In most of North America, nesting behavior is common any time between April and July. You may not think of it as a field mark, but it can be helpful in the identification process, since it narrows the possibilities.
You can start by noticing the signs of nesting. Almost any bird holding food or nesting material is likely to have a nest nearby. A bird showing extreme agitation, such as a distraction display, is likely to be defending a nest. A bird carrying a fecal sac or an eggshell as it flies from a nest, and courtship feeding, raptors perched side by side, or other evidence of pair bonds can also suggest nesting.
Any indication of nesting rules out species that are only nonbreeding visitors in your area, and this greatly simplifies identification. For example, in most of the eastern United States, any shorebird engaged in a distraction display must be either a Killdeer or a Spotted Sandpiper.
Another benefit of noticing nesting behavior is that it makes habitat a much more reliable clue. Birds move freely through many different habitats for most of the year, but nesting birds have stricter habitat requirements. For example, several species of sparrow can be found nesting in most areas, but their chosen nesting habitats overlap very little.
The actual nest site can also be distinctive. In the case of the swallows shown above, only Tree and Violet-green nest in cavities. Simply recognizing the birds as swallows and seeing their interest in a birdhouse (and knowing which species nests in your area) can be enough to identify them.
Finally, most species place their nest in a specific setting — on the ground, on a horizontal branch, in an upright fork high in a tree. If you find a nest, its location will help identify the species.
Watching nesting behavior is fun and rewarding on its own. Understanding behavior and using it to inform the identification process will help take your birding to the next level.
This article from David Sibley’s “ID Toolkit” column appeared in the May/June 2014 issue of BirdWatching.