The Origins of Birds, Feathers, and Flight

Feathered Dinosaurs and the Origin of Birds Class Aves: ~9700 extant species; exhibit feathers and wings;

keratinized beaks lacking teeth; air sacs aid respiration; hard-shelled eggs Shared Features with Reptiles: single occipital condyle, single middle-ear

bone, multiple lower jaw bones, uric acid excretion, amniotic egg, scales on feet; recognized as “glorified reptiles” by Thomas Henry Huxley in mid-1800s

Shared Features with Dromeosaurs (therapods): mobile S-shaped neck, three- toed foot, wishbone (fused clavicles), lunate wrist, hollow bones Evolution of feathers, keeled sternum, and loss of teeth evident in transitional

fossils from Jurassic: Archaeopteryx (discovered 1861, ten specimens as of 2007), Caudipteryx, Protarchaeopteryx, Sinosauropteryx, and others

The Origin and Development of Feathers Feathers homologous to scales; shaft develops from epidermal outgrowth

that rolls into cylinder, splits open; barbs harden as keratinizedThe Evolution of Flight in Birds (two competing hypotheses)

Arboreal (“Trees Down”) Hypothesis: flight evolved gradually from gliding (similar to kakapo in New Zealand); Archaeopteryx likely

was a glider, but note that ancestral dromeosaurs were likely ground-dwelling

Cursorial (“Ground Up”) Hypothesis: feathers and wings evolved for non-flight related functions (ex., assistance for incline running, sexual selection, thermoregulation), and then were co-opted for flight; note that no gliders today launch from the ground

Fig. 27.1

Fig. 27.3

Fig. 27.5

Diversity of Modern Birds, Wings, and FlightDiversity of Extant Birds: Terrestrial Flightless Birds (ostrich, emus, rheas, kiwis; all

derived from flying ancestors; many evolved on islands without predators); Swans, Geese, Ducks (broad bills); Loons, and Grebes (many diving forms); Galliformes: quail, grouse, pheasants, turkeys, peacocks, domestic fowl (ground nesting); Flamingos (bill strains zooplankton); Herons, and Storks (long neck and legs, waders); Raptors (diurnal birds of prey, incl. falcons, hawks, eagles, condors, and buzzards); Pigeons and Doves (short neck and legs); Parrots and Parakeets (hinged upper beak, tropical); Owls (nocturnal predators); Swifts and Hummingbirds (latter eat nectar); Woodpeckers and Toucans (nest in tree cavities); Seabirds and Shorebirds: include albatrosses, terns (long migrations), shearwaters (fly into water), gulls, sandpipers, pelicans (expandable throat), puffins, penguins (emperor penguin largest); Perching Songbirds: include 56 families (~60% of all bird species); warblers, mockingbirds, jays, ravens, and many more

Flapping Flight and Wing Morphologies Flapping Flight: thrust and stall control from distal end (wingtips); lift from medial wing

(2° feathers of forearm); alula (thumb feathers) & wing slots prevent stalling

Wing Morphologies Elliptical Wings: low aspect ratio (length to width); allows maneuverability; wings slotted between

the primary feathers (with alula, prevent stalling during sharp turns) High Aspect Ratio Wings: good for high-speed flight; found in birds that feed in air and migrators Dynamic Soaring Wings: mainly in oceanic birds (ex. albatross) High Lift Wings: slots and alula provide high lift at low speeds; found in raptors, owls (land soarers)

Fig. 27.2

Fig. 27.16

Fig. 27.17 and Fig. 27.18

Fig. 27.19

Modern Birds – Form and Function Feathers: include contour feathers (incl. flight feathers) and down feathers; molting

occurs periodically; colors due to melanin, lipochromes, and light scattering Skeleton: light-weight, pneumatized bones (ex. frigatebird with 7-foot wingspan has

four ounce skeleton); heavy leg bones balance body; kinetic (modified diapsid) skull; fused clavicles (furcula, or wishbone); rigid backbone (fused vertebrae, incl. caudal pygostyle); some forelimb bones fused in wing; keeled sternum; sharp talons in raptors (hawks, falcons, eagles); keratinized jaws beak

Muscular System: massive flight muscles include pectoralis and supracoracoidus (latter raises wing via pulley arrangement), both anchored to sternum; feet with tough tendons, incl. toe-locking mechanism (grip, perching); complex muscle groups in neck and tail (operate tail feathers)

Feeding and Digestion: flight allowed capture of insects in air; most are generalists, with many nectar specialists; high metabolisms force high food consumption; two stomach chambers, incl. gizzard for grinding food; some with crop (storage); beaks specialized for feeding mode (sea- birds and shorebirds include plunge diving, probing, skimming, spearing, and sweeping)

Circulation and Respiration: four-chambered heart; faster heart rate than mammals; nucleated red- blood cells; continuous, one-way airflow through parabronchi of lungs (vs. terminal alveoli); air sacs, with tubes extending into bones; lungs provide buoyancy and cool body during exertion

Excretory System: kidneys excrete uric acid; marine birds often with salt glands Nervous and Sensory Systems: cerebrum with small cortex but large dorsal ventricular

ridge (integrates flying, singing, courtship); large optic lobes and cerebellum; middle ear with single rod-like bone (columella); eye with distinctive internal pecten (provides nutrients and oxygen); raptors with ~100x more cones in fovea than mammals excellent vision

Fig. 27.5

Fig. 27.7 and

Fig. 27.8

Fig. 27.9

Fig. 27.10

Fig. 27.11

Fig. 27.23

Fig. 27.12

Fig. 27.13

Fig. 27.14

Modern Birds – Behavioral Ecology and Populations

Courtship, Mating, and Nesting Courtship commonly involves colorful plumage (especially males) and dances

(ritualized preening movements); male bowerbirds build and decorate bowers; males sing (or call) innately but improve song with experience

Mating: most species monogamous (but note leks of grouse) Nesting: nests often hidden or inaccessible; most brood eggs and gather food for

chicks (parents regurgitate food); altricial vs. precocial hatchlings

Flocking and Migrations Flocking: similar to fish schooling; anti-predation mechanism (ex. starlings);

most birds are solitary except for during the breeding season Migrations: typically to and from breeding/feeding grounds; triggered by the

lengthening days of late winter and early spring Migratory routes often fixed, range from narrow to wide; longest migration from pole to

pole (Arctic tern); navigation based on landmarks, celestial cues, and Earth’s magnetic field

Populations and Extinctions Introduced starlings and sparrows have out-competed native species in North

America; extinction of ~80 species since 1681, incl. dodo and passenger pigeon (both hunted to extinction); songbirds in decline (domestic and feral cats kill millions a year); DDT led to failed reproduction of pelicans and bald eagles; tropical deforestation major threat for migratory and tropical birds; pollution, nets, and longlines major threats for seabirds

Fig. 27.22 and Fig. 27.24

Fig. 27.26

Fig. 27.25

Fig. 27.27

Fig. 27.28

Fig. 27.20

Fig. 27.21

Fig. 27.29