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.
Georgann
Schmalz
Ornithologist, President Birding Adventures, Inc.
404-633-1527
jbgs@mindspring.com
birdingadventuresinc.com
