Bird species use a variety of different structures to help protect their eggs and growing young before they fledge. Many times when you think of a bird’s nest, you think of a structure in a tree made of sticks, moss, and various other plant materials. Several species though make their nests in cavities instead, whether it be natural holes in trees, excavated cavities from woodpeckers and cavities in river banks or other structures. Only about 12% of North American bird species are cavity nesters.
Many species will rely on natural rotten parts of a tree with softer wood to construct their nest, woodpeckers with their stronger bills don’t need to rely on this factor as much and may even chip away their cavity from living trees. Smaller species like chickadees can take up to almost two weeks to excavate their nests.
Not all cavity-nesting species construct their own nest, while species like woodpeckers will usually excavate their own nest with their chisel-like beaks, but other smaller species such as chickadee and nuthatches will also excavate their own nests. Secondary cavity nesters are species that will use natural tree cavities or those previously excavated by another species, species that do this include swallows and bluebirds. Overall, a wide variety of species are cavity nesters including woodpeckers, chickadees, kestrels, swallows and swifts, parrots, even some owls and waterfowl.
In addition, eggs or young are usually more protected in their cavities than open cup nest species, providing extra protection from not only potential predators but also from the rain, wind and extreme temperatures.
Some species use tactics to help add increased protection to their eggs and young. For example, the red-breasted nuthatch will smear sap from trees around the entrance to its nest to deter predators. Other species may smear bad-smelling insects near the entrance to deter predators also.
There are also species that will construct their own cavity. The cliff swallow is a good example of this. They will create a gourd shaped nest under overhangs of structures made entirely of mud they collect themselves. Mouthful by mouthful they create their own cavity nest with individual mud pellets, each nest needing about 900–1,200 individual pellets to complete their nest.
Unfortunately, due to the trend of removing dead trees and keeping yards looking ‘tidy’, it’s getting harder for cavity nesting species to find proper nesting locations and an increase in competition leading to increased territorialism. By allowing trees to stay and providing additional nest boxes for your local species, you can help your neighborhood birds find suitable nesting locations each spring.
An excellent source to see what cavity nesting birds are in your area with plans on creating nest boxes that can be made for them can be found at: https://nestwatch.org