The Web of Life (126-10-4) Bird Nests

 Bird Nests

Constructing a nest is a central experience in a bird’s life. ~ American ornithologist Lester Short

Many birds build nests. The commonly-held assumption has been that nest-building is an innate ability. It is not. While the urge may be inborn, birds must learn to build nests, becoming more skillful with practice.

Birds choose building materials that camouflage their nests. This includes selectively incorporating strategic amounts of contrasting colors, which provides a disruptive effect: breaking up the outline of the nest to help conceal it.

Birds are conscious of hygiene when building. They line their nests with vegetation that deters parasites. Urban birds sometimes use cigarette butts for that purpose.

Despite their efforts at exclusion, birds are never alone in their nests. Their constructions are small ecosystems, providing optimal conditions for a diverse community of tiny critters who use bird nests as a shelter, foraging site, and home to rear their own offspring.

While some unintended nest visitors are trouble, others are downright helpful. Fly larvae act as housecleaners by foraging on feces and uneaten food. The enhanced hygiene from this waste removal benefits chicks.

Bird nests often appear simple. They are nothing of the sort.

A single male bird, interested in attracting a mate, must carefully construct a suitable nest from available materials. This requires fitting pieces to create a remarkably sturdy structure and keep it in good repair with economical effort. Village weavers are exemplary.

 Village Weavers

The 1st step in making a village weaver nest is being able to tear off a 25–38 cm strip of grass. This is smartly done by perching on a stalk, biting through one edge of the grass blade, then flying with it in the direction of the tip. Doing so takes practice. Young birds tend to tear a strip in the wrong direction or cut strips too short.

The quality of grass matters. Green grass has the right moisture, strength, and pliability. Dry grass won’t do. Appreciating this must be learned.

A yearling village weaver builds a crude nest next to a 2-year-old. Physical maturation and trail and error learning make a difference.

Village weavers become more adept at sophistication in stitches and knots. With practice, weavers build a skill repertoire that affords sturdy construction with efficient material use.

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In birds with more elaborate nests, quality varies considerably among individuals. Adept nest builders get better at the craft as they age.

 Bowerbirds

Bowerbirds are medium-sized passerines. There are 20 species, distributed throughout the southeast Pacific, from northern Australian to New Guinea. Bowerbirds reside in a range of biomes, including rainforest, eucalyptus forests, acacia forests, and shrub lands.

A male bowerbird builds a bower to attract a mate, or at least decorates a cleared patch of ground, depending upon species. There are 2 basic bower types: maypole and avenue. Maypole bowers are made by placing sticks around a sapling; some bowerbirds add a roof. Avenue bowers comprise 2 walls of sticks bordering a path.

While nest-building has an innate impetus, as does many behaviors in every species, the craft of building a bower is acquired through extensive learning. Young birds practice for a couple of years, learning from their elders. Bower decoration is a culturally transmitted, and extended by personal aesthetic expression, both in color preference and construction technique.

A bower comprises a collection of aesthetically arranged twigs and sticks, decorated with various objects: pebbles, shells, acorns, bones, bark, berries, beetles, butterflies, flowers, feathers, or even pastel jelly-like fungus. The decorations tend to be colorfully vivid whenever possible, with different objects arranged in certain patterns, color-coordinated in a pastiche.

A bowerbird remembers his bower’s construction. If an arrangement is disturbed, the bowerbird puts items back in their place. Pilfering among males is common, as is recovery, at least by more dominant bowerbirds.

Avenue bowers are designed to be viewed by a female from a certain perspective. A male great bowerbird architects an avenue ending in a court: a stage where the male displays for a potential female mate. The avenue is lined with objects to create an optical illusion that dispels sense of depth; smaller objects up front, larger objects in the distance. This balances the visual display of the bowerbird along with his bower. A female’s attention lingers on a quality display.

Bowers are statements of individual prowess. Mating results demonstrate that the best birds have the best bowers.

Having built a bower of power, male bowerbirds call for females to come view their display. The male positions himself and performs for visiting females.

A male’s courtship dance on his bower stage can be quite intense: various songlike calls, synchronized with prancing and wing flapping, perhaps along with some fine feather fluffing. Many bowerbirds are superb vocal mimics, imitating other local birds and overheard animals. The vocal presentation also includes mimicry of natural sounds, such as waterfalls. A bowerbird production is altogether an impressive creative work of art.

A female looking for a mate may visit multiple bowers, returning to prospects of favor to view the show and inspect the bower several times. Several females end up selecting the same male. Also-rans may go without copulations.

Once a female has established a preferential male, she is likely to return the next year, with less inspection of others’ bowers. Conversely, a female not delighted with last year’s choice shops around.

Females have individual preferences, which may season with age. Young, inexperienced, female satin bowerbirds may find certain male productions too intense, provoking unease. They tend to prefer well-decorated bowers with bountiful blue objects, and their males showing a touch of finesse in their display.

Older females are better in considering holistic package quality: more tolerant of intense display, and less impressed with frill, as contrasted to overall artistic expression and quality bower construction.

Catbirds are monogamous bowerbirds that do not build bowers. Males raise chicks with their mate. All other bowerbirds are polygamous. A female builds her own nest and raises chicks by herself. A male’s bower is simply a statement of artistic achievement.