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1 CHAPTER Birds 27-1

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3 Profile Diversity Over 9,700 species have been described worldwide Only fishes have more species among vertebrates Birds live in all biomes, from mountains to prairies, on all oceans, and from the North to the South Pole Some live in dark caves, and some dive to 45 meters depth The bee hummingbird is one of the smallest vertebrate endotherms 27-3

4 Diversity The feather is the unique and essential feature or hallmark of birds Some feathers were also present in some theropod dinosaurs These feathers were not capable of supporting flight Obviously served in other capacities such as thermoregulation or mating behavior 27-4

5 Diversity Uniformity in Structure Despite 150 million years of evolution, birds are still readily recognized Forelimbs are modified as wings, although not all are capable of flight Hindlimbs are adapted for walking, swimming or perching All birds have horny, keratinized beaks All birds lay eggs 27-5

6 Diversity Driving force for this uniformity appears to be adaptations necessary for flight Wings Present for support and propulsion Respiratory system Must meet high oxygen demands and cool the body Bones Must provide a light but rigid airframe Digestion and circulation Must meet high-energy demands of flight Nervous system Must have superb sensory systems for highvelocity flight 27-6

7 Origin and Relationships History Discovery of the fossil of Archaeopteryx lithographica in 1861 linked birds and dinosaurs Skull resembled modern birds but had teeth rather than a beak Skeleton was reptilian with clawed fingers, abdominal ribs, and a long bony tail Feathers were unmistakably imprinted along wings 27-7

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9 Origin and Relationships Zoologists had long recognized that birds and reptiles shared many similarities Skulls that abut the first neck vertebra by a single ball-and-socket joint Single middle ear bone, the stapes Lower jaw composed of five or six bones Mammals have one mandibular bone Excrete nitrogenous wastes as uric acid Mammals excrete urea Similar yolked eggs Embryo develops on surface by shallow cleavage patterns 27-9

10 Thomas Henry Huxley classified birds with theropod dinosaurs Group of dinosaurs with a long, mobile, S-shaped neck Theropods belong to lineage of diapsid reptiles, the archosaurians, which includes crocodiles Fossil evidence is accumulating that support Huxley s theory Dromeosaurs, a group of theropods that includes Velociraptor, share many additional derived characters with birds Including a furcula (fused clavicles) and lunate wrist bones that permit swiveling motions used in flight Origin and Relationships

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13 Origin and Relationships Additional evidence linking birds to dromeosaurs comes from recently described fossils from late Jurassic and early Cretaceous deposits in China These fossils, including Proachaepteryx and Caudipteryx, are dromeosaurs-like theropods, but with feathers Feathers of dromeosaurs could not have been used for powered flight May have been used in social displays Additional theropod dinosaurs recently unearthed in China, such as Sinosauropteryx Covered with filaments that appear to be homologous with feathers 27-13

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15 Origin and Relationships Relationships Modern birds include Paleognathae with a flat sternum and Neognathae with a keeled sternum Original theories were based on the Paleognathae, or ratite lineage, never having attained flight Now rejected Flightlessness has evolved many times among bird groups Smaller birds can revert to flightlessness on islands that lack terrestrial predators 27-15

16 Origin and Relationships Larger flightless birds such as the ostrich and emu can outrun predators Flightless birds are free from weight restrictions of flight and some evolved to very large sizes 27-16

17 Structural and Functional Adaptations for Flight Feathers Structure Feather is a special bird adaptation that contributes to more power or less weight Hollow quill emerges from skin follicle and continues as a shaft or rachis Rachis bears numerous barbs Up to several hundred barbs are arranged to form a flat, webbed surface, the vane Each barb resembles a miniature feather Numerous parallel filaments or barbules spread laterally 27-17

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19 Structural and Functional Adaptations for Flight Up to 600 barbules in each side of a barb May be over one million in the whole feather Barbules from two neighboring barbs overlap Zip together with tiny hooks When separated, they are zipped back together by preening 27-19

20 Structural and Functional Adaptations for Flight Types of Feathers Contour feathers Provide the form of the bird Flight feathers are contour feathers that extend beyond body Down feathers Under contour feathers Barbules lack hooks and function as insulation Filoplume feathers Hairlike, degenerate feathers with a weak shaft and tuft of short barbs Powder-down feathers Herons and their relatives Disintegrate and release a talc-like powder to waterproof feathers

21 Structural and Functional Adaptations for Flight Origin and Development Bird feather is homologous to reptile scale Feather develops from an epidermal elevation over a nourishing dermal core Rather than flattening, feather bud rolls into a cylinder During growth, pigments are added to the epidermal cells Near the end of its growth, soft rachis and barbs transform into hard structures of keratin When the protective sheath splits apart, the feather protrudes and barbs unfold 27-21

22 Structural and Functional Adaptations for Flight Molting 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 Flight is unimpaired In many water birds, primary feathers are molted all at once Birds are temporarily grounded Most birds molt once a year, usually in late summer after the nesting season 27-22

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24 Structural and Functional Adaptations for Flight 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 Blue color of the blue jay, indigo bunting, and bluebird is from scattering of light by structure 27-24

25 Structural and Functional Adaptations for Flight Skeleton Bone Weight Compared with the Archeopteryx Modern birds have light, delicate bones laced with air cavities Termed pneumatized bones Very strong Total weight of a bird s feathers may outweigh skeleton 27-25

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28 Structural and Functional Adaptations for Flight Bird Skull As archosaurs, birds evolved from ancestors with diapsid skulls Skulls are so specialized Difficult to see the diapsid condition Skull is fused into one piece Braincase and orbits are large to hold a larger brain and eyes While the skull is lighter Legs are heavier than in mammals Lowers the center of gravity 27-28

29 Structural and Functional Adaptations for Flight Jaws In Archeopteryx Jaws contained teeth set in sockets Modern birds have a horny keratinous beak molded around bony jaws Most birds have kinetic skulls In some, the upper jaw is hinged to the skull 27-29

30 Structural and Functional Adaptations for Flight Vertebral Column and Appendages Vertebral column is very rigid Vertebrae fused except for cervical vertebrae Additional bony structures called uncinate processes are fused with the pelvic girdle Supports legs and provides rigidity for flight Ribs are mostly fused with the vertebrae, pectoral girdle, and sternum Except in flightless birds Sternum bears a large keel to anchor flight muscles Bones in the forelimbs Highly modified for flight Some bones reduced in number or fused 27-30

31 Structural and Functional Adaptations for Flight All elements of basic vertebrate limb are represented in modified form Most caudal vertebrae are fused into a pygostyle Fused clavicles form an elastic furcula that apparently stores energy as it flexes during wing beats 27-31

32 Structural and Functional Adaptations for Flight Muscular System Pectoralis muscles Depress the wing in flight and are attached to the keel Supracoracoideus muscle Raises the wing, is also attached to the keel Lays under the pectoralis muscles Pulls the wing up from below by way of a ropeand-pulley type of arrangement Having both muscles low in the body provides aerodynamic stability 27-32

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34 Structural and Functional Adaptations for Flight Main leg muscle mass is in thigh with connections by long tendons to feet and toes Toe-locking mechanism prevents a perching bird from falling off a branch while asleep Lost the long reptilian tail Substituted a muscle mound where tail feathers are rooted As many as 1000 muscles may control the tail feathers for steering in flight Neck is thoroughly interwoven with stringy muscles to provide great flexibility 27-34

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36 Structural and Functional Adaptations for Flight Food, Feeding and Digestion Insect Eaters Early in their evolution, birds were carnivorous Primarily feeding on the great variety of insects Modern birds have specialized to hunt nearly all types of insects in most habitats 27-36

37 Structural and Functional Adaptations for Flight Other Diets Other animals joined the diet of birds, including worms, molluscs, crustaceans, fish, frogs, etc Nearly one-fifth of birds feed on nectar Euryphagous species Eat wide variety of items Can switch to whatever is seasonally abundant Stenophagous species Specialists but are vulnerable if their food source is jeopardized Beaks of birds often reveal their food habits and vary between seed-eaters, insect-eaters, etc. Woodpecker has a straight, hard, chisel-like beak to expose insect burrows Long, flexible, barbed tongue seeks out insects in wood galleries 27-37

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39 Structural and Functional Adaptations for Flight Food Quantity Birds are voracious feeders Have a high metabolic rate Small birds need even more food per body mass Hummingbirds use oxygen 12 times faster than a pigeon and 25 times that of a chicken Eats 100% of body weight each day Blue tit about 30% Chicken about 3.4% Have rapid and efficient digestive systems A shrike can digest a mouse in 3 hours A thrush will pass berries through the tract in just 30 minutes 27-39

40 Structural and Functional Adaptations for Flight Because birds lack teeth Foods that require grinding are cut apart in the gizzard Salivary glands are poorly developed Lubricate food and tongue Few taste buds A long, muscular esophagus extends from pharynx to stomach Many have a crop that serves to store food at lower end of esophagus Crop of pigeons, doves, and some parrots, also produces a lipid- and protein-rich milk 27-40

41 Structural and Functional Adaptations for Flight Stomach consists of Proventriculus Secretes gastric juice Gizzard Grinds food Birds may swallow pebbles or grit to assist grinding in gizzard Birds of prey such as owls Form a pellet of indigestible material in the proventriculus and eject it Paired ceca at the junction of the intestine and rectum Serve as fermentation chambers End of the digestive system is the cloaca Also receives products from genital ducts and ureters 27-41

42 Structural and Functional Adaptations for Flight Circulatory System 4-chambered heart is large, with strong ventricular walls Share with mammals a complete separation of respiratory and systemic circulations Right aortic arch, instead of the left as in mammals, leads to dorsal aorta 2 jugular veins in neck have a cross vein shunt to continue circulation as head rotates Brachial and pectoral arteries to wings and breast are unusually large 27-42

43 Structural and Functional Adaptations for Flight Heartbeat relatively fast compared to mammals and inversely proportional to size Turkey heart beats 93 times per minute Chicken heart beats 250 times per minute A small black-capped chickadee heart beats 500 times per minute Red blood cells (erythrocytes) Nucleated and biconvex Mobile phagocytes are efficient in repairing wounds and destroying microbes 27-43

44 Respiratory System Differs radically from lungs of reptiles and mammals Bird Lungs Finest branches of the bronchi do not terminate in alveoli but are tube-like parabronchi Air sacs Extend into thorax, abdomen, and long bones Large portion of air bypasses lungs and flows directly to air sacs on inspiration On expiration, oxygenated air flows through lungs Continuous air flow Takes 2 respiratory cycles for a single breath of air to pass through system Most efficient respiratory system of any vertebrate Structural and Functional Adaptations for Flight

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46 Structural and Functional Adaptations for Flight An air sac system helps cool bird during vigorous exercise when up to 27 times more heat is produced Air sacs extend into bones, legs and wings, providing considerable buoyancy 27-46

47 Structural and Functional Adaptations for Flight Excretory System Pair of large metanephric kidneys is composed of many thousands of nephrons Each nephron has a renal corpuscle and a nephric tubule Birds use vertebrate pattern of glomerular filtration and selective resorption Urine flows through ureters to the cloaca Uric Acid Birds also use the reptilian adaptation of excreting nitrogenous wastes as uric acid 27-47

48 Structural and Functional Adaptations for Flight In shelled eggs, all excretory products remain within the eggshell Uric acid is stored harmlessly Uric acid has low solubility bird can use far less water to excrete wastes Concentration of uric acid occurs almost entirely in cloaca where water is absorbed Bird kidney is less efficient than a mammal kidney in removing ions of sodium, etc. Mammal kidneys can concentrate solutes to 4 25 times that of the blood 27-48

49 Structural and Functional Adaptations for Flight Avian kidneys concentrate solutes only a little greater than the blood concentration Marine birds excrete larger salt loads due diet and seawater they drink Salt glands located above each eye excrete highly concentrated solutions Salt solution runs out the nostrils Gulls and other sea birds have a perpetual runny nose 27-49

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51 Structural and Functional Adaptations for Flight Nervous and Sensory Systems A bird s nervous and sensory system must accommodate the problems of flight and a visual lifestyle Bird s brain has well-developed cerebral hemispheres, cerebellum and midbrain tectum Cerebral cortex is thin, unfissured, and poorly developed Core of the cerebrum, the corpus striatum, is enlarged into the principal integrating center 27-51

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53 Structural and Functional Adaptations for Flight Size of the cerebral hemisphere directly related to the intelligence of the bird Cerebellum is where muscle-position sense (proprioception), equilibrium sense and visual cues are assembled Optic lobes bulge to each side of midbrain and form a visual association apparatus Sense of smell is poorly developed except in flightless birds, ducks, and vultures Have good hearing and superb vision Best in the animal kingdom 27-53

54 Structural and Functional Adaptations for Flight Ear is similar to that 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 Allows birds to hear about the same range of sound as humans Bird ears do not hear as high a frequency as do humans, but surpass us in ability to distinguish differences in pitch and intensities 27-54

55 Structural and Functional Adaptations for Flight Eye is similar to mammal eye, but it is larger for a relative to body size Less spherical and almost immobile Bird turns its head rather than eyes 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 Juts into the vitreous humor May provide oxygen and nutrients to eye Herbivores must avoid predators Eyes placed to each side to view all directions 27-55

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57 Structural and Functional Adaptations for Flight Birds of prey have eyes directed forward Provides better depth perception Many birds have two foveae or regions of detailed vision Provides both sharp monocular and binocular vision A hawk has eight times the visual acuity of a human and can see a rabbit over a kilometer away An owl s ability to see in dim light is more than ten times that of a human Many birds can see partially into the ultraviolet spectrum Can see flower nectar guides 27-57

58 Structural and Functional Adaptations for Flight Flight History 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 2 hypotheses on the evolution of bird flight The ground-up (cursorial) hypothesis Based on running birds with primitive wings to snare insects The trees-down (arboreal) hypothesis Has birds passing through tree-climbing, leaping, parachuting, gliding, and finally powered flight 27-58

59 Structural and Functional Adaptations for Flight Feathers preceded flight and arose for thermoregulatory purposes No evidence for bird ancestors first being membrane-winged Debate about the origin of flight has not been settled 27-59

60 Structural and Functional Adaptations for Flight Bird Wing as a Lift Device Modified hand bones with attached primary feathers provide propulsion Lift is provided by the more medial part of the wing and secondary feathers of forearm Wing is streamlined with a concave lower surface Leading edge of the wing has small tight-fitting feathers Over two-thirds of the total lift comes from negative pressure from the airstream flowing a longer distance over the top of the wing, the convex surface 27-60

61 Structural and Functional Adaptations for Flight Lift-to-drag ratio Determined by angle of tilt and airspeed At high speeds, sufficient lift is generated so that wing is held at a low angle of attack, creating less drag At a point near 15 o, 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, or group of small feathers on the thumb, provides a midwing slot Slotting between the primary feathers provides a wing-tip slot 27-61

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63 Structural and Functional Adaptations for Flight Flapping Flight Requires a vertical lifting force and a horizontal thrusting force Thrust is provided by primaries at the wing tips Lift is provided by the secondaries Greatest power is provided by downstroke Primary feathers are bent upward and twist to a steep angle of attack On the upstroke, the primary feathers bend so that upper surfaces twist to produce thrust Powered upstroke is essential for hovering and fast, steep takeoffs 27-63

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66 Structural and Functional Adaptations for Flight 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. The small chickadee can change its course within 0.03 seconds 27-66

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68 Structural and Functional Adaptations for Flight High-Aspect Ratio Birds that feed on the wing or make long migrations have high-speed wings These wings sweep back and taper to a slender tip Reduces tip vortex turbulence Flat in section and lack wing-tip slotting 27-68

69 Structural and Functional Adaptations for Flight Dynamic Soaring Wings Albatrosses, gannets and other oceanic soaring birds have wings with long, narrow wings The high-aspect ratio of long, narrow wings lack wing slots and allow high speed, high lift and dynamic soaring They have the highest aerodynamic efficiency of any design, but are less maneuverable These birds exploit the highly reliable sea winds and air currents of different velocities 27-69

70 Structural and Functional Adaptations for Flight High-Lift Wings Vultures, hawks, eagles, owls and other birds of prey that carry heavy loads have wings with slotting, alulas and pronounced camber Produces high lift at slow speed Wings of these birds have an aspect ratio intermediate between elliptical wings and high aspect ratio wings Many are land soarers broad, slotted wings allow sensitive response for static soaring 27-70

71 Migration Migration and Navigation About half of all bird species migrate Can move between southern wintering regions and northern summer breeding regions Can exploit seasonal changes in abundance of insects and avoid bird predators Appearing one time a year prevents buildup of specialized predators 27-71

72 Migration and Navigation Migration also expands living space and reduces aggressive territorial behavior Migration favors homeostasis, allowing birds to avoid climatic extremes and food shortages 27-72

73 Migration and Navigation Migration Routes Most migratory birds follow established north-south routes Some use different routes in the fall and spring Some aquatic species make rapid journeys Others such as warblers take days to migrate 27-73

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75 Migration and Navigation 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 kilometers (11,200 miles) 27-75

76 Migration and Navigation Stimulus for Migration 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 27-76

77 Direction Finding in Migration 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 have a highly accurate sense of time Research indicates they can navigate by the earth s magnetic field May be related to magnetite found in the neck musculature of pigeons Migration and Navigation

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79 Migration and Navigation 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 sun as a compass An internal clock tracks position These experiments suggest use of the North Star as an axis at night Migration involves a combination of environmental and innate cues Natural selection culls individuals that make errors Only the best navigators leave offspring 27-79

80 Social Behavior and Reproduction Cooperative Behavior Sea birds often gather in huge colonies to nest and 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 27-80

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82 Social Behavior and Reproduction 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 27-82

83 Social Behavior and Reproduction Pelicans use organized cooperative behavior to feed Organized 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 27-83

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85 Social Behavior and Reproduction Reproductive System Testes are very small until the approach of the breeding season May then enlarge 300 times 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 In most birds, left ovary and oviduct develop and right ovary and oviduct degenerate 27-85

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87 Social Behavior and Reproduction Expanded end of the oviduct, the infundibulum, receives discharged eggs Special glands add albumin or egg white to the egg as it passes down the oviduct Farther down oviduct, the shell membrane, shell, and shell pigments are also secreted Fertilization takes place in the upper oviduct before albumin and shell are added Sperm remain alive in the oviduct for many days after a single mating 27-87

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89 Social Behavior and Reproduction Mating Systems Over 90% of bird species are monogamous Only mate with one partner each breeding season In a few species, such as swans and geese, partners are chosen for life Recent DNA analyses have shown many passerine species frequently are unfaithful, engaging in extra-pair copulations Nests of many of these species may contain 30% of young with fathers other than attendant male In monogamous birds, both male and females are equally adept at most aspects of parental care 27-89

90 Social Behavior and Reproduction 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 27-90

91 Some birds are polygamous Individuals mate with two or more partners each breeding season Polygyny Most common form of polygamy One male mates with many females Male grouse collect at a lek (collective display ground) where each has a small territory Vigorously defended The male grouse does not care for young Competition for females is intense and females appear to choose the dominant male for mating Polyandry in which a female mates with several males and the male incubates the eggs, is relatively rare in birds Social Behavior and Reproduction

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93 Social Behavior and Reproduction 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 27-93

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95 Social Behavior and Reproduction 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 parasite Lay eggs in other bird s nests 27-95

96 Social Behavior and Reproduction Precocial birds are able to feed and run or swim as soon as they are hatched 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 Causes of nesting failure include predators, nest parasites and other factors 27-96

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98 Bird Populations Factors Bird populations vary in size from year to year Birds of prey may cycle with the food supply Snowy owl populations vary with the rodents they eat When food supplies crash, birds may move elsewhere to locate alternative food Humans have introduced birds to new regions The starling and the house sparrow are both abundant now in the United States 27-98

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100 Bird Populations Since the dodo went extinct in 1695 More than 80 bird species have also become extinct due to human influence Causes of bird extinction include habitat destruction and hunting Modern hunting interests have helped recover wetlands No legally hunted birds are endangered

101 Bird Populations Recent Decline of Songbirds Some songbird 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 Loss of tropical forests also deprives about 250 migratory songbirds of wintering homes

102 Bird Populations Birds stressed in wintering grounds are in poor condition to make northward migrations Some species are adversely affected by deforestation Others such as robins, sparrows and starlings can accommodate these changes

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