Transcript Subphylum Vertebrata – Early Vertebrates and
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 keratinized
The 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
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Diversity of Modern Birds, Wings, and Flight
Diversity 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)
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Fig. 27.17 and Fig. 27.18
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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
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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