Birds of a Feather don't always flock together
By the end of August summer is drawing to a close. A hush comes upon the land. Most birds have stopped singing and the fall symphony of cicadas and katydids is just beginning. Insect singers are just emerging from their larval stages and the young birds are out of the nest. Family ties are broken; birds of different ages and species are forming wandering bands.
These vagabond bands include flocks of thousands of Red-winged Blackbirds and Common Grackles that move through the late summer and fall like giant vacuum cleaners. They will stay together in flocks throughout fall and winter.
Flocking up in winter is a common phenomenon. In October and November the winter visitors from the north arrive. Purple Finches form loose flocks with our resident House Finches. Red-breasted Nuthatches, kinglets, and Brown Creepers join flocks of resident chickadees and titmice. Cedar Waxwings will stay in flocks separate from other species. Pine Siskins will also arrive in and remain in discrete flocks. Masses of robins cover our yards, all seeming to stop by our feeders several times a day.
Why do birds do it?
There isn’t just one explanation. As with communal roosting at night, there are several related reasons. The advantages of joining a flock probably include the “many eyes effect” of detecting danger as well as the “learning effect” of taking advantage of what others have discovered or experienced. Flocking only in winter may also reflect both dietary differences between summer and winter and the inability or reduced opportunity to wander in flocks while caring for young.
In summer, most birds feed their young a high protein diet so that they will grow quickly. This means they must hunt for insects and in hunting, individual initiative is at a premium. In winter they can switch to high-energy food such as fruit or seeds — foods that are in widely dispersed but in large clumps which many pairs of eyes can locate more easily, sharing costs little, and risk to feeding alone and exposure in the open winter environment is minimized.
Seed and fruit eaters like Cedar Waxwings, for example, generally segregate into species-specific flocks. Most seed eaters are specialists on specific kinds of seeds and by joining a species-specific flock, relevant information is pooled.
Birds feeding on insects in the winter woods are an exception to the truism that “birds of a feather flock together.” All over the world insect-eating birds form conspicuous multi-species flocks.
This behavior raises two interesting questions. Why would birds solitary in summer dramatically change behavior in winter? And why would very different species of birds that feed on very different insects, (and almost never on seeds or berries,) follow each other around in winter only?
One theory for the insect-eaters is some birds act as beaters flushing prey for others. For example, woodpeckers on trunks of trees cause insects to fly from hiding places, making the insects available for capture. A second reason is safety in numbers. More eyes alert for predators like cats and hawks mean less individual attention diverted to vigilance and more to food finding.
In my woods, chickadees and titmice are the nuclei of roving multi-species flocks. They are always the most numerous, noisy, and conspicuous members of any flock, while inconspicuous Downy Woodpeckers, kinglets, Brown Creepers, White-breasted, Red-breasted, and Brown-headed Nuthatches will round out a flock.
There is little or no competition among the members of a multi-species flock of insect eaters because each species forages on different trees, different parts of the same tree, or different prey. And I doubt that the beater effect can be applied to winter flocks because cold insects are immobile and can’t be chased off a tree to become a meal for a flock member. That leaves the many-eyes hypothesis for predator protection as the most probable reason for the formation of winter flocks.
So why do birds flock in winter? I don’t know for sure, but I suspect flocking although possibly advantageous at any time of the year, is constrained in the summer when the birds are tied to a nest site. In winter, when a limited food supply becomes a factor for survival, flocking promotes survival, provided there is no competition for food. That is because it permits the birds to give almost constant attention to the search for food and little to vigilance for predators.
What do you think?