Birds of a feather flock together for a reason

Huge flocks of birds, the kind that darkens the sky, inspire awe and wonder. In March, for example, hundreds of thousands of sandhill cranes gather on Nebraska’s Platte River to rest on their way north. In May, similar numbers of shorebirds stop on the shores of Delaware Bay to refuel as they head north. And in the fall, flocks of ducks and geese stop at wildlife refuges across the country on their southbound journeys.

When alarmed, these flocks jump into the sky and twist and turn almost as a superorganism before setting down to resume feeding and resting. Last week, during the two coldest mornings of the young winter, a huge flock of starlings found my backyard. There were too many to count — maybe 1,000, maybe 5,000. But as they worked the yard, I couldn’t see any turf. They foraged shoulder to shoulder. And at the slightest disturbance, even my movement behind a pane of glass, they burst into flight as one, then quickly returned.

Starlings are usually gone by now, and that may have been a late pre-migratory gathering since I haven’t seen them since. Usually my starlings take off by mid-December and return in early March. But I’ve heard from a number of readers who have seen similar large flocks of starlings in recent weeks, so I’ve concluded their journey south was a bit delayed this year.

Such observations raise an interesting question:  Why do birds live in large groups, at least during certain times of the year?  

Flocks occur for many reasons; they are not random aggregations of birds. There must be advantages, such as safety in numbers, but there are compromises, as well.

Birds, such as the starlings in my yard or sandhill cranes in Nebraska, forage more efficiently in large flocks than they do as individuals. Food sources, be they patches of seeds or swarms of invertebrates, often occur in large, unpredictable, ephemeral patches. Many searchers find them more readily than solitary feeders. So members of a group typically have better access to food than individuals.

Furthermore, living in a large flock is safer than living alone. Though large groups are conspicuous and easy for predators to find, groups have many eyes and members are vigilant. They detect predators at greater distances and flee before a real threat materializes.

If a hawk or falcon attacks, groups are safer than individuals. Flocks under attack gather into tight formations and make incredibly precise swoops and turns. The result is that predators are confused; it becomes difficult to focus on one individual.  

Another reason evasive flocks form tight groups is that every individual tries to get to the center of the flock.  The center of a group is the safest position because predators usually attack individuals on the edge of a group. A flock member on the edge is easier to separate from the group.

Large roosting flocks enjoy similar advantages. Owls might visit huge roosting flocks of crow, robins, starlings and blackbirds, but they take only individual birds. So over the course of a winter a great horned owl might take 100 birds from a large roost, but if there are 50,000 birds in the roost, odds favor survival. And again the safest position in the roost is in the middle, so winter roosts tend to be highly concentrated. No individuals want to be on the edge. Typically older, more dominant birds take the safest interior spots, leaving younger, less dominant birds to the more vulnerable, peripheral positions. This results in constant bickering and competition for the best positions until all settle in for the night.

Another advantage to winter roosts is that flock members can huddle shoulder to shoulder on cold nights to reduce heat loss.

So next time you see a large flock of birds, know that there is method to their madness.

Cardinal follow-up

Since last week’s column, I’ve been inundated with e-mails and letters about cardinal numbers. Most report that cardinal numbers are down, but about 20 percent report normal or greater numbers. I’m forwarding the information on to Project FeederWatch, and if I ever come up with an explanation beyond normal population fluctuations, I’ll let you know.

X Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via e-mail to my Web site, http://scottshalaway.googlepages.com.