Birds' Nests: How on Earth Do They Do It?
For some birds, a new home requires more than just leaves and sticks.
|While hanging under its nest, a male weaverbird puts on a display for interested females.
The raw materials of their project—the twigs, pine needles and trash collected on a wintry walk that morning—lay on the living room floor before Audubon’s editor in chief David Seideman and his eight-year-old daughter, Addie. In recent months, they had discovered three nests blown from bare branches onto Brooklyn sidewalks. Fascinated by these small creations, which incorporated bits of twine and pieces of plastic in addition to more natural materials, Addie decided to enter her own “city bird” nest in the upcoming science fair at her school. But after several minutes of painstaking attempts to tie and weave boughs together, her dad and friend Tom Amorosi, a forensics anthropologist a neighbor, fetched a metal mixing bowl—and a lot of Elmer’s glue.
“Even though we have fingers and hands, it didn’t hold together,” says Seideman. “I have more admiration for [bird’s] amazing architectural skills. They have only beaks.”
And birds have developed a variety of ways to use their beaks to build nests. Village weaverbirds from Africa, for example, nip large leaves such as elephant grass and then fly off, tearing fine strips as they go. Once at their nesting site, males stitch the strips together under branches to make their homes. During the whole process, weaverbirds invest an estimated 200 miles of flying. Recent research comparing the anatomy and behavior of many species of birds has found that the more aerobatic birds with strong forelimbs—like weaverbirds, swallows, and swifts—are the ones that build complex nests that help keep their vulnerable fledglings safe.
Other birds have resorted—like Seideman—to “gluing” nest materials together. The fluffy long-tailed tit from the United Kingdom searches for building materials in both its traditional woodland habitat and in gardens with a mix of deciduous trees and undergrowth. A pair spends about three weeks collecting about 3700 pieces of lichen, moss, feathers and spider cocoons. It takes twenty-eight miles of flying for a pair to gather just the feathers, which are used to cushion the eggs. But it’s the outer cup that is masterfully assembled. Using a microscope, ornithologist Mike Hansell studied that structure and found that the lichen and spider cocoons dovetail like Velcro, adhering the component parts.
Mud and spit bind other nests. American robins cement their relatively simple, symmetrical cup to the branch with several hundred beak-fulls of mud. The nest itself is made of up to 350 twigs and grasses collected during spring over the course of 4-5 days. (See Journey North to pretend that you’re a robin preparing for your new blue additions, or look at a series of photographs that track the progression of a single nest.)
Black-billed magpies also use mud to strengthen their nests, which consist of a dome of sticks with two entryways. The mud lines the inside of the structure, along with finer and more comfortable materials like rootlets. The male and female take a combined two thousand trips to make their sanctuary. While the process might seem exhausting, spread out over the roughly 40 days of construction, the extra energy required is not more than one percent of the birds’ daily food intake.
Some birds are more opportunistic and flexible than others when it comes to selecting nest materials. For example, bluebirds prefer grass but will also make do with what is locally available, from pine needles to plastic Easter grass. Changes in American habits are also reflected in what bluebirds scavenge. The drop in smoking, for instance, has cut the amount of cellophane tabs (used to open cigarette boxes) found in nests. The chipping sparrow is another example. In the past, it incorporated enormous amounts of horsehair in its nests—which explains its formerly common name, the hair bird. But with the advent of the automobile and decline in available horsehair, the bird switched to other materials like fine grasses.
Back in Brooklyn, Addie finished her creation by poking police caution tape and hair from her dog Sheba into the nest. Once dry, the nest was about ten inches in diameter, shallow and appropriately urban.
Still, Seideman muses, “I don’t know what bird would live in it. It was the product of the mixing bowl.” But it did get top marks from the science fair judge.
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