Goshawks are one of the few good things about November. They move South at this time of year from their summer range in our northern forests. The numbers reported around Chicago fluctuate from year to year. Some years a good part of the population seems to come south to our comparatively balmy climes. Other years we see few birds.
The year-to-year differences are created by the cyclic rise and fall in the populations of their principal food sources in the north woods. The snowshoe hare is the most important of those, though goshawks will readily eat any bird or mammal that falls within their preferred size range and in summer will even eat a few insects.
Their preferred size range usually leaves out the smallest possible prey: mice, shrews, wood warblers, probably because animals that small don't provide enough calories to recoup the energy expended in capturing them. At the high end they go after almost any mammal smaller than a fox, eating weasels, woodchucks, and squirrels, as well as rabbits and hares.
Their common name means goose hawk, which is perhaps a slight overstatement of their hunting prowess--though a skilled and experienced goshawk might occasionally nail a goose. Their favored prey among birds runs from ducks, pheasants, and grouse at the large end to blue jays and blackbirds at the small end.
The cycle of the snowshoe-hare population also exerts a major influence on lynxes. The numbers of these northern cats rise and fall in a cycle that exactly duplicates the hare's cycle but lags about a year behind it. Common sense says that predators control the populations of their prey, but the lynx-hare cycle demonstrates that numbers of prey species also control the numbers of predators. The two categories of animals are actually involved in a dialectical process in which each affects the other.
I added the goshawk to my life list on January 14, 1982, at the Chevalier Forest Preserve, which is along the Des Plaines River just south of the Kennedy Expressway. The Audubon hotline had reported a sighting of an immature bird there, and on a cold gray morning I went in search of it. A couple of inches of snow covered the ground, and the sky was spitting a few more flakes when I arrived.
Adult goshawks are plumed in shades of gray and white, but the immature birds are brown backed with tan breasts streaked with darker brown. I never expect to find any bird I go looking for, but that day I was in luck. I found the hawk perched on the topmost branch of a tree that formed part of the edge between a patch of woods and an open meadow. I took a long look through my binoculars and an even longer look through my spotting scope, and all that time the bird sat, staring at me with its baleful yellow eyes (if you want to know what "baleful" means, just get stared at by a goshawk).
And then it took off, flying not away from me--as almost any other bird would have done--but toward me. It launched itself from its high perch and glided down, passing directly over me, not more than 15 feet above my head. This was plainly an animal not accustomed to showing deference to anyone.
Goshawks are big. A large female can weigh more than three pounds. Large males are slightly smaller. Three pounds may not sound like much--a good-sized house cat weighs 10 to 15. But dynamite comes in small packages. Even golden eagles don't outweigh house cats, but in a fight with an eagle who would bet on a house cat?
Certainly goshawks look big. Sitting in a treetop perch, a goshawk looks beefy, barrel chested--implacable, as Faulkner used to say. Certainly implacable. Social creatures like humans and dogs have submissive behaviors available. If we give up in the correct way our adversary may kick us once in a while, but he will let us live. Hawks have no such behavior. They not only won't give up. They can't.
I mentioned that female goshawks are significantly larger than males. This is a common feature of birds of prey, found both among owls and among the diurnal raptores--hawks, eagles, and falcons. Hawks and owls are not even distantly related, so the trait must have evolved independently in the two groups. The size difference is most marked in species that live by active hunting. Scavenging species show little or no size difference between the sexes.
Some scientists have suggested that the size difference produces a diet difference. The females might seek larger prey than the males, and this would spread the effects of predation among more species, reducing its impact on any particular one. This is a nice idea, but studies of the eating habits of hawks don't support it.
A more likely explanation focuses on the aggressive nature of these birds. Fierce raptors like the goshawk tend to look on anything smaller than they are as something to eat. The males have to be somewhat more aggressive than the females, since they establish and defend territories, so it may be that larger, more powerful females are required to suppress the male's urge to kill and allow pair formation and successful parenting.
What about the female's urge to kill? Well, males bringing food to a nest often act scared. They fly in, drop the food, and get away as quickly as possible. Attempts to breed the European sparrow hawk (its scientific name is Accipiter nisus, the goshawk's is Accipiter gentilis) in captivity often fail, because if you put the male and female in an enclosure together she is likely to kill and eat him.
Accipiter is one of the largest genera of diurnal birds of prey, with about 50 species worldwide. We have only three species in North America. The goshawk is the largest. The sharp-shinned hawk, which is about the size of a blue jay, is the smallest, and the Cooper's hawk falls in the middle. All three of these birds were reported from the Chicago Botanic Garden last week.
Birders often have a hard time distinguishing between the sharp-shinned and Cooper's hawks because of the size differences between the sexes. A large female sharp-shin is very close to the size of a small male Cooper' s.
All three species share the general accipiter body plan. They are long-tailed birds with short, broad wings. The wing shape is very much like that of gallinaceous birds--grouse, pheasants, chickens, quail--and gives the birds an explosive start. The long tail is a rudder; it gives accipiters a high measure of maneuverability, a very useful trait for birds that often hunt in dense woods.
The goshawk lives in northern forests in Asia, Europe, and North America, but the Cooper's and sharp-shinned hawks are both exclusively New World birds. Sharp-shins nest in both North and South America, but the Cooper's is strictly a North American bird. In fact, it is almost entirely a U.S. bird. Its nesting range extends only a little way into Canada and Mexico.
Back in the days when free-range chickens were the only kind of chickens there were, the Cooper's hawk was mercilessly persecuted by farmers. It was the classic chicken hawk. It had been eating grouse and prairie chickens for millennia, and the sight of a yard full of fat, slow, stupid birds would have been awfully tempting.
That persecution, combined with habitat loss, has made the Cooper's hawk a rare bird over much of its range. It is on the endangered list in Illinois. The forest preserves of Cook County provide a refuge for it--as they do for so many of our endangered animals--and several pairs nested here this year.
The Cooper's hawk was a bird of the oak savannas in Illinois, and that habitat is nearly extinct in the state. Cooper's hawks do not tolerate the presence of sharp-shinned hawks near their nesting and hunting grounds. This intolerance, presumably combined with a shortage of good habitat, made the sharp-shinned hawk an uncommon nester in Illinois even before the destruction of most of our native landscape.
Maybe we should declare the Cooper's hawk our state bird. The cardinal currently holds that honor; it is certainly cute, but until well into this century it was confined to the southern half of the state. The Cooper's hawk has always lived all over Illinois. Besides, all sorts of states have the cardinal as a state bird, but nobody has the Cooper's hawk. Traditionally only countries get to have big, fierce birds as their emblems, but maybe as a symbol of federalism we could allow states to be mean too.