FLOCKING BIRDS

A question that is often asked is why birds flock. And why do some of them form mixed flocks?

It is certain that birds flock for evolutionarily sound reasons:  to avoid predation and to find and capture prey.

By flocking, birds have a better chance to avoid predators than when they are by themselves. First, with more birds together, there is a higher level of alertness, as individuals are likely to be looking in different directions, and not all of them will have their heads down at the same time. In fact, it has been found that birds spend more time feeding and less time looking around when in flocks. Some shorebirds that feed by constantly probing the substrate, for example godwits, may flock with other species that forage with heads up, for example curlews, for the added vigilance.

Second, when birds fly in flocks, they make it more difficult for aerial predators such as falcons to catch them, because the falcon has to concentrate on a single bird, while a group promotes confusion. Furthermore, a bird in a flock of 20 has only a one in 20 chance of being caught, whereas a bird by itself has a statistically more serious problem. When a Bald Eagle flies over a flock of coots, they scatter in all directions.

Third, birds in flocks can actually intimidate predators. Starlings form a swirling superentity and dive on any hawk that approaches them; the tactic works quite well.

There are other advantages. Birds may discover food by associating in groups. Vultures and gulls roost in flocks, then spread out to feed while keeping a distant eye on one another. A vulture that drops to the ground is at the center of a contracting circle of birds heading in its direction. Gulls and terns have been attracted from boats by a handkerchief thrown in the air, simulating a bird dropping to the water.

Birds may also flock to capture prey more effectively. Pelicans in a line drive fish in front of them. Auklets surround a herring school and control its movements just as border collies herd a flock of sheep (why don’t falcons do this with shorebirds?).

Presumably birds form mixed-species flocks for the same reasons. If a flock of 100 is more effective than one half its size, then it makes sense for 50 blackbirds and 50 starlings to forage and fly together. For the most part, birds of similar size and habits flock together, so you’re unlikely to see a mixed flock of murres and juncos.

One of the most commonly seen mixed flocks is a winter feeding flock. In this area, it usually contains chickadees of one or more species, often Red-breasted Nuthatches, and sometimes Golden-crowned and/or Ruby-crowned Kinglets. The flock may be joined by a Downy Woodpecker or Brown Creeper or Townsend’s Warbler or Hutton’s Vireo. By moving through the woods together, these birds may help one another spot particularly good feeding areas, and they are surely more alert to predators as a group than if they were foraging individually.

I wonder if it’s possible to explain any of our own social behavior by this reasoning. Or can we explain bird behavior by what we know of our own? Perhaps some birds flock just to check out members of the opposite sex for the next breeding season.

Dennis Paulson

A COOT IS NOT A DUCK!

Next time you’re at the water, whether salt or fresh, watch for a ducklike bird with a gray body, darker head and neck, and white, chickenlike bill. You’re seeing an American Coot (Fulica americana). Coots are often thought to be waterfowl (i.e., a duck), but they are not. They are in the rail family, in the same avian order as cranes. These birds are only distantly related to waterfowl.

Coots are nonetheless interesting, even though they’re not ducks. They are rails that look like ducks because, over evolutionary time, they have come out of the marshes where rails are common and have become adapted morphologically and behaviorally for living a duck’s life. They swim like a duck and dive like a duck, but admittedly they don’t quack like a duck! The chickenlike bill is quite different from that of a duck. Ducks have sieve-like lamellae on the edges of their bill to allow a sort of filter-feeding, while coots just grab their prey items and swallow them.

To be able to swim well, coots have evolved lobed toes, much like those of grebes. Gallinules and moorhens, which are also rails and look much like coots, haven’t evolved the lobes, and they are rather intermediate between rails and coots, able to swim with their long toes even though they aren’t webbed or even lobed. Coots forage at the surface, dip below it somewhat as ducks do, and dive underwater in deeper water. They don't stay down very long, popping up like a cork after a brief visit to the nearest vegetation.

Coots breed locally in the Puget Sound area but more commonly in the dry interior, where they are on most freshwater wetlands. They prefer ponds and lakes with both dense marsh vegetation, from which they get material to construct their nests, and plenty of open water, where they feed. They migrate to larger lakes in the winter, where they form flocks, sometimes large ones. Poor fliers, they migrate at night, probably to avoid predation by bird-eating hawks.

Coots are much more tied to a herbivorous diet than are rails. Their principal foods in fresh water are pondweeds, algae, sedges, and grasses, although a wide variety of other plants are taken. Filamentous submergent plants seem to be favored overall. They also eat small numbers of freshwater invertebrates, especially in the breeding season. The young are fed almost entirely on animal matter. When seen in small numbers on salt water, they may be utilizing green algae such as sea lettuce.

Coots also leave the water to graze on land, and they are commonly seen doing so adjacent to city park lakes. They retain some of their rail heritage in being better walkers than ducks, and when disturbed, they can run over land. They are much less well adapted to flight than ducks are, and they have to make long take-off runs to get off the water, and then fly awkwardly. When Bald Eagles harass them, they cannot escape by flying, and a pair of eagles can tire out a diving coot fairly quickly and then share the meal.

Coots are feisty birds, very territorial in breeding season and inclined to chase just about any other birds of their own or other species away from their nesting areas. They display with both their white bill and white undertail coverts, then they may fight fiercely, locking feet and pecking each other. They do manage to get along with their mates, and they make big, sloppy floating nests out of marsh vegetation. They lay a lot of eggs, a typical clutch size being around 7 but often up to 10 or more. The young are semiprecocial, able to get around on their own but still having to be fed by the adults. They are strikingly colored in comparison with the drab adults.

Dennis Paulson