While I was walking a wooded area near a Michigan pond one spring, the forest floor stopped me cold. It was carpeted with White-throated Sparrows — the largest sparrow flock I had ever seen — that apparently had dropped in overnight. The birds were foraging and affirmed the old adage, “Birds of a feather flock together.”
Part of a bird’s feeding time is spent looking around and being alert for predators. A single bird must balance the two critical activities and is vulnerable to a sneak attack. If a second or third bird joins the first, and all three randomly check for predators, the total amount of time watching for threats increases significantly. It doesn’t take many individuals grouped together to ensure at least one of them is keeping watch almost all of the time. And when a bird detects a predator, its call alerts the entire flock.
It’s easy to see why flocks form. Not only does feeding in a flock allow more time for each bird to feed (and less individual time looking for predators), a flock will discover more food sites than an individual can.
White-breasted Nuthatches, for example, are often part of mixed flocks that include Tufted Titmice and Black-capped Chickadees, which seem to be flock leaders. In an experiment in which titmice and chickadees were removed by trapping, the nuthatches increased their vigilance for predators and reduced their feeding time, leading to decreased vigor.
As another testament to the value of flocking, flock sizes are larger when food is scarce and/or opportunities to forage are limited. During winter in the high latitudes of Norway, temperatures are low and daylight hours few. Willow Tits (look-a-like relatives of our chickadees) forage almost constantly to consume enough food (up to 10 percent of their body weight) to make it through the night. Feeding squabbles, commonly seen in large flock behavior, are almost non-existent during the extreme conditions.
The benefit of flocking to individual birds, however, can vary with social status. Adult male White-crowned Sparrows are dominant over females and younger birds of both sexes. In a study where food was placed at varying distances from cover, adult males utilized the sites closest to cover. If subordinates were using the sites, the males would displace them or otherwise interfere with their feeding. Subordinates were willing to feed at sites more distant from cover, where the risk of predation was greater but the likelihood of negative social interaction was less.
A study of Laughing Gulls in South Africa compared the reaction time for detecting an approaching “predator” (a hawk model on a wire) to flock size. In general, larger flocks detected the predator earlier, but when feeding time was considered, an optimum flock size was revealed. Small flocks of less than four birds detected the predator more quickly than did birds of larger flocks, but the birds in small flocks were more skittish and uneasy, and consequently, spent less time feeding. Large flocks of more than 15 birds had a slower reaction time to the predator, probably because they were having more feeding squabbles and were generally less attentive. Similarly, shorebirds in California were at less risk from a falcon attack when in medium-size flocks.
Flocks in flight can also deter predation. Peregrine Falcons attack single birds and will take a bird from the edge of a loose flock. If a Peregrine stoops on a loose flock of starlings, however, the flock tightens and loose singles from the edge squeeze into a solid oval. The likely confused falcon usually veers off with empty talons. Starlings, when threatened, display amazing flight patterns, such as tightening and expanding a flock or splitting or merging flocks, sometimes with artistic grace. (For an appreciation of the behavior, search for “starling flocks” on YouTube.)
One of the most elusive questions about flocking is how birds communicate within a flock. Anyone who has watched a flock of acrobatic shorebirds twist and turn in flight has almost certainly wondered about the phenomenon. The dark backs of birds in a flock change to white in an instant as the birds tilt and bank, showing their underparts. To the human eye, the changes are simultaneous — all birds turning at the same time. So how do they know when to turn? Which bird or birds turn first?
By using a series of high-speed cameras, researchers in Italy have tracked the movements of hundreds of individual starlings within a flock and analyzed the data with sophisticated software that creates a three-dimensional flock structure. This permits determination of spatial relationships between individuals. Each starling pays attention to the movements of six or seven neighboring birds — not necessarily the closest ones. The number of flock members tracked by an individual may be related to its cognitive ability, as laboratory studies have shown that pigeons can discriminate between up to six different objects. A few birds within the flock initiate movement, which spreads through the flock like a wave and in a fraction of a second.
Flocking is a widespread behavior that provides important survival benefits for birds. While the overall goal is the same for all species, the nuances and particular aspects are as varied as the number of species involved, offering further evidence of the amazing biology of birds.
Eldon Greij’s column “Amazing Birds” appears in every issue of BirdWatching magazine. This article appeared in the August 2017 issue. Eldon is professor emeritus of biology at Hope College, located in Holland, Michigan, and the founder of Birder’s World magazine.
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