Class Aves:! He is certainly a wise man who to-day can tell a bird" from a reptile, with only the fragments of an ancient" form before him.! - Othniel C. Marsh (Head of Yale s Peabody Museum)," The New York Herald, 19 January 1890! Over 9,000 spp. (among vertebrates, only bony" fishes are more diverse)! Live in all biomes, from mountains to prairies, on all" oceans, and from close to the North and South Poles! Some live in dark caves, and some dive to 45 m depth! The bee hummingbird is one of the smallest endotherms! Uniformity in structure: despite 150 MY of" evolution, birds are still readily recognized! Feathers are the unique and essential feature of birds! Forelimbs are modified as wings (not all can fly)! Hindlimbs are adapted for walking, swimming or perching! All have horny beaks and lay eggs! The driving force for this uniformity appears to be the" adaptations for flight; mammals (the other endotherms) have developed far more diverse forms! Wings are present for support and propulsion; ones must provide a light but rigid airframe! The respiratory system must meet high oxygen demands and cool the body! Digestion and circulation must meet the high-energy demands of flight! Nervous system must have superb sensory systems for high-velocity flight! Origin and Relationships! Discovery of Archaeopteryx linked birds" and reptiles! First A. lithographica fossil feather found in 1860! Skeleton with feather impressions found in 1861: other" specimens had been found earlier, but did not show feathers" and were described as dinosaurs (Compsognathus sp.)! The skull resembled modern birds but it had teeth! Archaeopteryx lithographica The skeleton was reptilian with clawed fingers," abdominal ribs and a long bony tail! Zoologists had long recognized similarities" between birds and reptiles:! Skulls that abut the first neck vertebra by a single ball-and-socket joint! Single middle ear bone, the stapes! Lower jaw is composed of five or six bones; in mammals" Sinosauropteryx prima there are two mandibular bones! Lay similar yolked eggs; the embryo develops on the surface by" shallow cleavage patterns! Excrete nitrogenous wastes as uric acid; mammals excrete urea! Thomas Henry Huxley classified birds with theropod dinosaurs! Belong to the same lineage of diaspid reptiles, the archosaurians, that includes crocodiles! Fossil evidence from Spain, China, etc. is accumulating that Huxley s theory is correct! Sinosauropteryx prima, a Cretaceous theropod discovered in China in 1996, had feathers! Fig. 1. Inferred evolutionary relationships of major vertebrate groups hypothesized from collagen {alpha}1(i) and {alpha}2 (I) protein data by using a Bayesian approach Origin and Relationships! C. L. Organ et al., Science 320, 499 (2008) Relationships! Modern birds include Superorder Paleognathae (ratites, 5 Orders) with a flat sternum and Superorder Neognathae (23 Orders) with a keeled sternum! Neognathae radiated into" most of the modern orders" during the Eocene! Living ratites include:! Tinamous (42 spp., Neotropics)! The ostrich (1 sp., Africa)! Kiwis (3 spp., New Zealand)! Rheas (2 spp., South America)! Emus and cassowaries (5 spp.," Australia and New Guinea)! Original theories were based" on the Paleognathae or ratite lineage never having attained flight! This is now rejected; flightlessness has evolved many times among many bird groups! Smaller birds (e.g., rails, pigeons) can revert to flightlessness on islands that lack terrestrial predators (increase in body size may follow)! Flightless birds are free from the weight restrictions of flight and some evolved to very large sizes (the moas of New Zealand were about 3.5 m tall and 500 kg)! Larger flightless birds such as the ostrich and emu can outrun predators! All living birds >25 kg are ratites, but there were large flightless predatory birds earlier in the Cenozoic (replacing non-volant theropods?)! Feathers! The feather is a special adaptation that" contributes to more power or less weight! The hollow quill emerges from the skin" follicle and continues as a shaft or rachis! Up to several hundred barbs are arranged" on the rachis to form a flat, webbed vane! Each barb resembles a miniature feather;" numerous parallel filaments or barbules" spread laterally! There are up to 600 barbules in each side of a" barb (may be >1 million in the whole feather)! Barbules from two neighboring barbs overlap;" they zip together with tiny hooks! When separated, they are zipped back" together by preening! Contour feathers provide the form of the bird; flight feathers extend off the wing in flight! Down feathers are under contour feathers; their barbules lack hooks (for insulation)! Filoplume feathers are hairlike, degenerate feathers with a weak shaft and tuft of short barbs! Powder-down feathers on herons and their relatives disintegrate and release a talclike powder to waterproof feathers! Page 1
Origin and Development of Feathers! The bird feather is homologous to the reptile scale! The feather develops from an epidermal elevation over a nourishing dermal core! Rather than flattening, the feather bud rolls into a cylinder! During growth, pigments are added to the epidermal cells! Near the end of its growth, the soft rachis and barbs transform into hard structures of keratin! When the protective sheath splits apart, the feather protrudes and the barbs unfold! Molting! The fully-grown feather is a dead structure; shedding or molting is an orderly process! Except in penguins, molting is a gradual process that avoids leaving bare spots! Flight and tail feathers are lost in pairs, one on each side, to maintain balance! In some species, replacement is continuous; therefore flight is unimpaired! In many water birds, primary feathers are molted all at once and the birds are temporarily grounded! Most birds molt once a year, usually in late summer after the nesting season! Color! Feather color may be due to pigments or to structural color! Pigments, or lipochromes, color red, orange and yellow feathers! Black, brown, red-brown, and gray colors are from the pigment melanin! The blue color of the blue jay, indigo bunting and bluebird is from scattering of light by structure! Skeleton! Compared with Archaeopteryx, modern birds have light, delicate bones" laced with air cavities! These are termed pneumatized bones; they are nevertheless strong! The total weight of a bird s feathers may outweigh its skeleton! As archosaurs, birds evolved" from ancestors with diapsid" skulls! Bird skulls are so specialized" that it is difficult to see the" diapsid condition! The skull is fused into one" piece; the braincase and orbits" are large to hold a larger brain" and eyes! Skull is lighter, but legs are" heavier than in mammals;" lowering the center of gravity! Most birds have kinetic skulls;" in some, the upper jaw is hinged" to the skull! In Archaeopteryx, both jaws contained teeth set in sockets! Modern birds have a horny keratinous beak molded around bony jaws! Skeleton! Vertebrae and Appendages! The bird vertebral column is" very rigid; vertebrae are fused" except for the cervical vertebrae! Additional bony structures" called uncinate processes are" fused with the pelvic girdle to" support legs and provide" rigidity for flight! Ribs are mostly fused with the" vertebrae, pectoral girdle and sternum! Except in flightless birds, the sternum bears a" large keel for anchorage of flight muscles! Bones in the forelimbs are highly modified for flight, with some bones reduced in number or fused! All of the elements of the basic vertebrate limb are represented in modified form! The bird s legs have undergone less modification since their function remains walking, etc.! Muscular System! The pectoralis muscles depress the wing in" flight and are attached to the keel! The supracacoracoideus muscle raises the wing," is also attached to the keel, lays under the pectoralis" muscles, and pulls the wing up from below by way" of a rope-and-pulley arrangement! Having both muscles low in the body provides" aerodynamic stability! There are air tubes between the flight muscles" to cool them during flight! The main leg muscle mass is in the" thigh with connections by long tendons to the feet and toes! A toe-locking mechanism prevents a perching bird from falling off a branch while asleep! Birds have lost the long reptilian tail and substituted a muscle mound where tail feathers are rooted! As many as 1000 muscles may control the tail feathers for steering in flight! The neck is thoroughly interwoven with stringy muscles to provide great flexibility! Food and Feeding! Early birds were carnivorous" (primarily insectivorous)! Modern birds have specialized to hunt" nearly all types of insects in most habitats! Other animals joined the diet of birds," including worms, molluscs, crustaceans," fish, frogs, etc.! Nearly one-fifth of birds feed on nectar! The beaks of birds often reveal" their food habits and vary between" seed-eaters, insect-eaters, etc.! Euryphagous species eat a wide variety of items and can switch to whatever is seasonally abundant! Stenophagous species are specialists but are vulnerable if their food source is jeopardized! Woodpeckers have a straight, hard, chisel-like beak to expose insect burrows; its long, flexible, barbed tongue seeks out the insects in the wood galleries! Contrary to the saying to eat like a bird meaning to eat little, birds are voracious feeders; they have a high metabolic rate and small birds need even more food per unit body mass! Digestion! Birds have rapid and efficient digestive systems! A shrike can digest a mouse in three hours! A thrush will pass berries through the tract in just 30 minutes! Because birds lack teeth, foods that require grinding are cut apart in the gizzard! Salivary glands are poorly developed but lubricate both the food and the slender tongue! There are few taste buds, but birds can taste to some extent! A long, muscular esophagus extends from pharynx to stomach! Many birds have a crop that serves to store food at the lower end of the esophagus! The crop of pigeons, doves and some parrots, also produces a lipid- and protein-rich milk.! The stomach consists of a proventriculus that secretes gastric juice and a gizzard that grinds food! Birds may also swallow pebbles or grit to assist in grinding in the gizzard! Birds of prey such as owls form a pellet of indigestible material in the proventriculus and eject it! Paired ceca are at the junction of the intestine and rectum; they serve as fermentation chambers! The end of the digestive system is the cloaca, which also receives the products from the genital ducts and ureters! Page 2
ZOO 2040!Biology of Animals!Topic 17! Circulatory System! Respiratory System! The four-chambered heart is large, with strong ventricular" walls! Birds share with mammals a complete separation of" respiratory and systemic circulations! The right aortic arch, instead of the left as in mammals," leads to the dorsal aorta! The two jugular veins in the neck have a cross vein" shunt to continue circulation as the head rotates! The brachial and pectoral arteries to the wings and breast" are unusually large! The cardiac rate is relatively fast compared to mammals" and, like mammals, is inversely proportional to heart and" body mass! Bird Lungs! Air sacs extend into the thorax, abdomen, and" even the long bones! A large portion of the air bypasses the lungs" and flows directly to the air sacs on inspiration! On expiration, this oxygenated air flows through the lungs; therefore there is continuous air flow! Thus it takes two respiratory cycles for a single breath of air to pass through the system! This is the most efficient respiratory system of any vertebrate! Bird red blood cells (erythrocytes) are nucleated and biconvex! Mobile phagocytes are active and efficient in repairing wounds and destroying microbes! Nervous/Sensory Systems! A bird s nervous and sensory systems" must accommodate the problems of" flight and a visual lifestyle! The bird s brain has well-developed" cerebral hemispheres, cerebellum and" midbrain tectum! A pair of large metanephric kidneys is composed of many thousands of nephrons! Each nephron has a renal corpuscle and a nephric tubule! Birds use the vertebrate pattern of glomerular filtration and selective resorption! Urine flows through ureters to the cloaca! Uric Acid! The cerebral cortex, a chief coordinating" center in mammals, is thin, unfissured and" poorly developed! The core of the cerebrum, the corpus striatum, is enlarged into the principal integrating center! The size of the cerebral hemisphere is directly related to the intelligence of the bird! Birds also use the reptilian adaptation of excreting nitrogenous wastes as uric acid! In shelled eggs, all excretory products remain within the eggshell; uric acid is stored harmlessly in the allantois! Since uric acid has low solubility, a bird can use far" less water to excrete wastes! Concentration of uric acid occurs almost entirely in" the cloaca where water is absorbed! A bird kidney is less efficient than a mammal kidney" in removing Na+ and other ions. Mammalian kidneys" can concentrate solutes to 4-25 that of the blood; avian" kidneys concentrate solutes only a little greater than the" blood concentration! An air sac system helps cool a bird during vigorous exercise when up to 27 times more heat is produced! The air sacs in the bones, legs and wings, provide considerable buoyancy to the bird (the usually warmer air acts like a hot air balloon).! Excretory System! The finest branches of the bronchi do not" terminate in alveoli, but are tube-like parabronchi! A turkey heart beats 93 times per minute! A chicken heart beats 250 times per minute! A small black-capped chickadee heart beats 500 times per minute! The bird respiratory system differs radically" from the lungs of both reptiles and mammals! The cerebellum is where muscle-position sense (proprioception), equilibrium sense and visual cues are assembled! Sense of smell is poorly developed except in flightless birds, ducks and vultures! Birds have good hearing; the bird ear is similar to the ear of mammals! External ear canal leads to an eardrum! Middle ear contains a rod-like columella that transmits vibrations to the inner ear! Inner ear has a short cochlea;! Birds hear about the same range of sound as humans (do not hear as high a frequencies, but surpass us in ability to distinguish differences in pitch and intensities)! Marine birds must excrete larger salt loads due to" the food they eat and seawater they drink! Salt glands located above each eye excrete highly" concentrated salt solutions that run out the nostrils! Flight! Nervous/Sensory Systems! The optic lobes bulge to each side of the" midbrain and form a visual association" apparatus! Birds have superb vision, the best in" the animal kingdom! The early airspace was an unexploited habitat, with flying insects for food! Flight also provided rapid escape from predators and ability to travel to better environments! Hypotheses on the evolution of flight! The ground-up hypothesis is based on running birds with primitive wings to snare insects! The trees-down hypothesis has birds passing through treeclimbing, leaping, parachuting, gliding and, finally, powered flight! Feathers preceded flight and arose for thermoregulatory purposes! There is no evidence for bird ancestors first being membranewinged! Many birds have two foveae or regions of" detailed vision; this provides both sharp" monocular and binocular vision! Hawks have 8 the visual acuity of a human and can see a rabbit over a km away! An owl s ability to see in dim light is more than 10 that of a human! Birds eyes are similar to those of mammals, but are relatively larger! Birds eyes are less spherical and almost immobile; a bird turns its head rather than its eyes! The light-sensitive retina has both rods and cones:" diurnal birds have more cones; nocturnal birds" have more rods! A pecten is a highly vascularized organ attached" to the retina its function is not certain! Vegetarians must avoid predators and have" eyes placed to each side to view all directions! Birds of prey have eyes directed forward to" provide better depth perception! Many birds can see partially into the ultraviolet" spectrum, seeing flower nectar guides! Page 3
The Avian Wing as a Lift Device! The Avian Wing as a Lift Device! The modified hand bones with attached primary feathers provide the propulsion! Lift is provided by the more medial part of the wing and secondary feathers of the forearm! A wing is streamlined with a concave lower surface! The leading edge of the wing has small tight-fitting feathers! Over 2/3 of the total lift comes from negative pressure from the airstream flowing a longer distance over the top of the wing, the convex surface! Lift-to-drag ratio is determined by the angle of tilt and the airspeed! A wing can carry a given load by high speed and small angle of attack, or low speed and larger angle of attack! As speed decreases, lift is increased by increasing the angle of attack, but this also increases drag! At a point near 15, the angle of attack becomes too steep and stalling occurs! Stalling is delayed or prevented by a wing slot along the leading edge to direct rapidly moving air across the leading surface! In some birds the alula (group of small feathers on the thumb ) provides a midwing slot! Slotting between the primary feathers provides a wing-tip slot! Flapping flight requires a vertical lifting and horizontal thrusting forces! Thrust is provided by primaries at the wing tips and lift is provided by the secondaries! Greatest power is provided by the downstroke! Primary feathers are bent upward and twist to a steep angle of attack! On the upstroke, the primary feathers bend so that their upper surfaces twist to produce thrust. The powered upstroke is essential for hovering and fast, steep takeoffs! Basic Forms of Bird Wings! Elliptical Wings! Birds that must maneuver in forested habitats have elliptical wings! Elliptical wings are slotted between primary feathers to prevent stalling at" low speeds, etc.! High-Speed Wings! Birds that feed on the wing or make long migrations have high-speed wings! These wings sweep back and taper to a slender tip; this reduces tip vortex " turbulence! They are flat in section and lack wing-tip slotting! Soaring Wings! Albatrosses, gannets (frigate birds), and other oceanic soaring birds have" long, narrow wings! The high-aspect ratio of long, narrow wings lack wing slots and allow high" speed, high lift and dynamic soaring! Have the highest aerodynamic efficiency of any design, but are less maneuverable! Exploit the highly reliable sea winds, and air currents of different velocities! High-Lift Wings! Vultures, hawks, eagles, owls and other birds of prey that carry heavy loads" have wings with slotting, alulas and pronounced camber. This produces high" lift at slow speeds! Many are land soarers; their broad, slotted wings allow sensitive response for static soaring! Migration! About half of all birds migrate! They can move between southern" wintering regions and northern" summer breeding regions." Temperate zone birds are thus" threatened by tropical" deforestation 1000s of km away" from their summer homes!! They can exploit seasonal changes" in abundance of insects and avoid" bird predators! Appearing one time a year prevents buildup of specialized predators! Migration also expands living space and reduces aggressive territorial behavior! Migration favors homeostasis, allowing birds to avoid climatic extremes and food! Migration Routes! Most migratory birds follow established north-south routes. Migratory paths appear to follow patterns of continental drift! Some use different routes in the fall and spring! Some aquatic species make rapid journeys; others (e.g., warblers) take 50-60 d to migrate! Smaller species migrate at night and feed by day; others are daytime migrants! Many birds follow landmarks; some fly over large bodies of water! Some have very narrow migration lanes; others have wide migration lanes! The Arctic tern circles from North America to coastlines of Europe and Africa to winter quarters, a total of 18,000 km! Direction Finding in Migration! Migration involves a combination of environmental and innate cues! The long days of late winter and early spring stimulate development of gonads and fat! Long day length stimulates the anterior lobe of the pituitary! Release of pituitary gonadotropic hormone sets in motion a complex series of physiological and behavioral changes resulting in gonadal growth, fat deposition, migration, courtship, mating behavior and care of young! Experiments suggest birds navigate chiefly by sight! Birds recognize topographical landmarks and follow familiar migratory routes! This pools navigational resources and also experience of older birds! Birds also have a highly accurate innate sense of time and an innate sense of direction! Research indicates they can navigate by the earth s magnetic field; this may be related to magnetite found in the neck musculature of pigeons! Sun-azimuth Orientation! German ornithologists used special cages to show birds navigate by sun at day and stars at night! Planetarium experiments revealed they use the sun as a compass; an internal clock tracks position! These experiments suggest use of the North Star as an axis at night! Social Behavior! Social interactions of birds are most noticeable during breeding season; they stake out territory, select mates, build nests, incubate and hatch eggs, and rear young! Pelicans use organized cooperative behavior to feed! Sea birds often gather in huge colonies to nest and to rear young! Land birds, except for birds such as starlings and rooks, tend to seek isolation for rearing their brood! Birds that isolate during breeding may congregate for migration or feeding. There are many advantages for flocking together:! Mutual protection from enemies! Greater ease in finding mates! Less opportunity for an individual straying during migration! Mass huddling for protection against low night temperatures during migration! Page 4
Reproductive System! Bird testes are very small; may enlarge up to 300 " as breeding season approaches! Before discharge, sperm are stored in a greatly" enlarged seminal vesicle! Males of most species lack a penis; mating involves" bringing cloacal surfaces in contact (cloacal apposition)! In most birds, the the right ovary and" oviduct degenerate! The expanded end of the oviduct, the" infundibulum, receives the discharged eggs! Special glands add albumin or egg white to" the egg as it passes down the oviduct! Farther down the oviduct, the shell membrane," shell, and shell pigments are also secreted! Fertilization must therefore take place in the" upper oviduct before albumin and shell" are added! Sperm remain alive in the oviduct for" many days after a single mating! Some ducks will continue to lay eggs" as long as the clutch is too small! Mating Systems! Over 90% of bird species are monogamous; they only mate with one partner each breeding season! In a few species, such as swans and geese, partners are chosen for life! A smaller number are polygamous; individuals mate with two or more partners each breeding season! Because of their high monogamy rate, compared to mammals, birds lack a built-in food supply and require the parental care from both parents to provision the young! Females enforce monogamy by selecting males that will not divide their time with another female! Bird Territories! A male sings often to announce his presence to females and drive away males! Females wander about to select a male that offers the best chance of reproductive success! Usually a male can defend an area that provides just enough resources for one nesting female! Polygyny! The most common form of polygamy is polygyny where one male mates with many females (the reverse is called polyandry)! Male grouse collect at a lek where each has a small territory to display to males! The male grouse does not care for young! Competition for females is intense and females appear to choose the dominant male for mating! Nesting and Care of Young! Nearly all birds lay eggs that must be incubated by one or both parents! Eggs of most songbirds require 14 days for hatching; those of ducks and geese may require a month! Often the female performs most of the duties of incubation; rarely the male has equal or sole duties! Some birds merely lay eggs on bare ground or rocks! Others build elaborate nests using mud, lichens," brush, etc.! Nests are often carefully concealed from enemies! Woodpeckers, chickadees, bluebirds and others nest in tree" hollows and other cavities! Cuckoos and cowbirds are nest parasites; they lay eggs in" other bird s nests! Altricial birds are naked and helpless at birth and" must be fed in the nest for a week or more! Nesting success in altricial birds is very low; sometimes barely" 20% of nests produce young! Precocial birds are able to feed and run or swim as" soon as they are hatched! Causes of nesting failure include predators, nest" parasites and other factors! Avian Extinctions! More than 70 bird spp. have become extinct because of humans since the 16th Century. The background extinction rate is estimated at 1 species per century! Analyses of fossil deposits from islands of the Caribbean and the Pacific, Indian, and Mediterranean Oceans indicate that the arrival of humans led to large-scale extinctions of birds and other animals! Fossils preserved in a cave on Maui show that bird species decreased by 2/3 after humans arrived 7750 YBP. This was also the case on other Polynesian islands! Over 15 spp. of giant birds in Australia and New Zealand were" eliminated by humans! 20 bird species in the U.S. (especially Hawaii) and its Territories (especially Guam) have gone extinct in the first 20 years after the establishment of the Endangered Species Act in 1973 (out of 113 total)! 2 species went extinct before they were listed! 9 species went extinct after they were listed! 9 species were never listed as either Threatened or Endangered! New Zealand Moas! At one time, as many as 11 species of giant, flightless" moas ( elephant birds ) inhabited New Zealand! Relatives of ostriches, emus, rheas, cassowaries and" kiwis, the largest were >450 kg and were 3 m tall! Human beings from Polynesia (Maori) colonized" New Zealand 700 to 1,000 y BP! Although the Maori had oral histories of moas, Europeans" (who arrived in 1770) did not learn of them until the 1830s," when scientists found bone and eggshell fragments that" indicated that giant birds had recently inhabited the islands! While moa populations may already have been declining" when the first humans arrived in New Zealand, the most likely explanation for their extinction is overhunting! Numerous archaeological sites contain evidence of moa hunting! Other anthropogenic factors that may have contributed to the moas extinction include predation of eggs and young by dogs and rats that came to the islands with the Maoris! The Maori also practiced extensive habitat burning, because it benefited their food plants, and this likely greatly altered the moas habitat! The Dodo! The dodo was a turkey-sized pigeon that" lived on the island of Mauritius in the" Indian Ocean! With no mammalian predators, it lost the need" and ability to fly, living and nesting on the" ground and eating fallen tree fruits! Mauritius was uninhabited until discovered in 1505 by the Portuguese! The island quickly became a stopover for ships engaged in the spice trade! Weighing up to 23 kg, the dodo was a source of meat for the sailors, and large numbers of dodos were killed for food! Later, when the Dutch used the island as a penal colony, pigs and monkeys were brought to the island! Many of the ships that came to Mauritius also had rats aboard, which escaped onto the island! By 1681, the dodo had become extinct due to predation on its eggs and meat by humans, and human-introduced mammals! Two dozen of the 45 other bird species in Mascarene Islands have become extinct since 1600!! Page 5
The Passenger Pigeon! In the early 19th Century, the number of individual passenger" pigeons was estimated to be equal to that of all other species" of North American birds combined!! At that time, a single flock could number 2 billion birds, and there" were many such flocks in the U.S. and Mexico! Audubon observed a migrating flock over Kentucky in 1813, and" reported that the sky was black with birds for three days!! Their nesting colonies in northeastern deciduous forests could" be 20 miles across, with so many birds per tree that the branches" broke under their weight! Extinction of passenger pigeon was likely precipitated by two factors:! The loss of eastern forests and the hickory, beech, and chestnuts they produced (which drove the birds into becoming a pest on planted fields)! Uncontrolled commercial hunting for their meat, which was in high demand by European and American settlers prior to the establishment of a cattle industry in the U.S.! Their migratory and nesting behaviors made them easy to hunt in large numbers: they were netted, shot (using special firearms that included a forerunner of the machine gun that fired shotgun shells), and dynamited and smoked out of trees at night with sulfur torches! The Passenger Pigeon! The growth of commercial hunting of passenger pigeons was" facilitated by the railroads, which made it profitable to transport" the birds by the boxcar-load to cities! In one year in Michigan alone, a billion birds were harvested! By 1850, several thousand people were employed in the passenger" pigeon industry. In New York in 1855, one operation processed" 18,000 per day.! Although several thousand survived in 1880, it was no" longer profitable to hunt them once they were widely" dispersed across the U.S.! This scattered distribution may also have contributed to their extinction, possibly by interfering with their breeding (e.g., large colonies may have stimulated breeding behaviors)! Smaller flocks may also have had trouble competing with other birds for nesting sites, and their nesting sites were likely severely reduced along with the eastern deciduous forests.! Some species (e.g., the elephant seal) have been able to recover from a low number of individuals, but the passenger pigeon continued to decline! The last flock was killed by a freak storm over Lake Michigan in the early 1900s! Although commercial hunting did not directly kill the last passenger pigeon, it sent the species into a death spiral from which it could not recover. Captive breeding efforts were not successful, and the last individual ( Martha ) died in captivity at the Cincinnati Zoo in 1914.! Recent Declines of Bird populations are hard to monitor, and many greatly fluctuate in size from year to year! Populations of birds of prey (e.g., snowy owls) may cycle with the food supply! When food supplies crash, the birds may move elsewhere or eat other foods! Humans have introduced birds to new regions; the starling and the house sparrow are both abundant now in the United States! Nevertheless, many species that were abundant 40 years ago are in decline! Agriculture has utilized once-fallow fields! Fragmentation of forests in the United States exposes nests to nest predators! House cats are formidable predators that kill many songbirds! The loss of tropical forests also deprives about 250 neotropical migratory songbirds of their wintering homes! Birds stressed in their wintering grounds are in poor condition to make northward migrations! Penguins nesting success is declining because global warming is affecting the distribution of their food-moving their prey farther away from the coasts and into colder mid-ocean waters! Page 6