Naturally Curious: Making a Nest, Or Claiming One, In a Cavity
When excavating a nesting cavity, black-capped chickadees remove wood chips in their bill and scatter them away from the immediate area, so as not to alert predators. (Mary Holland photograph)
The waste of nestlings comes pre-packaged in “fecal sacs.” The parents keep the nest clean by removing these sacs by eating them or dropping them far from the nest. (Mary Holland photograph)
Both parents help feed their young. They take turns catching moths, spiders and caterpillars and bring them back to the nest hole for their young. (Mary Holland photograph)
This nestling is seconds away from fledging. Judging from the intensity of the look in its eyes, it could be an intimidating moment. (Mary Holland photograph)
There are roughly 85 species of cavity nesting birds in North America. Cavities are the safest nest sites for raising young in that they provide protection from predators as well as weather. This lessens the pressure on birds to raise their young as quickly as possible, and the young of cavity nesters leave their nest, or fledge, at a comparatively later stage of growth than young birds not raised in cavities. Rather than feeding their young the maximum amount of food so that they will grow as quickly as possible, hole nesters can use the same amount of food to feed more young at a slower rate. Thus, the size of their clutch is often larger than the average songbird’s. The cavity-nesting birds we see regularly have other characteristics in common, including the fact that most of them are insectivorous. They also are usually among the first species to lay eggs in the spring.
Some birds make their own holes and are referred to as primary cavity nesters. Woodpeckers (downy, hairy, pileated, yellow-bellied sapsucker, northern flicker) are responsible for most of these nest holes. Many more species of cavity-nesting birds are unequipped to excavate their own hole, and instead use natural cavities or holes that were made by primary cavity nesters. These birds are called secondary cavity nesters. They are at a distinct disadvantage from those birds that can make their own cavity, for there aren’t as many nest sites as there are mated birds, and there is severe competition within and between species for the finite number of cavities that do exist.
One adaptive strategy in the competition for these holes by secondary cavity nesters is to be a permanent resident, like the tufted titmouse and white-breasted nuthatch, so that you have the jump on returning migrants in the spring. Most secondary cavity nesters, however, are migratory, and their strategy is to arrive back as early as possible in the spring and claim a cavity before any other bird can do so. Thus, many of our secondary cavity nesters are among the earliest birds to arrive from the south in the spring — these include species such as tree swallows, American kestrels, eastern bluebirds, wood ducks and mergansers.
Upon observing the limited number of potential nesting holes, humans came up with the idea of providing additional cavities for birds in the form of nest boxes. The eastern bluebird population declined drastically in the mid-1900s due to a lack of habitat, pesticides and nest predation. This led to the creation of many Bluebird Trails all over the country that had nest boxes posted at close intervals all along the trails. These boxes have been readily accepted by bluebirds (and swallows) and this strategy has brought the widespread decline in the bluebird population to a halt.
The black-capped chickadee, another secondary cavity nester, is also an opportunist when it comes to nesting. It will use an old woodpecker hole, a natural cavity, excavate its own hole or even a nest box. Most often, however, a chickadee chooses to create its own cavity. One look at a chickadee’s small bill tells you that it is not capable of any serious drilling into healthy, living wood. In fact, it can make or enlarge a cavity only if it is located in soft, rotting, punky wood, which its bill can handle. If building their own nest, chickadees will find a dying tree or stump (usually birch, aspen or sugar maple) shortly before laying eggs, and begin excavation. After excavating and removing wood chips, but before the eggs are laid, the female builds a nest of soft material inside the cavity.
A plea to you from all cavity nesters — primary, secondary and all others — please leave your snags standing, and don’t harvest them. They are prime habitat for hole nesters and if for no other reason, the birds rid your woods of thousands of insects, including many forest pests.
Mary Holland is the author of “Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England” and “Milkweed Visitors.” She has a natural history blog which can be found at www.naturallycuriouswithmaryholland. wordpress.com.