COOPER HAWK. Accipiter cooperi (Bonaparte)

Field characters.—Similar in all respects to Sharp-shinned Hark (which see), except that size is about double and end of tail is distinctly rounded (pl. 44g). Voice: Of adults a rather harsh kluk, kluk, kluk, kluk; of young a shrill quick, quick, quick, many times in rapid succession, and also a far-carrying swee'-ew or psee'-ur.

Occurrence.—Moderately common resident, chiefly in Upper Sonoran and Transition zones, on both slopes of Sierra Nevada. Partial to growths of tall trees in vicinity of streams. Observed up to 7700 feet (Dark Hole) on the west slope and to 8000 feet (Walker Lake) on the east side.

The Cooper Hawk is a larger replica of the Sharp-shinned Hawk in both form and structure, and it also closely resembles its smaller congener in habits. Its greater size enables it to prey upon larger birds such as quail and young grouse, but it is guilty of killing all manner of smaller birds as well, even down to those of the size of the Yellow Warbler.

In flight the Cooper Hawk exhibits the rounded wings and the relatively long tail characteristic of the bullet hawks (Accipiter and Astur), but the end of its tail is slightly rounded, a character which serves well to distinguish it from the Sharp-shin. (See pl. 44). It also indulges in more soaring and circling during flight than its smaller relative. From the Goshawk it differs in much smaller size as well as in its brown rather than gray effect of under surface.

As indicative of the stealthy nature of the Cooper Hawk, we recite our experience with a family of these birds. In the course of our field studies on the floor of the Yosemite Valley, we many times passed a dense stand of young yellow pines and black oaks situated between the foot of the Yosemite Falls trail and the Ahwahnee footbridge. We did not note anything there, however, except the usual assemblage of small songsters. But on the morning of July 25, 1915, 3 young Cooper Hawks were discovered in this thicket. Their characteristic calls drew our attention and we located the birds through finding a large amount of white excrement spattered about on the ground and shrubbery. This excrement, moreover, gave a decisive clue to the situation of the forsaken nest over head. The thicket of trees had been passed repeatedly during the preceding six weeks by members of our field party while searching for nests of small birds without our once catching sight of the old hawks, who must of course have been going to and fro many times a day.

The nest was about 60 feet up in a tall slender black oak growing in a dense thicket of oaks and pines about a hundred feet from a small meandering, willow-bordered stream in a meadow. In silhouette the nest could easily be mistaken for one of the many clumps of mistletoe which grew in several of the oaks in the vicinity. It was composed of sticks, and placed against the main trunk on smaller horizontal branches giving the needed support.

The three young hawks were perched about 30 feet above the ground in trees near the nest. Since their wing and tail feathers were not fully out of the sheaths, the birds could not have been long from the nest. Yet when frightened they were able to fly away far enough to hide more or less successfully in the forest.

Fig. 38. Pellets and other debris picked up from beneath nest of Cooper Hawk in Yosemite Valley, July 25, 1915. About 2/5 natural size. See text for analysis.

One of the young hawks was shot and upon examination was found to have in its gullet the scalp, eyes, brain, one kidney, and some other parts of an Allen Chipmunk (Eutamias senex), a Canadian Zone species, not known to occur anywhere on the floor of the Yosemite Valley, which is itself in the Transition Zone. On this same day one of the parent birds of this family was seen circling high overhead in the direction of Yosemite Falls and may then have been going to forage above the rim of the Valley. It is a well-known habit with this hawk, never to forage in the near vicinity of its nest, but to seek its prey far afield, presumably so as to avoid any risk of disclosing the location of its own brood. We may thus explain the apparent foraging of a pair of hawks in another life zone, while their nest was located in the zone to which the species characteristically belongs. The bird observed July 2, 1915, at Dark Hole, in the basin of the upper Yosemite Creek, a point well within the Canadian Zone, may well have been one of the pair nesting nearly 4000 feet lower, altitudinally, on the floor of Yosemite Valley.

On the ground below the nest in question we found a large amount of evidence relating to the food habits of the Cooper Hawk. Some of this material, comprising picked bones of victims, scattered feathers, and pellets of indigestible material regurgitated by the hawks, was preserved for subsequent detailed examination (fig. 38). The pellets we find to consist of feathers of birds, and skin and hair of mammals, all of which had been eaten along with the flesh of the victims. Later, when the processes of digestion had removed the meat, the residue had formed into dense pellets and had been disgorged. A great deal of this mass of material, of course, was in a condition to defy recognition; but the following species were identified, in each case to the extent indicated. Chipmunk: Much hair and some skin. Red-shafted Flicker: Single feather from breast. Sierra Grouse: A single, characteristically marked feather from a young bird; a Canadian Zone species, like the Allen Chipmunk mentioned above. Blue-fronted Jay: Bones of one wing with two typical feathers attached; also scattered feathers. Sacramento Spurred Towhee: One covert from the right wing of a juvenile bird. Western Tanager: Several feathers. California Yellow Warbler: Several feathers. Audubon Warbler: Several feathers from adult birds. Western Robin: One claw and part of a toe, the latter with a dark horny sheath indicative of an adult bird; also feathers from juvenile bird. There were also remains of June beetle, ladybird beetle, and of other insects, which may have been taken only incidentally because of their presence in the gullets or stomachs of the avian victims. These hawks are not known to hunt for insects.

Like other hawks the Cooper Hawk is often subjected to attack from kingbirds. At Pleasant Valley we saw one mobbed in flight by Western Kingbirds and Brewer Blackbirds until it took off in rapid retreat, and near Coulterville one seen flying across a cañon was harried by kingbirds until it was driven down close to the brush and there lost to sight.