As birds move from the non-breeding season and nesting season, I enjoy watching the changes in social behavior of the birds. In particular, I look for how birds react to other members of their own species.
Social behaviors are so varied that we will only consider the songbirds, variable enough in their own right. During the nesting season, most male songbirds establish and vigorously defend a territory. The only other birds allowed on the territory are female mates (and any visiting female that is looking to mate).
Territorial social behavior is worthwhile when resources, usually food, are at some intermediate level. There is no reason to devote energy to defending an area with an inadequate food supply. If food is superabundant, there is enough for everybody and again defending that resource makes no sense. That’s why you can see tons of birds at your feeders that you generously keep stocked.
With moderate food, a songbird pair can find enough food for them and their young but not enough to share. So vigorous defense is the right behavior. The male will defend a territory that is just big enough to provide sufficient food. Keeping other males out is an added benefit of the territory.
Outside the breeding season, flocking behavior frequently occurs. A flock may consist of just a few birds up to thousands. In Africa, the red-billed quelea (a sparrow) may occur in flocks of a million birds.
Flocking confers two advantages. A flock of birds has lots of eyes to detect predators. Vigilance against predators increases as flock size increases.
Flocking also increases food acquisition. In nature, the foods that birds prefer often occur in patches. A productive patch of winterberries is just the ticket for American robins and waxwings. An isolated birch tree bears enough seeds to provide a few meals for goldfinches and redpolls. Those patches will be found if there are enough birds looking.
Flocking and territoriality are not incompatible behaviors. During the winter a pair of black-capped chickadees will expand its territory from a couple of acres during nesting to 10-25 acres. These resident adults are joined by eight to 20 other black-capped chickadees. These birds are all juvenile birds and all are unrelated to the resident adults. The residents kick their kids out of the house in the summer only to have other chickadees’ kids descend on them.
These flocks are usually mixed-species flocks as other birds join the merry band, usually just one or two of each species. Common species are red-breasted and white-breasted nuthatches, tufted titmice, golden-crowned kinglets and downy woodpeckers.
Yet another social behavior, mate selection, occurs in these flocks. There are usually an equal number of male and female juvenile chickadees in each flock. Over the course of the winter, the young birds form pair-bonds.
In late spring, the flocks break up and the resident male will contract the territory to a smaller territory. The newly formed chickadee pairs will use a couple of acres of the now unoccupied winter territory to begin a family of their own.
You can see evidence of this switch-over at your bird feeders. All during the winter, you may see a dozen or more chickadees at your feeder. These are birds from one winter flock. Within a few weeks, you will see chickadees in ones or twos at your feeder. The winter flock has dissembled.
Our migratory nesting birds show a variety of social behaviors between nesting grounds and wintering grounds. In Jamaica, American redstarts and black-throated blue warblers maintain individual territories. Pairs do not migrate together so it is each bird for itself during the winter.
On the other hand, it is common to see chestnut-sided warblers, common yellowthroats and Cape May warblers in mixed-species flocks roaming widely, showing no territorial behavior. One trick to successful tropical birding is to listen for mixed-species flocks and get on them quickly before they move on.
Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu
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