Evolution of Bird Characteristics

This page has two parts: background discussion of bird evolution, followed by classification groups and photographic examples of current birds.

Bird Evolution

Birds form a monophyletic class, Aves, the only modern therapod dinosaurs. From the late Mesozoic, evidence exists for at least eight subclasses of Aves. Only one such subclass survived into the Cenozoic: Neornithes, the modern birds. This subclass has expanded to fill nearly all the niches that the prior eight subclasses exploited, including tree perchers, waders, divers, flightless ground birds. There are likely some examples of co-evolution between Neorinthes and the extinct Aves sub-classes.

Modern birds are adapted for flight, homoiothermic, carnivorous, feathered, with toothless beaks. It may not always have been so. Dinosaurs evolved from reptiles, who exhibit a range of cold-blooded thermophysiology modes. Recent studies of dinosaur eggs reveal that many types were buried just as crocodile eggs are today. Buried eggs, not meant for incubation, tend toward high-density, non-porous shells. But the discovered eggs of some therapods reveal porous shells that were likely incubated, demonstrating a hypothetical shift to homoiothermy along the development line of Aves.

Early examples of Aves were toothed, just as were other dinosaur cousins. There is evidence from fine bone structure that the earliest instances of Aves may not have been fully homoiothermic, though it is not clear how a cold-blooded dinosaur could have generated the metabolic energy necessary for flight. It is probable that early flight may have involved not much more than hopping and gliding, so that other flight-support adaptations had time to evolve as the complexity of flight evolved.

Feathers are an adaptation of reptilian scales. But paleontological evidence dismisses the notion that feathers were specifically motivated by flight. Early in the evolution of therapod dinosaurs, feathers appeared on both flying and ground-based species. Thus it appears, it was from a larger grouping of feathered therapods that Archaeopteryx evolved as ancestor to Aves. That feathers were useful for flight ultimately allowed Aves to leave the ground. But feathers likely existed for more primitive uses in order for this happy coincidence to occur. Feathers also supply insulation and a means of sexual display. The latter function may have been the prime motivation for feathers.

The early birds must have gained some advantage from their necessarily weak flight abilities. Observing hatchlings learning to use their wings gives some insight into how it must have been for these pioneers. Flight would have increased the abilities to catch airborne insect prey, to escape from danger, to overcome obstacles, and to attack prey from unexpected angles. These advantages offset the added danger and energy expenditure associated with flight.

There are two theories of how flight began: arboreal and cursorial. Arboreal proponents have the earliest birds using their limbs to climb trees and other elevations, then gliding down as many animals still do. The cursorial adherents believe the earliest birds were bipedal runners, using wings to gain speed, hop, and remain briefly airborne. Both may be correct, but no arboreal glider uses wings or appendages to increase airborne distance. They remain purely trajectory gliders. A more likely beginning would involve a strategy that used the wings in a flapping motion to gain some advantage. Also, from the earliest period, there have been Aves orders containing birds that do not fly, yet have wings. This again suggests a ground-based strategy for the beginning use of wings.

The study of ground-based superprecocial hatchlings shows that even though their wings won’t enable the bird to achieve flight, they are used to climb steep inclines, where wing power is needed to retain traction as the slope reaches 70 degrees or steeper. Getting off the exposed flat ground is the prime survival directive for these baby birds, and even on their first day they can climb steep, even inverted, inclines using wing power for brief periods, which exerts a downward force through their feet of up to three times their body weight. The wing motion when generating ground traction is effectively at a 90 degree rotation from that in flight. This is possibly the primordial wing motion of the bird class.

High energy consumption is the cost of flying. For flying objects, such as birds and planes, physics dictates a U-shaped power curve that plots expended energy with airborne velocity. The energy required to become airborne at slow speed is relatively high, then decreases to a minimum at the optimal-energy velocity, then increases again as velocity increases. Evolutionary pressures would cause flying animals to optimize their energy usage.

As expected, birds are able to flatten their energy curve by morphing their shape to optimize flight, thus decreasing the energy output required to get airborne and also extending the range of energy-efficient velocities. Part of such morphing is the continuous flexing of the rib cage to optimize breathing.

A final consideration of bird characteristics is again an issue from physics, the effect of body mass on flight capability. There appears today a maximum body mass for which flight is possible. Several orders of class Aves have species of this maximal flight size, but there are no current or past examples of a larger flying creature.

To become airborne, a bird can use its legs to increase forward velocity and simultaneously use its wings in a flapping motion to further increase forward velocity and to provide lift. Small birds do not need the forward running component to become airborne. They are able to move their small wings quickly enough to take off vertically. But larger birds with larger wings cannot move the mass of their wings rapidly enough to lift off. Thus they employ running assist. But at some mass limit, the bird’s body is not able to do the work required to lift off. They cannot run fast enough and move their large wings with high enough frequency.

Bird behavior is influenced by size, since the world appears different to large and small birds. Smaller birds, due to their ability to move directly vertically through wing power, inhabit a true 3-dimensional space. Their maneuverability compensates for their lack of size in the predator-prey struggle. Larger birds are more like us, living in a flatter space. Their space includes both the ground and the sky, but for them also, sudden vertical moves are forbidden by physics. That smaller birds should dominate the current species of Aves is due largely to physics.

Current Bird Groupings

Modern birds account for 10^4 species, falling into approximately 30 orders (orders are being expanded as DNA reorganizes the relationships between groups). 60% of all species reside in one order, the perching birds (Passeriformes).

Birds are classified in two super-orders. (Orders below are characterized by a representative species).

Super-order Paleognathae comprises the following orders, including ratites or large flightless birds:

  • Apterygiformes – kiwi
  • Struthioniformes – ostrich
  • Rheiformes – rhea
  • Casuariiformes – cassowary
  • Tinamiformes – crested tinamou

Super-order Neognathae comprises the remaining orders. These can be related by the following groupings, plus two currently ungrouped orders. The groupings do not form a scientific classification, but are suggestive of closely related orders.

  • Orders that stand separate from the others:
    • Galliformes – pheasant
    • Anseriformes – duck
  • Group 1: Primitive Water Birds
    • Podicipediformes – grebe
    • Gaviformes – loon
    • Sphenisciformes – penguin
  • Group 2:
    • Pelecaniformes – pelican
    • Procellariformes – albatross
  • Group 3:
    • Gruiformes – crane, rail, coot
    • Charadriiformes – gull
    • Ardeidae – heron
    • Columbiformes – pigeon
  • Group 4:
    • Ciconiiformes – stork
    • Psittaciformes – parrot (between groups 4/5)
    • Falconiformes – falcon (between groups 4/5)
  • Group 5:
    • Accipitriformes – eagle
    • Cathartiformes – New World vulture
    • Strigiformes – owl
    • Cucuiiformes – cuckoo (between groups 5/6)
  • Group 6:
    • Caprimulgiformes – nightjar
    • Apodiformes – swift, hummingbird
    • Trogoniformes – trogon (between groups 6/7)
  • Group 7:
    • Coraciiformes – kingfisher
    • Piciformes – woodpecker
    • Passeriformes – other perching birds: jay, crow, sparrow, finch, warbler, flycatcher, …

Personal Note: I am a casual observer of birds. By observation, I mean capturing a digital image suitable for identifying species, gender, and maturity. Following is a table of my observations, organized by order. My observation images can be viewed at https://picasaweb.google.com/e2foo2/Birds#5713223622542952210.

Common Name Species Genus Family Order
Red-tailed Hawk jamaicensis Buteo Accipitridae Accipitriformes
Long-winged Harrier buffoni Circus Accipitridae Accipitriformes
Black-chested Buzzard Eagle melanoleucus Geranoaetus Accipitridae Accipitriformes
Bald Eagle leucocephalus Haliaeetus Accipitridae Accipitriformes
Snail Kite sociabilis Rostrhamus Accipitridae Accipitriformes
Osprey hallaetus Pandion Pandionidae Accipitriformes
Egyptian Goose aegyptiacus Alopochen Anatidae Anseriformes
American Wigeon americana Anas Anatidae Anseriformes
Green-winged Teal carolinensis Anas Anatidae Anseriformes
Eurasian (Common) Teal crecca Anas Anatidae Anseriformes
Cinnamon Teal cyanoptera Anas Anatidae Anseriformes
Red Shoveler platalea Anas Anatidae Anseriformes
Mallard Duck platyrhynchos Anas Anatidae Anseriformes
Chiloé Wigeon sibilatrix Anas Anatidae Anseriformes
Gadwall strepera Anas Anatidae Anseriformes
Greylag Goose anser Anser Anatidae Anseriformes
Lesser Scaup affinis Aythya Anatidae Anseriformes
Ring-necked Duck collaris Aythya Anatidae Anseriformes
Common Pochard ferina Aythya Anatidae Anseriformes
Tufted Duck fuligula Aythya Anatidae Anseriformes
Canada Goose canadensis Branta Anatidae Anseriformes
Black Brant Goose nigricans Branta Anatidae Anseriformes
Bufflehead albeola Bucephala Anatidae Anseriformes
Muscovy Duck moschata Cairina Anatidae Anseriformes
Snow Goose caerulescens Chen Anatidae Anseriformes
Kelp Goose hybrida Chloephaga Anatidae Anseriformes
Upland Goose picta Chloephaga Anatidae Anseriformes
Coscoroba Swan corscoroba Corscoroba Anatidae Anseriformes
Whooper Swan cygnus Cygnus Anatidae Anseriformes
Black-necked Swan melancoryphus Cygnus Anatidae Anseriformes
Mute Swan olor Cygnus Anatidae Anseriformes
Black-bellied Whistling Duck autumnalis Dendrocygna Anatidae Anseriformes
Harlequin Duck histrionicus Histrionicus Anatidae Anseriformes
Hooded Merganser cucullatus Lophodytes Anatidae Anseriformes
Patagonian Crested Duck specularioides Lophonetta Anatidae Anseriformes
Common Merganser merganser Mergus Anatidae Anseriformes
Ruddy Duck jamaicensus Oxyura Anatidae Anseriformes
Lake Duck vittata Oxyura Anatidae Anseriformes
Common Eider Duck mollissima Somateria Anatidae Anseriformes
Northern Shoveler clypeata Spatula (from Anas) Anatidae Anseriformes
Flying Steamer Duck patachonicus Tachyeres Anatidae Anseriformes
Turkey Vulture aura Cathartes Cathartidae Cathartiformes
Black Vulture atratus Coragyps Cathartidae Cathartiformes
Andean Condor gryphus Vultur Cathartidae Cathartiformes
Piping Plover melodis Charadrius Charadriidae Charadriiformes
Semi-palmated Plover semipalmatus Charadrius Charadriidae Charadriiformes
Killdeer vociferus Charadrius Charadriidae Charadriiformes
Southern Lapwing chilensis Vanellus Charadriidae Charadriiformes
Blackish Oystercatcher ater Haematopus Haematopodidae Charadriiformes
Northern Jacana spinosa Jacana Jacanidae Charadriiformes
Brown-hooded Gull maculipennis Chroicocephalus Laridae Charadriiformes
Black-headed Gull ridibundus Chroicocephalus Laridae Charadriiformes
Kelp Gull dominicanus Larus Laridae Charadriiformes
Glaucous Gull hyperboreus Larus Laridae Charadriiformes
Western Gull occidentalis Larus Laridae Charadriiformes
Black-necked Stilt mexicanus Himantopus Recurvirostridae Charadriiformes
American Avocet americana Recurvirostra Recurvirostridae Charadriiformes
Black Skimmer niger Rynchops Rynchopidae Charadriiformes
Black Turnstone melanocephala Arenaria Scolopacidae Charadriiformes
Western Sandpiper mauri Calidris Scolopacidae Charadriiformes
Marbled Godwit fedoa Limosa Scolopacidae Charadriiformes
Long-billed Curlew americanus Numenius Scolopacidae Charadriiformes
Whimbrel phaeopus Numenius Scolopacidae Charadriiformes
Wilson’s Phalarope tricolor Phalaropus Scolopacidae Charadriiformes
Greater Yellowlegs melanoleuca Tringa Scolopacidae Charadriiformes
Willet semipalmata Tringa Scolopacidae Charadriiformes
Cattle Egret ibis Bubulcus Ardeidae Ciconiiformes
Black Stork nigra Ciconia Ciconiidae Ciconiiformes
Wood Stork americana Mycteria Ciconiidae Ciconiiformes
Black-faced Ibis melanopis Theristicus Threskiornithinae Ciconiiformes
Rock Dove (Pigeon) livia Columba Columbidae Columbiformes
Eurasian Collared Dove decaocto Streptopelia Columbidae Columbiformes
Mourning Dove macroura Zenaida Columbidae Columbiformes
Belted Kingfisher alcyon Megaceryle Cerylidae Coraciiformes/Alcedini
Groove-billed Ani sulcirostris Crotophaga Cuculidae Cuculiformes
Greater Roadrunner californianus Geococcyx Cuculidae Cuculiformes
Southern Crested Caracara plancus Caracara Falconidae Falconiformes
Yellow-headed Caracara chimachima Milvago Falconidae Falconiformes
Chimango Caracara chimango Milvago Falconidae Falconiformes
Indian Peafowl cristatus Pavo Phasianidae Galliformes
Common (Ring-necked) Pheasant colchicus Phasianus Phasianidae Galliformes
American Coot americana Fulica Rallidae Gruiformes
Andean Coot ardesiaca Fulica Rallidae Gruiformes
Eurasian Coot atra Fulica Rallidae Gruiformes
Light-footed Clapper Rail longirostris levipes Rallus Rallidae Gruiformes
American Bushtit minimus Psaltriparus Aegithalidae Passeriformes
California Scrub Jay californica Aphelocoma Corvidae Passeriformes
American Crow brachyrhynchos Corvus Corvidae Passeriformes
Stellar’s Jay stelleri Cyanocitta Corvidae Passeriformes
Gray Jay canadensis Perisoreus Corvidae Passeriformes
Dark-eyed Junco hyemalis Junco Emberizidae Passeriformes
Song Sparrow melodia Melospiza Emberizidae Passeriformes
California Towhee crissalis Melozone Emberizidae Passeriformes
Savannah Sparrow sandwichensis Passerculus Emberizidae Passeriformes
Sooty Fox Sparrow iliaca unalaschcensis Passerella Emberizidae Passeriformes
Spotted Towhee maculatus Pipilo Emberizidae Passeriformes
Clay-colored Sparrow pallida Spizella Emberizidae Passeriformes
Rufous-collared Sparrow capensis Zonotrichia Emberizidae Passeriformes
White-crowned Sparrow leucophrys Zonotrichia Emberizidae Passeriformes
Lesser Goldfinch psaltria hesperophilus Carduelis Fringillidae Passeriformes
American Goldfinch tristis Carduelis Fringillidae Passeriformes
House Finch mexicanus Carpodacus Fringillidae Passeriformes
Chilean Swallow meyeni Tachycineta Hirundinidae Passeriformes
Red-winged Blackbird phoeniceus Agelaius Icteridae Passeriformes
Tricolored Blackbird tricolor Agelaius Icteridae Passeriformes
Brewer’s Blackbird cyanocephalus Euphagus Icteridae Passeriformes
Great-tailed Grackle mexicanus Quiscalus Icteridae Passeriformes
Northern Mockingbird polyglottos Mimus Mimidae Passeriformes
California Thrasher redivivum Toxostoma Mimidae Passeriformes
Black-capped Chickadee atricapillus Poecile Paridae Passeriformes
Chestnut-backed Chickadee rufescens Poecile Paridae Passeriformes
Yellow-rumped Warbler coronata Setophaga Parulidae Passeriformes
House Sparrow domesticus Passer Passeridae Passeriformes
Phainopepla nitens Phainopepla Ptilogenatidae Passeriformes
Red-breasted Nuthatch canadensis Sitta Sittidae Passeriformes
House Wren aedon Troglodytes Troglodytidae Passeriformes
Varied Thrush naevius Ixoreus Turdidae Passeriformes
Mountain Bluebird currucoides Sialia Turdidae Passeriformes
Western Bluebird mexicana Sialia Turdidae Passeriformes
Austral Thrush falcklandi Turdus Turdidae Passeriformes
American Robin migratorius Turdus Turdidae Passeriformes
Rufous-backed Robin rufopalliatus Turdus Turdidae Passeriformes
Olive-sided Flycatcher cooperi Contopus Tyrannidae Passeriformes
Ground Tyrant Flycatcher TBD Muscisaxicola? Tyrannidae Passeriformes
Black Phoebe nigricans Sayornis Tyrannidae Passeriformes
Say’s Phoebe saya Sayornis Tyrannidae Passeriformes
Black-headed Tody-Flycatcher nigriceps Todirostrum Tyrannidae Passeriformes
Loggerhead Kingbird caudifasciatus Tyrannus Tyrannidae Passeriformes
Great Egret alba Ardea Ardeidae Pelecaniformes
Grey Heron cinerea Ardea Ardeidae Pelecaniformes
Great Blue Heron herodias Ardea Ardeidae Pelecaniformes
Striated Heron striata Butorides Ardeidae Pelecaniformes
Little Blue Heron caerulea Egretta Ardeidae Pelecaniformes
Little Egret garzetta Egretta Ardeidae Pelecaniformes
Snowy Egret thula Egretta Ardeidae Pelecaniformes
Tri-colored Heron tricolor Egretta Ardeidae Pelecaniformes
Yellow-crowned Night Heron violacea Nyctanassa Ardeidae Pelecaniformes
Black-crowned Night Heron nycticorax Nycticorax Ardeidae Pelecaniformes
Magnificent Frigate Bird magnificens Fregata Fregatidae Pelecaniformes
Brown Pelican occidentalis Pelecanus Pelecanidae Pelecaniformes
American White Ibis albus Eudocimus Threskiornithinae Pelecaniformes
Roseate Spoonbill ajaja Platalea Threskiornithinae Pelecaniformes
Chilean Flamingo chilensis Phoenicopterus Phoenicopteridae Phoenicopteriformes
Greater Flamingo roseus Phoenicopterus Phoenicopteridae Phoenicopteriformes
Northern Flicker auratus Colaptes Picidae Piciformes
Pileated Woodpecker pileatus Dryocopus Picidae Piciformes
Acorn Woodpecker formicivorus Melanerpes Picidae Piciformes
Downy Woodpecker pubescens Picoides Picidae Piciformes
Horned Grebe auritus Podiceps Podicipedidae Podicipediformes
Orange-fronted (half moon) Conure canicularis Aratinga Psittacidae Psittaciformes
Austral Conure ferrugineus Enicognathus Psittacidae Psittaciformes
Lesser Rhea pennata Rhea Rheidae Rheiformes
Humboldt Penguin humboldti Spheniscus Spheniscidae Sphenisciformes
Magellanic Penguin magellanicus Spheniscus Spheniscidae Sphenisciformes
Burrowing Owl cunicularia Athene Stringidae Stringiformes
Eurasian Eagle Owl bubo Bubo Stringidae Stringiformes
Double-crested Cormorant auritus Phalacrocorax Phalacrocoracidae Suliformes
Great Cormorant carbo Phalacrocorax Phalacrocoracidae Suliformes
Red-legged Cormorant gaimardi Phalacrocorax Phalacrocoracidae Suliformes
Brandt’s Cormorant penicillatus Phalacrocorax Phalacrocoracidae Suliformes
Jamaican Mango Hummingbird mango Anthracothorax Trochilidae Trochiliformes
Anna’s Hummingbird anna Calypte Trochilidae Trochiliformes
Rufous Hummingbird rufus Selasphorus Trochilidae Trochiliformes