Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [24]
There is some flexibility in nest construction (but not much). A tree swallow at my house in Vermont built a nest almost exclusively of feathers (87 large white ones presumably from chickens, and one small black one from a raven, to be exact). House wrens used the same nest box immediately after the swallows left the eggs that had rotted (after they stopped incubating during a week of cold, early spring weather). But the wrens built a nest of 675 twigs that filled the box nearly to the top. This nest had no feather lining, although for their second successful brood that they raised in the same box that season, the wrens replaced the nest that I had removed with one made of only 40 twigs and lined with 185 pieces of grass and rootlets and 52 feathers of many kinds of birds.
Male ravens build their nest by holding large thick sticks they break from trees with their bills. With rapid head-bill shakes, they vibrate the sticks against or on the substrate of ledge or tree branches at the chosen nest site, until they anchor there. After the nest platform is built, the bird habitually perches in the center, where fewer sticks then become anchored, and a nest cup with a high rim develops. Once the basket of sticks is completed, both members of the pair then upholster it with bark, moss, grass, and animal fur. Almost all nests in New England contain deer hair, sometimes a dense felt of it. Snowshoe hare, bear, and moose hair are also commonly used to cushion the eggs and help insulate them. The female alone incubates for twenty-one days even at the typically subzero temperatures in February and March.
Robins build a symmetrical hard mud cup that is surrounded by loose debris and lined with soft grass. The whole process involves instinctive responses. Two nests I watched being made were each almost completed in two days. On the first dawn, the female robin deposited a haphazard pile of grass, twigs, and birch bark strips at the future nest site. At dawn the next day, she started bringing mud, making one trip after another, at one-to three-minute intervals for five and a half hours. During the first hour or so the male followed the female but did no work; then he perched in the nearby woods and sang quietly, while she worked. No work was done in the afternoon. By the third morning the female made only a few trips, and she was unaccompanied by her mate. Without exception after depositing the mud, she squatted down and vibrated her body for a second or two; then she got up, turned a few degrees to right or left, and repeated the squat-vibration. She performed as many as sixteen of these routines after dumping a single billful of mud or other debris. Her nest-shaping did not seem to be attributable to a conscious plan. On one long beam under a shed, I found where a robin had built a two-cup nest, apparently because she got started turning at two similarly appearing spots on the initial pile of debris that had spread too far laterally on the smooth beam. I found a phoebe who had built a similarly misconstructed nest also on a long even beam where it might have been difficult to fix the precise nest location. In real estate, location is everything, especially in birds.
Although among geese and robins the female alone builds the nest, in some species (principally wrens and weaverbirds) the nest is built primarily by the male. Such male-made nests initially serve as sexual attractants. The female chooses the nest and through it, the builder, and she signifies her choice by adding the nest lining according to her own finicky specifications. I recall watching African spotted weaverbird males in a colony, all dangling from their half-finished nests and beating their wings and calling to show off. When prospecting