Most of our local breeding birds build open-cup nests, more or less like deep saucers with no roof. Robins, mallards, hummingbirds, warblers, and sparrows are examples of open-cup nesters.
In contrast, some of our local birds nest in cavities. Woodpeckers and nuthatches excavate their own nest cavities in trees and snags. Other species depend on pre-existing tree cavities, made by the excavators or by decay; chickadees, two species of swallows, several owls and ducks, and the kestrel are examples. Winter wrens burrow into mosswads or find existing holes in stumps and logs. Kingfishers, two species of swallows, and several species of seabirds, such as puffins, burrow into earthen banks.
Still others build their own "cavity" by making a roof on the nest. Cliff swallows construct a ball of mud, with an entrance. American Dippers build volleyball-sized covered nests of moss. Magpies, which occasionally may nest in Southeast (and are here in winter) build huge domed nests of sticks.
All told, roughly 20 percent of bird species observed here in spring nest in some kind of cavity. Why nest in cavities? Why not build open-cup nests like everybody else?
Studies have shown that, on average, cavity nests are safer than open-cup nests. Although open-cup nests are sometimes very successful in producing fledglings, it's a bit of a lottery, and success rates are sometimes as low as 5 percent for a particular population. The average rate of success is probably around 40 to 50 percent. Cavity nests usually do better, especially for species that excavate their own cavities (often 80 to 90 percent successful). Birds that use pre-existing holes have nest success intermediate between that of excavators and open nesters - these holes are usually older, and perhaps in softer wood, and often closer to the ground than freshly excavated holes. In addition, the birds that depend on pre-existing holes generally have smaller bodies than excavators and may be less able to defend their nests.
If cavities, even pre-used ones, are good, why don't more bird use them? The down side of using a pre-existing cavity is that suitable cavities are limited in number. The housing market is tight and competition can be fierce. The ability to excavate is probably limited by existing adaptations of many birds-they may be so specialized to their own particular ways of foraging that it is very difficult to evolve chisel bills and excavating expertise. For instance, it is hard to imagine a duck or a heron, specialized for an aquatic existence, somehow becoming able to dig holes in trees and pry insects out of woody tunnels.
Although cavity nests are generally safer than open nests, cavity nesting is no guarantee that parents will raise chicks successfully. Twice I have observed (while canoeing in western Ontario) a pine marten raiding a flicker nest high in a tree, carrying off the chicks one by one, as the parents screamed and flapped helplessly nearby. Other tree-climbing members of the weasel family are also threats to cavity nesters.
Black bears can climb well and are known to prey on woodpecker nests. There is evidence that they often key in on the calls of the chicks in the nest. In some cases they may approach the nest opening and snap up any adults or large chicks that try to scramble out or they tear out the doorway altogether. And sometimes they rip new holes in the sides of nesting trees, right beside the lower part of the cavity, where young chicks are squeaking. The softer the wood, the more vulnerable the nest.
Mary F. Willson is a retired professor of ecology.