TEXT│Y Unlike their reptile relatives, whose young hatch as well-formed, active miniatures of their parents, the warm-blooded young of birds require parental attention both as eggs and as hatchlings. Parental care helps to ensure survival of the young. Parental pair formation, and the preceding courtship, commits both parents to care of the young in some form, such as nest building, physical protection, and feeding of young birds.COURTSHIPRitualsCourtship refers to the various kinds of behavior that brings males and females together as mates and leads to copulation. These behaviors are stimulated by hormonal changes. Male birds act aggressively as they defend their territories against interlopers, including potential mates. Some courtship rituals include singing by birds, as a means of both defending territory against conspecific males and advertising the male╒s presence to a female. Flashing of colorful wing, tail, and body patches often supplements singing. Examples include the display by the males of bright crown patches in such birds as kinglets, the Eastern Kingbird, and the Ovenbird; the parading of brilliantly colored tanagers and orioles before the females and rival males; and the display of especially brilliant breast feathers by robins and meadowlarks.Many males do not sing in courtship, but have other means of attracting the attention of females or of rival males. Woodpeckers drum with the bill on tree limbs, and Ruffed Grouse drum with their wings. Prairie-Chickens and Sage Grouse gather on their traditional ╥dancing grounds,╙ where the males compete in communal courtship rituals. A Sage Grouse arena, or lek (from Swedish, ╥leka╙ for ╥play╙), may be as large as one half-mile long and several hundred yards wide, and may contain as many as 400 males, each about 25-50 feet apart The males maintain small courting grounds on the arena, where they display to females, making booming or tooting sounds associated with the inflation of their colorful neck pouches in a ritual called ╥arena courtship,╙ and defend their small stage from interloping, often younger, males who seek to establish their own courting grounds. The females appear at the grounds to select male partners. Copulation usually occurs on the territory. The females do not form pair bonds. Following copulation, they leave the area to nest some distance away..How Birds Court, or Win MatesAmong birds, the female makes the choice of mate by her acceptance of the male. In familiar North American territory-holding birds--warblers, thrushes, and songbirds in general--the male establishes his territory in spring or early summer and awaits the arrival of the female. He will then aggressively attack or threaten any bird of his kind that enters his territory, male or female, and usually drives away intruding males.The female is attracted by the male╒s aggressive tactics, and when he modifies them, his courtship of her begins. Now he pursues her, running over the ground after her in some species, or chasing her in courtship flights. The songs of the male may called her into his territory, where, usually within a few days, the pair bond between the two is formed.Courtship FeedingIn many different groups of birds, the male feeds the female during courtship and sometimes during her incubation of the eggs. One of the primary functions of courtship feeding is to help maintain and strengthen the pair bond. It is especially important in species in which both the male and female feed the young.In most courtship feeding, the female flutters her wings and opens her mouth to be fed. This posturing emulates a young bird begging its parent for food. Courtship feeding has been reported among birds of some forty families of sixteen orders. Birds that practice courtship feeding include doves and pigeons, many hawks and eagles, cardinals, jays, magpies, crows, goldfinches, redpolls, wood warblers, Northern Bobwhites, Tree Swallows, Bewick╒s Wren, titmice, chickadees, some species of nuthatches, Brown Creepers, bluebirds, Townsend╒s Solitaires, and Cedar Waxwings.COPULATION AND COPULATORY ORGANSMost species of birds lack penises so there is seldom true copulation. Exceptions include ostriches and other ratites, certain ducks, gallinaceous birds, and South American tinamous. Most males impregnate the females with a ╥cloacal kiss.╙ The male bird mounts the female and presses his cloaca (a combination of anal opening and genital pore) against hers. Transmission of sperm into the female cloacal opening follows, and the sperm travels up the oviduct where the eggs are fertilized. In some species copulation also helps to maintain the pair-bond. Pairs of Red-bellied Woodpeckers and House Sparrows, for example, may copulate as many as two months before the eggs are laid.EGGS AND EGG-LAYINGEggs range in size from the smallest--those of the Vervain Hummingbird of Hispaniola and Jamaica, less than 1/2 inch long--to the largest known-those of some of the extinct Elephant Birds of Madagascar, whose eggs were 13 inches long and 9 1/2 inches in diameter at their widest part. Each Elephant Bird egg weighed about 18 pounds and had a capacity of 2 gallons. One of them could hold the contents of 33,000 eggs of the Vervain Hummingbird; of 6 ostrich eggs; or 148 eggs of a domestic hen.The largest egg of any living bird is that of the ostrich: 6.8 x 5.4 inches. Egg sizes (in inches) of other large birds are: Wandering Albatross: 5.7 x 3.5 Mute Swan: 4.5 x 2.9 California Condor: 4.3 x 2.6 Trumpeter Swan: 4.3 x 2.8 Common Loon and American White Pelican: 3.5 x 2.2 In general, small birds lay eggs that are heavier in proportion to their body weight than those of larger birds. For example, a small wren lays an egg weighing 13 percent of her body weight, but an ostrich egg is less than 2 percent of her body weight. The Kiwi of New Zealand lays a very large egg relative to its size: 18-25 percent of the bird╒s body weight.Many birds lay white or nearly white eggs, as do reptiles, and it is thought that originally all birds╒ eggshells were white. Natural selection may have later favored colored eggs in birds, owing to the protective advantage of coloration and markings making them less conspicuous than white eggs.Birds that lay white eggs in open nests--some doves, hummingbirds, owls, and grebes, for example--begin to incubate with the laying of the first egg, thus hiding the eggs during the period that the bird is on the nest. Ground-nesting birds that lay white eggs--some ducks, geese, grebes, and many gallinaceous birds--cover their eggs with plants or grasses just before they leave the nest. This hides the conspicuous white eggs from the sight of foxes, skunks, raccoons, ravens, crows, snakes, dogs, and animals that would eat them. No eggs are entirely safe, however. Whether white and hidden or of protective colors, a significant number may be lost to predation.The word ╥egg╙ includes the ovum and associated materials (yolk, albumen) enclosed with shell membranes and shell--the finished product, so to speak. But this completed egg begins as one egg cell, or oocyte, within the bird╒s ovary. Only a small percentage of the oocytes in the ovary of the female bird ever mature into eggs. The ovary of a thirteen-day-old Red-winged Blackbird or European Starling contains about 100,000 oocytes, of which only about 50 will grow and be released from the ovary during her lifetime. Depending on the species, birds lay from one to twenty-three eggs in a clutch. The number of eggs laid for any species depends upon a variety of factors: food supply, nest site, latitude, and degree of development of the young at hatching. Altricial ground-nesting ducks, quail, grouse, pheasants, turkeys, and other game birds lay larger clutches than birds that nest in trees or on inaccessible cliffs and islands. The number of eggs in the clutch of each species has been adjusted by natural selection to the largest number of fledged young that a pair of birds is capable of producing. Many birds lay fewer eggs in a clutch in the tropics. Here the daylight is shorter during the breeding season and less food is available than in northern latitudes. Shorter days result in less effective time per day to collect food for young. There are also seasonal variations in clutch size. Some birds lay more eggs (have larger clutches) in the first nest of the season. Second or third clutches may contain fewer eggs (this is true, for example, of bluebirds).Clutch size in many birds is correlated with food supply, as shown by Rough-legged Hawks and Snowy Owls. These birds lay larger clutches when their main food (lemmings) is plentiful. Barn Owls may nest almost continuously during a year when meadow voles are at their peak abundance. They lay fewer eggs, or fewer clutches, or even fail to nest, when their staple prey is scarce. Even certain non-predatory birds, such as quail, lay smaller clutches in dry years or may not nest at all.Most birds are determinate egg-layers, i.e., they lay a certain number of eggs in a clutch and will not lay more even though one or more of their eggs are removed from the nest. Others, called indeterminate layers, will continue to lay eggs to replace eggs taken from the nest.NESTINGThe Purpose of NestsFeather and egg--flight and nesting--are quintessential elements of birds. One could say that birds nest because they fly, and fly because they nest. Efficient flight is enhanced by a reduction in body weight. Egg laying is, in part, an adaptation for such weight reduction.The production of an egg--a protective container holding a developing young complete with nutrients--is an adaptation that allows a female bird to produce offspring and then care for them outside her body. She need not expend energy carrying around her developing young within her body beyond the minimum development time to produce the egg. Indeed, most birds only have a single (usually left) ovary as a further adaptation for weight reduction. But the price of this lighter weight is the complex behavior required to care for the egg and the resulting hatchling. The nest is where such care takes place.Definition of a Nest The words ╥bird nest╙ conjure up a stereotype in most people╒s minds of a basket of grass and fiber, sized to fit in a human palm, fitted into the crotch of a branch. This picture may fit many kinds of nests, but there is a great variety of sizes, shapes, and types ranging from a scratch on the ground (terns) or the tops of a parent╒s feet (penguins) to tree-top aeries (eagles) and roof-top platforms (storks) weighing a ton and more.A nest is a special location for raising young--selected, augmented and maintained by parent birds for the purpose of egg protection, incubation, and parental care of hatchlings until they mature to independence. Nest site selection, nest building, maintenance, and parental behavior can be vastly different among the 9,000 or so species of birds.Choosing the Site for a NestMost birds build a new nest for each clutch of eggs, whether during the same breeding season or in subsequent seasons. Exceptions include large birds of prey such as eagles and Ospreys that return to the same nest year after year. The location of the nest may be selected by the female (as in grouse, pheasants, and wagtails) or by the male (terns, wrens, and sparrows) or by both (doves, crows, jays, and titmice).Building the NestAs with site selection, a pair may divide the labor of nest construction. Male and female kingfishers, woodpeckers, swallows, and waxwings share in nest building. Male Mourning Doves supply the material and the female builds the nest. With frigatebirds the reverse is true. In some species, females are the sole constructors with no help from males; examples include hummingbirds and Red-eyed Vireos. Male shrikes build their nests without help from females. Site selection is important for some species, but no nest is constructed. Nighthawks, nightjars, tropicbirds, and terns nest on the bare ground. Kingfishers nest in ground burrows and Fairy Terns balance a single egg on the bare limb of a tree or shrub. Nest LocationsThe essence of a nest is protection for eggs and young. A good site is a safe site. A pair of birds with a single nest within a large colony of nests of the same species gains the advantage of the mobbing behavior of other adults in routing off predators; e.g., Arctic Terns nesting in beach colonies mass together to drive away marauding crows.Many kinds of small birds in the tropics build their own nests near wasp nests to get protection from predators; e.g., Black-throated Warblers in Australia, and caciques in South America.The site and area selected for a nest by a bird or pair of birds depends on a list of factors, including but not limited to:(1) competition for space from other birds(2) proximity of building materials(3) proximity to feeding grounds(4) presence (or absence) of predators(5) the species of bird selecting the siteIn some species, American Robins for example, both site selection and the character of nest construction may depend on the experience of the nest builder. Young birds build a typical robins╒ nest but gain experience with each season. Location of a nest may vary within a species depending on the geographic area and ecological conditions. Brown Thrashers build nests in low bushes in the eastern United States but on the ground in the West. They nest in bushes in southern Michigan, and on the ground in northern Michigan.Given the great variety of bird species, it is hard to find an area that does not include a site for some kind of bird to build a nest, including: On The Ground: Many birds (plovers, young sparrows, ducks, and geese) build nests of grass, twigs or leaves on the ground. Others, such as night hawks, tropicbirds, and terns, limit their nest building to simple scrapes or depressions. Ovenbirds construct domed nests of grass and sticks on the ground.In The Ground: Kingfishers, puffins, storm-petrels, and Bank Swallows dig burrows and may construct simple nests within them. Megapodes, relatives of the fowl-like birds (guinea-fowl, grouse, pheasant) bury their eggs in sand, warm volcanic ash or ╥compost heaps╙ they construct from soil and rotting vegetation. Their young are extremely well-developed and fend for themselves after digging out soon after hatching. On The Water: Some waterbirds--coots, grebes, jacanas--build raft-like nests of mud and reeds attached to water plants. Greek legend held that kingfishers (their modern scientific name is ╥Halcyon,╙ the same as their Greek name) nested on the sea surface when days were calm and the wind was still--the easy time we still call ╥halcyon days.╙Off The Ground: Elevating a nest above the ground provides some protection from predators, affords views of surroundings, and assists take-offs for adults leaving on feeding forays. Elevation of nests can range from a few feet above ground in shrubs (boobies, frigatebirds) to 100-foot-high tree-tops (Ospreys) to 1,000-foot-high sea cliffs (kittiwakes, murres). While tropical Blue-footed and Masked boobies lay eggs on stony beaches, related red-footed boobies build nests in shrubs behind the beach berm, often adjacent to shrub-nesting frigatebirds. Trees, of course, are the most likely place to seek birds╒ nests. Most songbirds build cup nests wedged in branches. Woodpeckers excavate holes in trees; desert species like the Gila Woodpecker carve chambers inside giant saguaro cactus. Some birds (e.g., orioles and the related neotropical oropendulas) suspend purse-like, woven nests from branches. The surfaces of trees, cliffs, or building eaves or chimneys are attachment sites for ╥adherent╙ nests built by Barn and Cliff swallows. Many species will adapt to artificial nest-sites (boxes, birdhouses, etc.) when they are provided. Examples include ducks, loons, and purple martins, to name just a few.TimeMost small songbirds build nests in a matter of three to nine days. Construction time may also depend on the point in the season when building commences on a given nest. Goldfinch nests built early in the breeding season may take two weeks; nests started a month later may take less than a week. The more elaborate nests, like the pendulous bags of orioles, may take weeks to build.The Smallest Nest-The Largest NestThe White Stork of Europe and Asia (the baby-bringer of fable) constructs huge nests: 6 feet deep, 5 feet across, and weighing a ton. Pairs use their nest year after year. A Bald Eagles╒ nest in a Florida tree is the largest North American nest recorded. Occupied for decades, it measured 20 feet deep and 9 1/2 feet across. The nest of a Ruby-throated Hummingbird is about an inch across and an inch deep.INCUBATION AND EARLY PARENTAL CAREAll birds (with the exception of a few that seasonally hibernate) are ╥warm-blooded╙ (biologists use the term ╥homiothermic╙). That is, a bird╒s body temperature is constantly maintained within a certain range (usually 103 to 109 degrees Fahrenheit, depending on the species and on an individual bird╒s condition), regardless of the temperature of the environment. Developing young contained within eggs and hatchlings must be maintained at or near this temperature to develop successfully.A nest is the place where parents incubate eggs--keep them near the adult body temperature--and care for the young. To maintain temperature, parents transfer heat from their own bodies.Most birds (e.g., songbirds, birds of prey, shorebirds, fowl-like birds, and ducks) develop ╥incubation patches╙ that act as heat radiators. These bare, featherless areas of highly vascularized skin (i.e., a lot of blood circulates near the surface of the skin) transfer heat from the circulating blood of parents to the egg and newly hatched young.Other birds (e.g., tropical sea birds such as tropicbirds, frigatebirds, cormorants, pelicans, and boobies) form no incubation patches. Instead, heat is transferred from blood circulating in the webs of the parents╒ feet.The usual concern of adult birds that incubate eggs is keeping the eggs warm. However, tropical seabirds nesting on sun-baked islands must also cool nests to avoid overheating of eggs and hatchlings. For example, both adult and young boobies radiate heat from gular (or throat) pouches and by holding their webbed feet in the circulating, cooling air.Young Birds In The NestOrnithologists classify hatchling-birds in two general categories (although they realize there are stages in between). Birds that hatch in a helpless form, with little feather development and often with closed eyes, are called ╥altricial╙ (from a Latin word ╥altrix,╙ meaning ╥wet-nurse╙). Altricial young require considerable care and relatively elaborate nest construction. Their nests are usually located in protected areas, such as trees and cliffs, that are difficult for predators to reach. Examples of birds with altricial young include songbirds such as robins and cardinals; some waterbirds such as cormorants, penguins, petrels, and albatrosses; and other birds such as pigeons, woodpeckers, swifts, owls, and hawks.In contrast, precocial young are well developed at hatching and are able to move about and feed themselves. Most are only partly dependent (some not at all) on their parents for care. The word comes from the Latin ╥prae-cox╙ meaning ╥to ripen early.╙ Birds with precocial young usually have simple nests on the ground or near water. The mobility of the young birds helps to protect them from predators. Examples of birds with precocial young include ducks, loons, quail, grouse, and most shorebirds. Two other important terms used by ornithologists in describing young birds and their relationship to nests are ╥nidicolous╙ and ╥nidifugous.╙ While ╥altricial╙ and ╥precocial╙ refer to the physical development of hatchlings, these terms describe their behavior around the nest.Both words come, in part, from the Latin for nest (╥nidus╙). Nidicolous (from the Latin ╥colere,╙ ╥to dwell or inhabit╙) young remain in or at the nest and are cared for by parents. Nidifugous (from the Latin ╥fugere,╙ ╥to flee or escape╙) young leave the nest area soon after hatching.NESTS AND NEIGHBORSNo nest exists in complete isolation. At the least, a bird╒s nest is home to mites and insects with larvae that suck bird blood. Some birds exploit insect relationships. In Australia, parrots and kingfishers regularly build burrow nests in termite nests, presumably for the protection this setting affords. Many kinds of birds nest in colonies with other birds of the same species (Sooty Terns, murres) or sometimes in mixed colonies (e.g. gulls and terns, herons, ibises, and Anhingas.) Smaller birds sometimes nest close to large birds of prey, apparently for protection. For example, grackles may nest in the sides of Osprey nests; Cliff Swallows near Prairie Falcons. Although most nesting colonies of birds are on surfaces--a beach, marsh, or cliff-face--the Sociable Weavers of South Africa share colonial nests. Seventy or more pairs cooperate to build huge suspended tree-nests as big as haystacks. Each pair has its own chamber in this extensive set of nesting compartments. In North America, Purple Martins commonly nest in multi-roomed houses provided by bird fanciers. More than 270 pairs have been reported to share one large house. Some birds rely on other species to build nests for them. ╥Brood parasitism╙ is the term for the regular habit of some birds that lay their eggs in the nests of others. They thereby save the energy required to construct nests and in some cases to feed the young, relying on the host parents to do so. The Brown-headed Cowbird builds no nests of its own and lays its eggs in the nests of more than two hundred species of North American birds, especially Red-eyed Vireos, American Redstarts, Yellow Warblers, Song Sparrows, Rufous-sided Towhees, Warbling Vireos, andYellow-throated Warblers. European cuckoos provide a notorious example of brood parasitism reflected in the word ╥cuckold.╙WHAT A NEST TELLS ABOUT A BIRDIn summary, a found nest can reveal much about the natural history of the parents that built it and the young that were raised there. For example, an elaborate, container-nest was constructed with care and required time and energy to locate and build. The young were altricial and nidicolous and spent a considerable part of their early life in the nest.A flimsy nest on the ground was home to precocial nidifugous young that hatched well-prepared to venture forth. Parents probably spent much time protecting the nest and selected a site protected by camouflage or by proximity to a lot of other parents.A feather, a nest. an egg-shell: all evoke the basic essence of birds, and each can tell us much about the birds that produced them.
