Location is everything when it comes to the security of a bird nest. If a pair of birds can't protect their eggs and developing young from predators and severe weather, they stand to lose weeks of effort, massive energy reserves, and an opportunity to pass their genes on to the next generation. To protect their reproductive investment, birds build nests in a wide variety of locations, each specific to the species, and each with its own set of advantages and disadvantages.

Some birds hide their nests in the smallest branches, amid a tree's dense outer foliage. Such locations are well concealed and difficult for predators to detect, but they are also vulnerable to high winds and can't support the nests of larger, heavier species. Larger branches, lower in the tree, provide better support for nests, but usually have less foliage to conceal them and thus must be defended aggressively against predators. Nests built on or very near the ground are often well concealed, but are easily accessible to myriad ground-dwelling predators and may also be vulnerable to flooding.

Cavity nests, such as those excavated by woodpeckers, are perhaps the best-protected and safest nest sites of all. Though the entry holes are fairly easy for predators to spot, they are quite difficult for all but a few enemies to access. They are also well protected from severe weather conditions. Excavating these holes, however, requires tremendous effort and a set of characteristics unique to woodpeckers, including a sturdy skull, a chisel-like beak, strong leg bones, four powerful toes (two forward and two back) on each foot, and stiff tail feathers that help prop the bird against the tree.

Interestingly, the majority of birds that nest in tree cavities are not woodpeckers. Because most woodpecker species prefer to build a new nest each year -- a strategy that helps them avoid the invertebrate parasites that invariably infest nests of all types -- abandoned cavities become available every season. This provides an opportunity for dozens of species that biologists call secondary cavity nesters. These species have evolved reproductive strategies that allow them to take advantage of the security that cavities provide, even though they lack the specialized physical characteristics necessary to create these safe havens.