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By Root 1290 0

females entered a nest to inspect, the male owner went wild in frenzy. In the northern New England woods, perhaps one of the best examples of this is the winter wren, which builds its nest hidden deep under the roots of trees that have been blown over by the wind. The wren’s nest is a snug little cavity with walls camouflaged with a lattice of moss and conifer twiglets. It has a quarter-inch entrance hole at the front. A male wren may build several of these nests in an area (I found a sedge wren who built six in his territory), but only the one chosen by the female will be lined with fur and feathers. The “false nests” apparently not only give the female more choice but may also prevent unmated males from moving into the neighborhood.

Weaverbirds of many species literally weave hanging fiber bags that resemble our textiles. However, one species, the sociable weaver (Ploceus cucullatus) of Africa (a close relative of our local weaverbird, the common suburban English sparrow) builds communal nests that in size and shape superficially resemble human-built thatch-roofed huts. Generation after generation of birds add to these apartment houses until they may weigh several tons and fill out the entire top of a large tree. The apartment complex shields the individual nests within from the sun and from extreme temperature fluctuations in the bird’s desert environment of southern Africa where nights are commonly cold and days are commonly hot.

Aside from protection from the elements, an important ultimate or evolutionary reason for such astounding diversity of nests and nest locations is predation. By far the greatest vulnerability a bird experiences is often in the long egg and nestling stages. Nest predators include other birds, as well as snakes and mammals. Nest inaccessibility works as an adequate defense for some, but most species rely on hiding their nests to reduce predation. However, for effective hiding, much depends on what others are doing.

In the winter woods, much is buried and hidden, but more is revealed. Curtains of snowflakes drifted over my face, and the snow already piled into cushions muffled my footsteps. Two feet of fresh powder had already fallen on solidly frozen ground layered with fallen leaves. Recently those leaves had obscured the views through an otherwise opaque forest of green. Now, at the end of January, I could see through the forest from one large maple tree to another, to beech trees, root tip-ups, and beyond to an open bog bordered with viburnum thickets. I was looking for bird nests that had in the summer been almost impossible to see because they were then enveloped in leaves. They would now show up as dark silhouettes with white caps. (The ground nests obviously will be missed, but most are almost invisible even without a covering of snow.)

We often find nests by sheer luck. But like nest predators, we improve our chances if we know where and what to look for. That is, present success depends on previous success. Similarly, predators also develop search images based on their previous experience. But by focusing on the familiar nests they may miss those that are different or at other than the usual locations. Variety of nests and nest sites is often a key to nest survival in many species. A simple thought experiment shows why. Suppose every species of small songbird were to tuck its nest behind the loose dead flaking bark of dead balsam fir trees, as brown creepers do. If every bird species were to choose such a site, then it would not be a hiding place. Instead, these sites would soon indicate where to find eggs and tender nestlings, in the same way that the Golden Arches now show us where to find a hamburger. What applies to the nest site applies to the nest itself as well.

Red-eyed vireo nest in sugar maple in the forest understory.

Programming for precise nest construction yields species-specific nests, and the more different or exotic the nest appearances are for different species, the less any one would stand out to predators. Like a nest predator, during my winter walks I