Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [28]
Goldfinch nest in red maple sapling.
The three bird nests had, I suspect, been taken over by deer mice. Unlike other mouse and squirrel nests that serve as nurseries for young in the spring and as shelters from the winter elements for adults, these partially arboreal mice had cleverly rebuilt nests to serve as grain bins for winter food larders. Bird nests are often passed on among other bird species. For example, wood ducks, buffleheads, and mergansers depend on the old nest holes made by pileated and formerly ivory-billed woodpeckers. By providing safe nesting sites, woodpeckers are thus keystone organisms for a vast assemblage of birds the world over, including many owls, parrots, parids, flycatchers. But the recycling of the bird nests for food storage by the mice was, as far as I know, a previously undescribed behavior.
Cedar waxwing nest in arrowwood.
The mice made me wonder why almost all the other bird nests I found were destined to be abandoned. Why don’t birds reuse their nests for winter shelter? Even the nonmigrants that stay all winter such as chickadees, goldfinches, purple finches, and blue jays don’t reuse their nests or build new ones for shelter or for food storage. Yet most birds are skilled and eager builders. They construct amazingly elegant and functional nests with which to attract mates and in which to incubate their eggs and rear their young, but would they build even a crude shelter from the killing cold? Nothing doing! Well, almost nothing doing. There are, as almost always in the winter world, blatant exceptions.
The verdin (Auriparus flaviceps) is one. This mouse-gray bird which is halfway in size between a kinglet and a chickadee is a year-round resident of the southwestern United States and northern Mexico, where temperatures in winter can drop easily to 0°C at dawn. Not only is the verdin an apparent exception to most birds, it is a conspicuous exception because it builds numerous bedroom nests at any time of year.
The verdin’s breeding and roosting (or bedroom) nests are both globular structures of about 20 centimeters in diameter. They are shaped like a small squirrel nest, but have only one entrance. Breeding nests take about six days to construct, while roosting nests are finished in just four days. Verdin nests are elaborate structures of three layers. The outermost layer is made of tightly interlaced thorny twigs that solidly anchor the nest into the tree; typically a mesquite, smoke tree, paloverde, or catclaw acacia. The middle layer is a compact mass of matted leaves, feathers, lichen, moss, and other various materials. Finally, the nest interior, especially in winter nests, is lined with soft fluffy material such as feathers from various birds, plant down, and spiderwebs. The winter roosting nests are larger and better insulated than summer nests.
Verdin nests are not only used by verdins. Glenn E. Walsberg, at Arizona State University, discovered a verdin nest in a shrubby desert wash that, during December 1989, was being used as a communal roosting place for fifteen to sixteen black-tailed gnatcatchers (Polioptila melanura). For five nights Walsberg observed these birds as they dispersed during the day, arriving back at the nest in the evening. Walsberg again counted them as they left the nest the next dawn, but this time the nest remained unoccupied for two nights. Obviously, the birds had alternate roosting sites and were traveling en masse from site to site.
Walsberg recorded internal nest temperatures near 30°C at night, even as air temperatures outside the nest dipped to near 0°C at