If you live in a temperate climate and pay attention to your local birds, you have probably noticed that their preferences for companionship change with the seasons. In spring, pairs stick close by each other and three’s a crowd—any third-wheel interloper is likely to be chased off in a flurry of angry wingbeats. But in fall and winter, the birds suddenly become community minded, travelling around in flocks of dozens of their fellows. In Chicago in the winter you can find trees liberally decorated with the round orange forms of fluffed-up American Robins, bearing more than a passing resemblance to Christmas tree ornaments. Even in the Bay Area, not generally known for its seasonal variation, huge flocks of quietly chirruping Dark-eyed Juncos make it clear that (mild, occasionally rainy) winter has arrived.
Why flock in winter? Or, why flock only in winter?
There are some big advantages to flocking. A constant concern for a small bird is keeping out of the way of the animals that would like to eat you, and being in a large group makes you less likely to be lunch in several ways. First, the more eyes around you, the larger the chance that someone will spot the hawk early and give you warning—and the less time you, personally, have to spend staring around, being vigilant. If you have ever watched a flock of Canada Geese grazing, you may have noticed that one or two geese hold sentry duty for a while, keeping their heads up and eyes out for danger, while everyone else feeds; periodically some other goose takes over, and the former sentries get to eat.
Having more eyes on the lookout might seem like a minor advantage, but in a harsh winter, when food is hard to find and your tiny bird body is using up fuel like crazy just to keep from freezing, every advantage makes a difference. 30% more time eating, rather than being vigilant, might mean the difference between survival and starvation.
Even in a flock, birds have to balance foraging with vigilance. Take these American Goldfinches eating seeds from a tree: they have to duck their head to get the seed out of the spiky seed pod, but as soon as the seed is free, they raise their head. They then look around while eating the seed.
The more other goldfinches are around, the longer every goldfinch can spend head-down in a seed pod.
If a predator still manages to sneak up on you, flocking gives you other advantages too. The more other individuals there are around you, the less likely that you will be the one who is too slow or too unlucky, and gets eaten: this is known as the predator dilution effect. Essentially, having lots of others around you dilutes the risk of being taken by a predator.
In addition, large groups of animals can be very difficult to hunt, as the complex motion of many individuals makes it hard to focus on one individual long enough to catch it. This is one reason why fish school: it’s much harder to pick one fish out of school than to catch a lone fish. Bird flocks enjoy a similar advantage, so being in a flock may reduce the chances of anybody being caught by a predator.
So why don’t birds live in flocks all the time? A few species do, breeding in flocks as well as spending the winter in flocks; but most species split off into pairs for the breeding season, suddenly becoming aggressive to any unwelcome intruders, although they would happily have flocked with them a month earlier.
Emlen (1952) imagines birds as being motivated by two opposing “forces,” an attractive one that drives them to seek each others’ company, and a repulsive one that drives them to defend territories and maintain what we would think of as “personal space.” He describes captive doves trying to settle down for the night:
Each bird… sought a perch close to friendly companions, but not too close, and the difficulties involved in satisfying both the appetite for companionship and the aversion for crowding often kept the birds busy for more than an hour.
Emlen’s two “forces,” though vague, are a good way to remember that there are both benefits and costs to flocking. In the reproductive season, the costs become considerably greater. Other birds of the same sex may try to steal away your mate, or secretly mate with them. Better to keep everyone away than risk raising babies that aren’t genetically your own.
Nesting sites are often limited and in high demand, so you don’t want to allow others near your own nest lest they try to take it.
And for most songbirds, the distribution of food changes from winter’s clumps of seeds and berries to more evenly-distributed insects. Just as a winter tree covered in berries encourages foragers to group together in that tree, a more even distribution of insects across the landscape encourages foragers to spread out.
These dramatic seasonal changes in the lives of (some) birds is one of their more foreign aspects, for us humans. Humans are remarkably aseasonal creatures: we can form romantic partnerships and have babies at any time of the year, and we don’t have different sets of “spring friends” and “winter friends.” Birds change from passionate one-on-one courtship and then diligent partnered parenting to a highly social—but entirely sexless—time of hungry companionship; we humans merely exchange our sandals for boots.
Emlen JT. 1952. Flocking behavior in birds. The Auk 69:160-170.