By Jerry Sullivan
In my first 25 years of birding I saw exactly no Cooper's hawk nests. In the past week I've seen two. Sometimes you just get lucky. Or maybe we can all get lucky. We may be witnessing a modest comeback by a species that has long resided on the endangered list in Illinois.
The two nests were in Swallow Cliff Woods, a Cook County forest preserve near Palos Park, and in Miami Woods, another forest preserve in Morton Grove. This species has been reported from both locations in previous years, so the discovery wasn't a total surprise. What was unexpected was the opportunity to get a really close look.
This opportunity will be gone within a couple of weeks, as the emerging leaves conspire to conceal the treetop nests, but for now the birds are somewhat exposed. It surprised me that they seemed indifferent to this exposure. Most of my looks at Cooper's hawks have been either extended views of migrating birds passing high overhead or split-second glances at flying hawks disappearing rapidly into the depths of the woods.
Both of these recent sightings were major departures from that pattern. The first clue to the Miami Woods nest was a bird flying toward us, calling loudly--a rapid ki-ki-ki that all the authorities say is given only around the nest. This bird landed high in a tree not more than 100 feet away and just sat in full view while we got a long look at it.
I was with Judy Pollock, a birder from Evanston who is conducting a nesting survey at Miami Woods this year. We were laying out survey points for her when the Cooper's showed up. The call and the unusually tame behavior strongly suggested that we were near a nest. So we started scanning the treetops around us, and suddenly there it was--a shallow bowl of sticks in the crotch of a tree at least 30 feet up. And perched on the rim of the bowl was the other member of the Cooper's hawk pair. Based on comparative sizes, I would say the first bird we saw was a male and the bird on the nest was a female.
Hawks and owls both show a strong tendency toward size differences between the sexes, with the females consistently larger than the males. In Cooper's hawks and other members of the genus Accipiter the differences are quite large. Some evidence suggests that these differences could be ecologically significant. The small males might take smaller prey than the females and thus spread the pair's demands on the environment. You would suspect that there must be some significance to the size difference, since the pattern is so widespread in the two unrelated orders of raptorial birds.
At Swallow Cliff Woods Roger Keller from the Palos Restoration Project and I were leading a group of volunteers on a workday when we saw first one hawk, then a second, and finally the nest. This one was about 100 meters off the trail--the Miami Woods nest was no more than 50 feet from a heavily used bike trail. Both of these preserves get a lot of use by humans. Maybe the hawks are less skittish about people than they used to be. Certainly Cooper's hawks have had good reason to shy away from humans during the past couple hundred years.
Cooper's hawks, like the other members of their genus, are primarily bird eaters. They will eat pretty much anything they can catch, and the list of the animals they are known to have swallowed includes grasshoppers, crickets, frogs, snakes, and a rich assortment of mammals. They have even been known to grab minnows out of shallow streams. But birds are their bread and butter, and they will grab anything from a pheasant down to a warbler.
And the evidence is very strong that they love chickens. Why wouldn't they? Chickens offer a lot of meat, and they are typically penned up--a circumstance that would make them very easy to catch. Bent's Life Histories of North American Birds calls the Cooper's hawk a bloodthirsty villain, noting, "It is essentially the chicken hawk, so cordially hated by poultry farmers, and is the principal cause of the widespread antipathy toward hawks in general."
So Cooper's hawks have had a couple of centuries to make the connection between human beings and shotguns and to learn to be wary around people. Maybe 80 years in the forest preserves are changing that. Then again maybe not. We shouldn't push it. If you happen upon a nest, keep your distance. And after you have a good look, move on. The birds will get enough accidental disturbance; they don't need any deliberate botheration.
Accipiters--the others in North America are the smaller sharp-shinned hawk and the larger goshawk--hunt like cats. They lurk on concealed perches and capture their meal in a sudden rush. Their wings are short and broad, aerodynamically an ideal shape for quick starts. Their long, slender tails make excellent rudders, allowing them to make sharp, quick turns while chasing birds through the woods. Some authorities claim that Cooper's hawks don't hunt in the area right around their nest, so the songbirds in the immediate vicinity might be safer than they would be without the hawks.
As field marks, the combination of short, broad wings and long tail is distinctive. A Cooper's hawk high overhead looks quite different from a falcon, with its slender, pointed wings, or a soaring buteo, with its long, broad wings. Size can be a tricky mark for identifying birds in the field, especially with the size range between the sexes in accipiters. But if you see an accipiter about the size of a crow, you can be pretty safe in calling it a Cooper's.
By the way, the Cooper's hawk, Accipiter cooperii, was named by Charles Lucien Bonaparte based on skins sent to him by Charles Cooper, a New Yorker who, among other achievements, named and described the evening grosbeak in 1825. Bonaparte was a nephew of the emperor and became a noted American ornithologist. Bonaparte's gull is named after him. The 19th-century American ornithologists--with a whole continent of birds to name and describe--took care to immortalize one another as often as possible.
There is reason to believe that the Cooper's hawk and the sharp-shinned hawk are mutually intolerant. If a woodland contains a nest of one it will probably not have a nest of the other. Habitat preferences could affect this distribution as well. Cooper's hawks may prefer more open woodlands than the sharp-shinned. The decline of the Cooper's hawk in Illinois may be partly attributed to the loss of the open oak woodlands that were typical of the state at the time of settlement. This reading would suggest that the denser woods of today are not good Cooper's hawk habitat. Both the Miami Woods and Swallow Cliff Woods nest sites are in areas being restored to a more open condition. That could be important, although two pairs of birds are a slim base for drawing any major conclusions.
Pesticides also had an impact on Cooper's hawks. We have strong, nonanecdotal evidence of eggshell thinning of the sort created by DDT. The reduction in the amount of persistent pesticides in the environment is almost certainly helping Cooper's hawks survive.
There were probably no eggs in the nests we saw, but by the end of the month there should be some. Eggs are usually deposited at one- or two-day intervals, and the most common clutch size is four or five eggs. Most birds don't start incubating until the last egg is laid. That way all the eggs will hatch at approximately the same time. Both sexes share in incubation, which lasts about 24 days. After hatching it will be another five weeks or so before the young leave the nest. Even then, they will be slow, clumsy fliers, still dependent on their parents for food. The young will not be on their own until the end of summer. In the meantime, if we can be discreet in our attentions we can enjoy the progress of their nesting without disrupting it.