The Teamwork Of Late Winter: Bird Flocks
Except when mating and nesting, bohemian and cedar waxwings are simply highly social birds. They seek food and water supplies large enough for the entire flock. They will sometimes dispatch a few birds at a time out of a larger flock to a food source, rather than allow a frenzied competition of individuals.
Late winter is a time of year when we're more likely to see birds of the same species in flocks, rather than alone or in pairs. This was most dramatically illustrated for me in late February, as I approached my hike up a neighborhood hill. I observed several massive cottonwood trees absolutely full of birds in a way I hadn't noticed before. Hundreds of Bohemian waxwings were chattering, indifferent to the comings and goings at the nearby school. Then suddenly, seemingly without cause, they took to the sky with a magnificent display of numbers and acrobatic skill, producing an effect more akin to a school of tropical fish.
Why do birds actually get together in large numbers, and why do some species do so more often than others? Answers to these questions are varied. One of the most obvious occasions for birds of a kind to flock together is migration, during which birds are able to use formations to fly with greater efficiency. Larger migration flocks may also be useful for deterring predators in flight.
Protecting nests and young may be one reason the magnificent great blue heron flocks communally on nesting grounds, or rookeries. On a Montana Natural History Center trip along the Clark Fork River, I observed several herons flying in and out of various nests in their rookery within a large ponderosa pine tree. Several birds fished the river, while others tended the home front. Defending their young may also explain the dozen or so violet-green swallows I observed in Missoula's North Hills swarming an American kestrel, which was resting a bit too close to their nesting sites.
Some species that summer in Montana, such as red-winged blackbirds and European starlings, may also flock together in winter to offer greater opportunities to find food. And Dustin Rubinstein, an evolutionary biologist at the University of California, Berkeley, argues that starlings may also group together to maximize breeding opportunities when food and rainfall are unpredictable.
These explanations for social behaviors in birds make sense, but why would some species take advantage of these behaviors, while others inhabiting the same areas do not? For some species, social behavior may bring elevated risks of disease, attract predators, or create greater competition for mates. It also seems to be the case, however, that some bird species, like other animal species, are simply more social - which brings me back to the waxwings.
Except when mating and nesting, bohemian and cedar waxwings are simply highly social birds. They seek food and water supplies large enough for the entire flock. They will sometimes dispatch a few birds at a time out of a larger flock to a food source, rather than allow a frenzied competition of individuals. Waxwings also behave more cooperatively than other species for water and bathing sources. And even more remarkably, they will sometimes pass berries from bird to bird down a line until a berry is finally swallowed. Finally, they not only eat together, but they may also become intoxicated together, indulging their tendency to gorge themselves on overripe berries. In short, waxwings seem to display characteristically more social and cooperative behaviors than most other bird species for reasons that are not readily explainable.
The waxwings’ extra measure of social proclivity may yet find an explanation in evolution. For now, however, I am content to wonder and be inspired that in the daily competition and struggle for survival among birds, we can find social behavior and even cooperation among the chattering waxwings of Montana’s cottonwoods.
"Field Notes" is produced by the Montana Natural History Center.