The birds are resident on a winter territory. Species such as chickadees, tit-mice, nuthatches, and woodpeckers defend territories during the winter as well as the breeding season. Although breeding territories are usually occupied by a single pair, in winter a group of birds may defend a territory. We used FeederWatch data to investigate variation in winter group size in the Black-capped Chickadee, a common feeder bird. Other population biologists, studying individually marked birds, found that winter flock sizes vary greatly, from an average of 8.05 chickadees per flock in western Massachusetts to 3.3 chickadees per flock in southern Illinois.
Larger flocks tend to occur in northern latitudes, or where supplemental food is offered.
Flocks tend to be smaller when a variety of parid species occur in the same area. The results are suggestive, but they're based on just a few studies, from a handful of localities. The data collected by Project FeederWatch offer a golden opportunity to examine whether these trends exist on a continentwide scale. We describe here the results of some preliminary analyses of variation in chickadee flock sizes. We looked at differences in group sizes over the course of one winter, from year to year, and from region to region.