HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

24. Window on Bird Politics

171 24 Window on Bird Politics B usy birds come and go, helter-skelter, about my pile of sun- flower seeds. One comes to expect this when birds have been conditioned by easy calories. When I watch casually, this is all I see. But when I critically examine the scene, I discover that my bird feeder serves as a window into a subtle world of dictators, parasites, and liars. Read this chapter on a cold winter day, feet propped up and hot chocolate at the ready, overlooking your stocked bird feeder. You will find your backyard birds engaged in intriguing politics. In the winter, my feeder attracts single-species flocks of northern juncos, American goldfinches, evening grosbeaks, and purple finches, and mixed-species flocks featuring black-capped chickadees, tufted titmice, white-breasted nuthatches, and occasionally downy woodpeckers and brown creepers. A mixed-species flock includes individuals of two or more species. Although pairs of cardinals and of rufous-sided towhees occasionally visit, flocks bring most of my customers . The two reasons birds form into flocks — greater protection against predators and higher feeding efficiency — provide insight into the action at a feeder. Let us take a look at each. Many of us have seen on television a film in which a lioness fixates on and then attacks an isolated zebra, the one that strayed from the herd or behaved differently. The pruning of marginal individuals favors the evolution of centripetal instincts. For this reason, flocks of small birds tighten in the presence of a raptor. This point was driven home not long ago when I saw a flock of starlings flying in a highly synchronized formation. Based on their 172 tightness, I thought a predator might be nearby. Sure enough, a few seconds later, a sharp-shinned hawk glided into view. Under my breath, I hoped a stray would trigger a stoop, but on that day the squadron was disciplined. In many bird species, when one member of a flock sees an aerial predator, it gives a special alarm. Chickadees and titmice respond to the cry by diving into bushes and remaining quiet for several minutes; downy woodpeckers and white-breasted nuthatches simply freeze on the tree trunk. Regardless of the type of response, alarm calls present us with an interesting evolutionary problem. Some flocks include both genetically related individuals and unrelated members of the same species. In such flocks, alarm calls alert not only genetic relatives , which would make the call explainable by kin selection, but also are heard by unrelated members of the same species. Thus, a warning caller could increase the fitness of unrelated flock mates. This would represent genetic altruism, reproductive selflessness, a phenomenon unexplainable by natural selection. How then do we account for alarm calls? Perhaps the individual that sees the hawk and sounds the alert is manipulating its flock mates into heading for cover even though the responders do not know the predator’s position. Then the caller could head for the far side of the flock. Thus, straightforward natural selection may have favored the behavior of alarm calling because the caller increases his own chance of survival to the detriment of its flock mates. On the other hand, we know that some flocks reflect the action of kin selection. During winter, black-capped chickadees and tufted titmice, the nucleus of natural mixed-species flocks, forage with their mates and offspring. The high pitched alarm calls of chickadees and titmice are ventriloquistic, making the caller hard to locate. These observations suggest that chickadees and titmice may benefit from kin selection. The great tit has added an intriguing layer to this story In Denmark in the winter, great tits give alarm calls without a predator being nearby, especially when food is scarce. Hearing this false alarm Hollows, Peepers, and Highlanders 173 call, a bird leaves its feeding site for shelter, giving the manipulator access to the food. In this population, all great tits give false alarm calls — each individual is both manipulator and manipulatee. I have long wondered if this is happening at my feeder. Several times per day, even though I see no predator, the juncos, sparrows, and other ground-feeding birds explode into the air and fly away. It is not unusual for one or two individuals to remain on the spot and continue feeding. Cheating has its limits, though, because false alarm calls are effective only...