Nesting and the Nesting Season
October 25th, 2022 by georgann
Most birds have a nesting season when food, temperature, weather and a protective habitat are at an optimum for a successful brood. In response to these environmental clues and its own internal stimulus, a bird prepares it to nest at the most optimal time. In the temperate zone this occurs only once a year, while in the tropics many birds nest repeatedly throughout the year.
It is no coincidence that the favorable environmental conditions match a bird’s physiological breeding condition. Birds are aroused from their non-breeding refractory period by hormones secreted from the hypothalamus, pituitary gland and sex organs, which, in turn have been stimulated by the lengthening of the daylight and an internal biorhythm. In some cases, the environmental stimulus for breeding appears rather obscure to the human observer. Tropical species are more stimulated by the alternation of wet and dry seasons than by day length as in temperature zone species.
In the northern temperate zone, as the day lengthens and the sun swings north during early spring months, all the necessary factors for successful nesting come into play simultaneously. Neotropical migrant species, such as thrushes, warblers, flycatchers and tanagers, have crossed the Gulf of Mexico and have reached the southern United States by early April. Many of these migrants begin nesting but most must advance northward as spring brings milder weather farther and farther north. These birds must wait until food is abundant, the vegetation lush enough for nest concealment and the days long enough to find food.
Sub-arctic and arctic nesting species tend to have larger clutch sizes than their tropical counterparts. In the northern latitudes, the season is short and there is only time to nest one time. More eggs laid usually means more fledglings. Tropical species lay fewer eggs in a clutch, but nest more than once during the year.
Once the urge to breed is set in motion and migration complete, the male of each species sets up a territory, sings, and attracts a female to a likely nest site. In most cases, after mating the female selects the exact time for nest building and egg laying.
The nest is designed to conserve heat and to protect and conceal eggs and young from unfavorable weather and predators. Safety of eggs and young is a primary consideration when choosing a nesting site. Alexander Skutch, studying tropical birds, suggests that birds depend upon four factors for safety: invisibility, inaccessibility, impregnability and invincibility.
Invisibility is naturally achieved by hiding the nest as much as possible. Killdeer rely upon camouflage as they lay their eggs next to stones on the ground. Northern Cardinal, American Robin and Blue Jay nests are difficult to find amid the vines and summer foliage. Only patient watching for birds carrying nesting materials or food can reveal exactly where the nest has been placed.
Nests built at the ends of flimsy branches or attached to thin dangling vines are inaccessible to most predators. Oropendolas and related birds of tropical America hang their long nests from the ends of branches. Cliff nesters, such as murres and gannets, and tunnel excavators, such as puffins and kingfishers, place their eggs in places that are physically difficult for most predators to reach. Cactus Wrens in the southwestern United States delicately build their nest amid thorns and needles of desert cacti. This is, of course, comes with its own risks since a bird can get impaled entering or leaving its own nest.
Woodpeckers are the consummate artists of impregnable nests; the male finds an appropriate nest site within his territory, “sells” it to his mate by using courtship displays and calls, and helps her excavate the cavity. Carolina Chickadees, Tufted titmice, and nuthatches and wrens have evolved cavity nesting and often take advantage of pre-owned woodpecker homes.
Only exceptionally bold or strong birds can protect their nests by driving away predators, thereby making the nest invincible. Eagles, hawks and other birds of prey display this protective behavior pattern. Western Kingbirds and many species of hummingbirds will fiercely dive-bomb would-be disturbers and attempt to drive them away. Often birds will use other animals to help make their nest invincible. In the tropics, nests of some trogons are built in close vicinity or actually in the nests of wasps and ant colonies. In other cases, smaller birds will nest close to more powerful birds. Cliff Swallows are often found nesting close to the eyrie of Prairie Falcons and Western Kingbirds nest near Swainson’s Hawks.
The male bird may build a nest alone, the female may build by herself, or both may cooperate in building. Male hummingbirds have rarely been observed involved in nesting activities including caring for or feeding the young. Female phalaropes, jacanas and tinamous lay their eggs in a nest that the male has built by himself. He then continues with incubation and caring for the young. Among the nonpasserine families, herons, hawks, pigeons, woodpeckers, kingfishers, gulls and terns, nest building is almost always a combined effort. The male may place leaves and twigs into the nest or his mate may wait on the nest and receive his materials, preferring to arrange them by herself. In the passerine order, males and females may take turns building, the male may watch the female and give her frequent encouraging chirps, or he may make ineffectual attempts at helping. Male tityras from Central and South America have been seen to follow their mates faithfully back and forth, all the time carrying the same piece of nesting material, but never actually adding it to the nest.
The types of nests and materials used in construction are as varied as the sites and skills used. The simplest nest is an unceremonious scrape on the ground or clearing on a roof of a flat building. Most ducks and geese make a small depression on the ground by pushing aside grass and leaves. These scrapes may or may not be lined with feathers. Common Nighthawks nest on the flat gravel roofs of buildings.
Platform nests are somewhat more complicated than scrapes. Architecturally simple, they lack the inner cups of more evolved nests. The nests of Mourning Doves and Rock Doves are platforms of small twigs and branchlets. Two to five eggs somehow manage to stay in the flat nest without rolling out. The same pair of Bald Eagles may use their large platform nests for over thirty years. Each year they add new branches and sticks to the nest which can eventually reach six feet in diameter and weigh up to two tons. On occasion, Great Horned Owls will nest in the side of an eagle nest.
Some of the most complex nests can be found in the weaver finch family. Interwoven grasses and rootlets and tied into place to form pendulous structures.
Montezuma Oropendolas of Central America probably hold the record for the longest pendulous nests, averaging 35 inches in length.
Chimney Swifts glue together small broken twigs and stems into a semi-cup-shaped nest and then adhere it to the inside of a chimney with their sticky saliva. Mud gathered little by little from puddles and riverbanks make up Barn Swallow’s and Cliff Swallows’ nests. American Robins stick rootlets and grasses into a middle layer of mud that has been pushed into a bowl by the female’s breast, belly and feet.
The majority of birds build cup-shaped nests of rootlets, vines, dried leaves, leaf skeletons, moss, bark of plants, grasses, tendrils and down materials from thistles and fern stems. The Ruby-throated Hummingbird lines her tiny nest with fern down and surrounds it with lichens glued in place with spider webbing.
Animal materials are used more often as decoration than as substantive components. Feathers and hairs may line the inner cup making a soft, warm nesting place. The downy feathers from the breast of Eider Ducks may eventually be gathered, cleaned and processed into pillow stuffing. Horse hairs and sheep wool found in Chipping Sparrow nests might be gathered from barbed wire and fences. One account tells of a gentleman being repeatedly visited by a Tufted Titmouse that pulled pieces of hair from the top of his unprotected head. Titmice have also been known to gather hair from squirrels, woodchucks and opossums.
Various cavity nesting birds use the cast skins of snakes and lizards. House Wrens, Bewick’s Wrens and Great Crested Flycatchers weave the skins into the wall of the nest. It is questionable that these reptilian skins scare away predators since these birds will use other similar materials such as cellophane, plastic or thin clear paper.
In place of mud, cow dung has been used by nesting grackles. Sometimes, the bird’s own droppings can add support to the nest framework. On the island of Trinidad, the Oilbird makes its nest largely of regurgitated fruit and other material.
Man-made objects are often a favorite of birds. These artificial materials may be used in nest construction for their added support or perhaps because the birds are attracted to their uniqueness or color. Blue Jays and American Crows use shiny artifacts including watches and rings in their nests. Although not a nesting structure but a courtship arena or lek, the bower of the Satin Bowerbird of eastern Australia attracts females with the aid of colorful glass, pottery, scraps of rags, paper and jewelry stolen by the male.
The Northern Mockingbird is a familiar nester in the southeastern United States. The outside layer of its nest is mostly grass and stems often loosely intertwined with cellophane, yarn and string. For whatever reason, American Robins invariably fasten a long, dangling piece of string, cellophane or tinsel from the side of the nest. One account tells of an American Robin nest with a leg from a pair of panty hose boldly hanging down.
Once a nest is built, the occupants will, with some luck, successfully raise a brood of young. The incubation of the eggs and the feeding and protection of the nestlings is a tremendous job, usually requiring the full attention of both parents. Nest building behaviors are generally suppressed by the other nesting activities of as incubation and feeding. Nest repair, however, may continue during the incubation period. Cliff and Barn Swallows will add mud to support a loose or broken wall. Hanging nests are often repaired by weaving fresh vines into its place of attachment. Pigeons and anis will continually add sticks and twigs to their nests.
There is a fine line between nest repair and continued building. In some species, the urge to build carries on into the period of incubation and, rarely, into the care and feeding of the young. Becards, bushtits, and some hummingbirds keep adding so much material to their nests throughout incubation that the nests grow quite large and bulky. Adelie Penguins continually add stones to their nest for a more apparent motive–extra stones raise the egg above the danger of floodwaters from melting ice.
After the nestlings have fledged, the parents may immediately reuse the same nest. Mourning Doves have been known to lay five sets of eggs in the same nest during one season. If the nesting has failed, a new site is often chosen since the old nest was probably too exposed or unsound. Thin, delicate nests are usually rebuilt during the season. Most birds do not use the same nest year after year. Exceptions to this are eagles, hawks, and other birds that rebuild on the foundations of last year’s nest.
A certain amount of nest pirating takes place among birds. Piratic Flycatchers of tropical Central America patiently wait until oropendolas, smaller flycatchers or trogons have completed their nests. The flycatcher begins to annoy their victims and distracts them from the nest. Then the flycatcher flies into the abandoned nest, removes any eggs already laid and takes over, forcing the original owners to build a new structure elsewhere.
Some birds will use the nest as a dormitory for a few days or weeks before nesting. Woodpeckers and Eastern Screech-Owls often place their eggs in a hole in which they have been roosting. In Guatemala, the Blue-throated Green Motmot digs a hole in June or July but will not lay eggs until the following April. On the other hand, some birds will lay eggs before their nest is finished. This can be disastrous for the first egg or two that may not be adequately protected.
Most birds lay their eggs within two to four days after the nest is completed. It is believed that the very act of nest building stimulates the pituitary gland to release follicle-stimulating hormone to mature an ovum and luteinizing hormone to induce ovulation. Generally, birds lay one egg a day until the appropriate clutch size is reached. Indeterminant egg layers innately know how many eggs should complete their nest and finish laying when that number is reached. Domestic hens are indeterminant layers and will consistently replace eggs that are removed every day. The majority of birds, however, are determinant layers. They do not replace lost eggs unless their entire clutch is destroyed. Unfortunately, some of the species that only lay one egg are determinant layers and will not replace their lost egg but instead wait until the next year to nest again.
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