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By Root 1258 0

species, because those species are almost never associated with each other in the absence of chickadees. I have never found creepers with woodpeckers, nuthatches with creepers, although all three are often alone. They either seek out and follow chickadees, or chickadees follow them, and the latter seems unlikely because a flock of chickadees cannot follow a half-dozen other species at the same time. Chickadees are also always by far the most numerous, the most noisy, and the most visually conspicuous members of any mixed-species flock. They make the most distinct “target.” It is hardly possible to miss a chickadee flock, but it is hard to find the often quiet, unobtrusive, and hidden brown creepers, nuthatches, kinglets, or the downy woodpecker or two that may be with them.

The insect-eating winter birds minimize competition among themselves because each species forages on different trees, different parts of the same tree, or different prey. However, I’m doubtful that the beater effect, which may appropriately apply in the summer or in tropical forest flocks of presumably nonbreeding birds, would apply for these winter groups. Frozen insects are immobile and will not be chased off to become a target to a flock member. That leaves the many-eyes hypothesis for predator protection as a reasonable alternative.

I suspect flocking is advantageous at any time of the year, but it 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 may also become evermore advantageous, provided there is no competition for food. That is because it permits the birds to pay more constant attention to searching for food, and less on vigilance for predators.

Kinglets in a mixed-species flock in winter (with downy woodpeckers, brown creepers, red-breasted nuthatches, and chickadees).

In the Maine winter woods I almost always find the golden-crowned kinglets in groups of two to five individuals. Despite their small flocks, the group cohesion in kinglets is remarkable. The Austrian ornithologist Ellen Thaler, studying captive kinglets near Innsbruck, found that the birds make special calls when approaching their sleeping place. These calls attract members of the troop of kinglets foraging together. A second assembly call draws the group into a cluster. Once together, the birds in the center of the cluster hunch their heads down into their shoulders with their bills pointing up. The birds at the edges tuck their heads back and to one side under their wing feathers. The group takes about twenty minutes to get into position in warm weather, but they bunch up in only five minutes when it is cold, although mated pairs and siblings always bunch up with each other in seconds. Apparently the kinglets recognize family members by voice and they are less inhibited to huddle with them than with strangers.

The average number of kinglets per winter troop near my cabin in Maine is probably too small for either the many-eyes or the selfish-herd hypothesis to apply. But if huddling is necessary for surviving cold nights, as can be deduced from physiological studies showing considerable energy savings of two birds huddling together, then a couple may be enough. However, body warmers are not likely to appear magically at dusk if each bird is moving at breakneck speed all day through the dense woods, looking for the nearly invisible caterpillars it likes to eat. Attracting and keeping vocal contact with others throughout the day may be a key component of their winter survival, especially in dense coniferous woods where kinglets are not only rare but almost invisible. The availability of body warmers at dusk cannot be left to chance; losing only one or more members per troop might doom the rest to freezing to death on some cold nights, especially after a day of poor foraging. If so, then it is no surprise that no kinglet winter flock, even a tiny one, is ever silent for more than several seconds at a time. The birds try to keep in contact. If they should get separated from