will grow in ensuing years.
New Atlantic puffin colonies have been successfully seeded using mirrors and two-dimensional cutouts resembling birds of the species. The birds are not fooled, but they are curious enough to linger, and a chance encounter with another curious bird may get things started.
Similarly, common murres nearly always remain in their colony. Abandoned colonies are seldom recolonized. One such colony in California was decimated by gill net fishing and off-shore oil spills. The colony went from two thousand birds to none in five years. Occasional visitors appeared, but none would take up residence in the apparently cursed place. Again, an audio system, a group of hand-painted decoys, and mirrors were used to simulate a colony. At nesting season, decoy eggs and chicks were added. The trickery worked, with an initial six pairs in year one; the thriving colony numbered 200 pairs eight years later.
Various tern colonies have similarly been re-established. A colony of Caspian terns took up residence on artificial Rice Island and was decimating the Columbia River salmon fishery due to an appetite for juvenile salmon during nesting season. The colony had previously occupied East Sand Island 25km downstream, where birds eat from the marine fishery. But East Sand Island had been a convenient site for dumping river dredgings, causing the terns to move upstream.
Wildlife managers succeeded in renovating East Sand Island and removing the glaucous gull predators that had moved in. Then after planting winter wheat on Rice Island to reduce its nest site potential, they pulled out all the tricks from their bag and convinced the Rice Island colony to move back downstream in a period of three years. This achievement helps ensure the future of this colony, for if push came to shove, fishery preservation would trump tern colony preservation, which at 8000 birds represents 10% of the global population of this species.
Between 2003 to 2007, nest mortality of the colonial American white pelican increased from 4% to 44% due to West Nile virus that had been introduced to North America in 1999. Half the world’s population of this species breeds in four colonies on our northern plains. Although pelicans are long-lived and so can ride out a few years of bad reproductive success, their reproductive rate is small, meaning that genetic resistance to WNV may spread too slowly to save these colonies. This is further evidence of the large downside of coloniality, susceptibility to extinction by disaster events.
Populations of colonial seabirds on islands off our Pacific coast have plummeted in recent decades. Warmer and more frequent El Niño years, hypothesized the result of human-accelerated global warming, have increased the number of zooplankton biomass crashes that occur naturally due to climate variability.
On California’s Farallon Islands, the population of Cassin’s auklet declined 75% from 1972-2002. Their main diet is krill. During the most severe El Niño years, adult survival decreased from 80% to 40% annually, and few of the eggs were even hatched.
For other species along the coast, rhinoceros auklets, tufted puffins, common murres, it is the same fate. Warm water means a crash in nesting success. When parents have to range more than a day’s time in search of food, it is fatal for the nest. This is a worldwide problem. Warming means colonies must shift to more northerly sites, but as seen, colonies are not easily movable.
With penguins, many of whose success already hangs by a climatic thread, any large change spells doom. The Indian Ocean’s Possession Island is home to a third of the global king penguin population. Studies show that the population reaches a survival rate minimum some two years after a sea surface temperature (SST) maximum (food minimum). A SST increase of .2ºC correlates with a 10% decrease in colony survival rate. This is the forecast SST increase in each of the next several decades at current rates of climate warming.
All birds face a delicate balancing act between nesting success and health. The greater the nesting effort, the greater the strain on the adult bird, resulting in diminished health and vitality. This balancing act is seen most vividly in migrating songbirds.
Given that a migrating bird’s hopes to survive to breed again next season are typically about 50% (observed for hooded warblers), why doesn’t the bird strive for maximum nesting success in the current year? Double-brooded females average a 60% increase in reproductive success.
A brood takes 4 weeks from building nest until hatching, and 3 more weeks of feeding the chicks. The entire nesting season is just 12 weeks. Thus a second brood is only possible if the male takes over sole care of feeding the first chicks until they fledge. Some females arrive on the nesting ground early. Those with a one week headstart are much more likely to attempt two broods.
A mid-August cutoff for nesting is necessitated by rapidly increasing risk after, for that is when birds start to molt. They are somewhat incapacitated for several weeks, during which they find some dense patch of vegetation and lay low. Birds can feed their young and moult at the same time, but the extra energy it takes means the bird may be even more helpless during the heaviest molt and when feathers regrow. The burden of the second brood falls mostly to the female.
The risk of delaying molt and caring for a second brood is that the start of migration can be delayed, with the bird being in lesser physical condition for that grueling 3000km flight. A delayed start and weaker bird means arriving at the wintering grounds late. Late arrival for such a territorial bird means getting a second-rate territory, or even no winter territory at all.
Blood testing and feather isotope testing evidence from hooded warblers and the wood thrush demonstrate that multi-brooded parents are more stressed than single-brooders. They start their moult late and some do not even finish molt until on their wintering grounds. The risk-reward calculation becomes even more complex due to dwindling amounts of acceptable wintering habitat that will increasingly favor early arrivals.
Fifty billion songbirds migrate each year, but most are small and there has not been a small enough device to attach to them to help track their movements. That has recently changed with the creation of an ultralight device that detects light and dark and includes a clock. The device is mounted before migration in the fall, and retrieved in the spring when the bird returns. Then each migration day’s light data can be downloaded.
The timing of light and darkness serves to locate the bird’s location that day. Longitude is estimated by the timing of light and dark. Latitude is estimated from day length. The two weeks before and after the equinoxes are thus black-out periods for latitude. Initially, purple martins and wood thrushes were selected to test out the devices, because of their very different habits. Martins live in the open, fly in daytime, end up in South America, and are gregarious year-round. Thrushes live in forests, migrate at night, end up in Central America, and defend territories year-round.
The first test, 2007-2008, saw two devices return in spring from a 13,000km migration; three more devices returned the following year. It had been thought that martins were lazy and would undertake the migration in a leisurely fashion. Contrarily, it was learned they flew 1600km from PA to the Gulf coast, then 800km more across the Gulf of Mexico to near Merida on the Yucatan Peninsula in just 5 days. They are remarkable flying machines. All five tracked birds ended up near Manaus, Brazil, their overwintering location. They also all averaged 300km a day going northward, with one rocket bird averaging 500km per day. Spring migration is faster than in the fall for all species.
The thrushes took a similar route but stopped in Central America. Most took two weeks to complete the 3600km fall migration. One thrush flew 1000km longer to avoid flying across the open gulf water, arriving at her destination weeks late. Perhaps she had been in poor condition to start the trip.
Adaptability of bird migratory habits has been observed. Blackcaps from Germany/Austria have begun increasingly to winter in Britain rather than Portugal over the last 30 years. It appears to be a genetic trait that is responsible, changing the bird’s instinctive navigation process based on night skies. At current rates, perhaps in 50 years all blackcaps will winter in Britain.
Two changes in environment may have triggered the switch in wintering grounds. Climate change in central Europe that softens the early spring weather may be an enabler of British overwintering, by not stressing early arrivals from Britain back on the nesting grounds. Also, an increasing number of winter bird feeders in Britain serve to increase winter survivability.
There are further selection criteria favoring the British gene mutation. Shorter migration distance favors British birds with early access to nesting sites. Coordinated arrival times also favor British birds mating with one another. The more northerly wintering sites have more rapidly varying day lengths which stimulate hormone activity in birds, causing an earlier spring migration. British birds are ready to breed 20 days earlier than their Portuguese counterparts.
Birds are adapting to urbanization. European blackbirds were formerly forest dwellers that migrated to the Mediterranean for winter. Now, they are choosing European cities as their permanent dwelling places. They are increasingly less timid and able to live among humans. This habit change allows them to begin breeding three weeks earlier than their ‘wild’ counterparts.
Counteracting the advantages of warmer city environments and greater food availability are increased predation and accidental death exposure, and various sources of pollution. There has been seen some evidence that the urbanization tendency and concomitant weaker migration instinct of European blackbirds is genetically based.
Will Birds Succeed in a Rapidly Changing Environment?
The scale of the human footprint on our planet is causing environment change at a very rapid pace, at a hyper pace in the polar regions due to polar amplification. Bird survival is favored in those species with wide environmental tolerance and the ability to adapt to high human density and urbanization. The more highly specialized birds will likely not adapt in time and hence now face extinction. And the changes we effect reach even the most isolated areas of the globe. Nowhere is excluded.
Human factors of change include greenhouse gasses that are warming our planet, huge quantities of pesticides and antibiotics that are fostering genetic resistance of pathogens, over-fishing, acid rain, and conversion of forests to agriculture. Birds have shown some resiliency to such change to date, and some genetic adaptability. But many species are seeing precipitous drops in population, making survival through this century doubtful.
Evolved behavioral flexibility appears to be necessary for ongoing survival. Some species such as the American robin are showing the needed tendencies: to explore novel habitats, food sources, nesting sites; to develop physiological flexibility for maintaining low stress levels while wing to elbow with us in our concrete jungles and exposed in our vast plantations. Robins appear to be leading the other thrushes in such adaptability. Secretive birds such as the wood thrush do not yet appear within grasp of the required survival skills.
Yet the wood thrush did survive the cutting down of the eastern forests in the 19th century. But the far more numerous and far more exploratory and adaptable passenger pigeon could not. Hunting is commonly thought to be the reason, but only an estimated 5% of the population was killed directly by humans. Their extinction is tied rather to their super-colonial behavior that needed forests to survive. The small forested patches that were not cleared could not support super-colonies, and they would not breed in small numbers. Deforestation happened so quickly that there was no time to evolve a changed colonial strategy.
Flexibility is not nirvana by itself, however, because new behaviors and new territory types bring adapting birds into conflict with new predators and new competitors, creating ecological traps. Hooded warblers are specialists of the forest gap. But as the gaps grow ever larger, these birds experience heavy nest failure due to cowbird predators who raid nests. Brown cowbirds are prairie birds that moved east as the forests were logged there, encountering a wealth of new bird species with no evolutionary defenses, easy targets for parasitic cowbird attacks.
Climate change disrupts the timing of food availability. Birds have evolved finely-tuned breeding timing to match the period of greatest food abundance. Great tits in Oxford time their breeding to a spring pulse of caterpillars. Being too early or too late in laying guarantees fewer offspring. Over the past 47 years, the mean laying date has advanced two weeks in response to warmer springs. Individual birds have the plasticity to adjust egg laying each year based on that year’s temperature at their locale.
Great tits in the Netherlands do not exhibit such complete flexibility; their egg laying is not advancing fast enough to time the peak caterpillar burst. Some females have made the full adjustment, but even strong selective pressure has not yet been able to create a fully flexible population.
Migratory songbirds are at the most serious disadvantage with respect to nesting timing because they can be far distant from their nesting site and not be able to sense a need to adapt. Some pied flycatcher populations have declined 90% when the caterpillars at their nesting territory come early. A 100 species study in Europe found the species in steepest decline were those that had been unable to shift their egg-laying timing to match food availability.
The successfully adapting populations have so far relied on their behavioral plasticity to adjust egg-laying timing. Birds are genetically wired to lay as early as possible. But we may be approaching the limits of the available plasticity. As environmental change accelerates, evolutionary change may be needed to reprogram behavior. The plasticity limit seems already to have been exceeded in the great tit and pied flycatcher populations of the Netherlands. These populations may continue to approach extinction status, as genetic change cannot keep up with rapid climate change.
An entire suite of interconnected behaviors may need change. Hormonal change related to breeding timing is triggered by day length (photoperiod) and a bird’s built-in annual clock. It is then fine-tuned by temperature, food supply, and social interaction with mates. Climate change affects all these parameters except photoperiod and built-in clock. Reprogramming of such intricate physiological processes may require millennia, not decades. There is no sign yet that migrating birds are able to evolve required changes in time to save them. In pied flycatchers, there has been no noted advance in spring arrival dates in decades.
Given median temperature projections for 90 years out, there may be expected 500 land species extinctions by 2100, with another 2000 species at risk. Sea birds face even worse prospects. Drops in sea food supply can persist through the entire breeding season. Although most are long-lived and can afford a bad breeding season occasionally, projections are for blocks of multi-year failures.
Ultimately, it may be human adaptability and plasticity that will save the soon-to-be lost species. Our Clean Air Act of 1970 caused atmospheric SO2 to drop 40%, lessening the effects of acid rain. Recovery of fresh water lakes is still decades away, but seems to be possible now. Many areas are recovering from the dramatic damage to wildlife caused by the now banned DDT. The FDA recently banned the dangerous pesticide carbofuran; crops to which it has been applied can no longer be imported. In 2009, our Congress set goals of reducing US production of greenhouse gasses by 17% in 2020 and 83% by 2050. As a nation, we are waking up to the need for more preservation of wilderness areas.
But the problems are global and we must use influence to get other nations like China and India to get in step. Nature is remarkably resilient. But the farther we allow the environment to degrade from acceptable, the longer and less complete any recovery will be.