By Lee Franks
The Cooper’s Hawk, Accipiter cooperii, a crow-sized (avg. length, 14”-20”, wing span, 29”-37”) woodland raptor, is a secretive, inconspicuous species. Generally, this bird inhabits deep woods, utilizing thick cover for both hunting and nesting. It has short, powerful, rounded wings and a relatively long tail that ensures maneuverability in dense woodland areas. It is superbly adapted for quick pursuit of woodland birds and mammals capturing a variety of prey, but mainly jays, robins, and squirrels.
They are considered residents within California, but migrants from the north substantially increase the population during the winter months. Also, birds from the northern reaches of the state drift southward, joining the migratory flow and increase the fall populations in central and southern California. Fall migration occurs from mid-September to mid-October. Spring migration is protracted and widespread.
Female Cooper’s are about one-third larger than males. As adults, both sexes are similar in plumage, but males, on average, are more brightly colored. The upper parts of the male are a dark bluish-gray, whereas, on the female they are brownish without the bluish cast. The under parts of the male are a reddish brown barring and on the female the barring is brownish without the reddish cast. Both sexes have a dark blackish-blue gray crown with cinnamon tips on the forehead, however the coloration on the male is more pronounced. Their eyes are red. Cooper’s wear juvenile plumage for about a year, from the time they leave the nest until the following summer. Juveniles differ from adults in having dark brown upper parts and pale buff under parts. Their breast is narrowly and sharply striped with blackish-brown on a white background. Their eyes are yellow.
The best field mark to use with this bird is the dark bluish-gray crown, as it contrasts nicely with a much lighter colored nape and upper back. Also, their tail feathers have four straight alternating bands of dark and light brown, along with a rather wide white stripe at the tip.
The breeding season begins in late March to April and extends through May and June. The male selects the nest site and does most of the nest building. They prefer to nest in extensive forest areas with trees at least 50-60 feet tall, rather than in isolated trees. Typically, the nest is built of sticks with a “cup” that is lined with bark flakes, and is placed in a main crotch or on a horizontal limb against the trunk. Males vigorously defend an area approximately 100 ft. in diameter around the nest site, although they may forage up to 2 miles away. Individuals occasionally use the same nest in successive years, but typically build a new nest in the area.
Only one brood is raised each year. The normal clutch size is 4-5 eggs. Egg laying occurs at 2 or 3 day intervals, usually in the morning. Incubation begins after the third egg is laid, and continues for 34-36 days. Most day time and all night time incubating is done by the female.
Hatchlings, weighing about 0.1 ounces and measuring 4 inches in length, are completely covered with white natal down. The female begins brooding immediately after hatching, ending when the young are 14 days old. The hatchlings growth is slow for the first 3 days, then rapid until day 23-24. They are fed directly by the female until they are about 20 days old and able to dismember prey. The male delivers prey to the female at a nearby perch and she brings food to the nest. If the female is absent, the male delivers to the nest but does not feed the young. The young lose their natal down and acquire juvenile feathering at 30 days, and depart the nest. After leaving the nest they can range 150-300 yards from the nest, but they return for prey deliveries and roosting for at least 10 days. The parents continue to bring food to the nesting area until the young are 8 weeks old and have learned to forage on their own.
A pair of Cooper’s nested in Edgewood in 2001 in a 70-foot-high tree, approximately 60 feet off the Sylvan Loop Exercise Trail. The tree is located 35 paces downhill from the 0.5 mile marker at a bearing of 28 degrees. The nest was built in late March and egg laying began early in the second week of April. Over the course of 10-12 days four eggs were laid. The male made repeated use of a certain tree off the Ridgeview Loop Trail, approximately 1.5 miles away, as a perch for placing prey (birds such as jays and flickers) for delivery to the nest site. The eggs hatched and the four young successfully fledged around Jun 24-25. They continued to return to the nest site until the end of July. the last observation of this family was made on August 16, 2001 when one of the young was caught in the act of hunting Stellar’s Jays unsuccessfully in the Day Camp area.
Feeding. The term “hawkeyed” accurately describes this bird. It can distinguish prey at something like 2 or 3 times the distance that we can detect the same creature. They have relatively large eyes. Evolution has arranged the structure of their eyes so that each eye functions very much like a telescope. The eye has a somewhat flattened lens placed rather far from the retina, giving it a long focal length which produces a large image.
Typically the bird relies on concealment and uses a series of brief perch-and-scan episodes to locate prey, but also flies close to the ground, using bushes to shield its approach. A sudden burst of speed is the usual pursuit when hunting from a perch. Occasionally it pursues prey on foot. Cooper’s that have been used for falconry employ an “attack and strike“ strategy which involves ceasing wing flapping about 10 feet from the prey, and about 4 feet from contact begin swinging the legs forward. At contact, the hawk sets its wings in a breaking position, seizing the prey with both feet. The hawk responds to movement of the captured prey by strongly grasping it, then relaxing grip, and then clamping down again. If near water, it will drown its prey, holding it under water until it ceases to move.
Sounds. Vocalizations are probably the primary means of communication, because pairs restrict their activities to relatively dense woodland vegetation where visual contact is limited. They are silent much of the year, but fairly vocal during breeding season. The female has a large repertoire of calls (short, simple vocalizations differing from songs), which is attributed to a greater need to convey more information, as they apparently control male-female interactions. Listen to some different calls here.
Flight. The usual flight behavior is several rapid wingbeats alternated with brief glides, actively using its tail for maneuvering, but much faster and more maneuverable when attacking prey. Cooper’s often fly close to the ground or below tree canopy when hunting of approaching/departing the nest, but may carry prey at altitudes of 300-400 feet. The bird will soar frequently. Flying birds use both legs to tuck prey to their belly, as they are often mobbed by smaller birds. Here’s a video of a Cooper’s Hawk in flight.
Sociality. Outside of the breeding season, the hawk is solitary. The sexes roost apart in the pre-incubation stage of nesting. The birds are monogamous, and some pairs re-mate while others are known to have new mates in subsequent years. The male leaves the nesting area to hunt for himself and his mate, but nevertheless remains near the nest and the female about 80% of the day in a guarding mode.
Mortality. Mortality is quite high during the bird’s first winter, approaching 78% as opposed to only 34% per year for adults 2-8 years old. The maximum reported age of a Cooper’s is 12 years. Predation is probably the most widespread cause of death. The principal predators are other raptors and the Great Horned Owl.
Juvenile birds are also prone to injury during their first year. On October 6, 2001, Wildlife Rescue, Inc. personnel released two Cooper’s back into the wild using Edgewood Park and Preserve. Both birds, a second-year male and a hatch-year female sustained concussions as a result of flying into windows. The female was in rehab for 2 months, and the male 18 months. During that time Wildlife Rescue, Inc. provided medical and support care in order to give these birds a second chance at surviving in the wild. Their personnel go to great lengths during rehab to reduce the chances of taming or habitualizing them with too much human interaction.
The October 6 release was only partially successful in that the male, who was in captivity the longest, flew into another house window in Redwood City on October 11. When he re-entered rehab his weight was down 10 grams (5%) from his release weight. Probably because he had spent 18 months in captivity where he was not exposed to live food, his hunting skills deteriorated. For this reason, Wildlife Rescue, Inc. decided to place him with a falconer to see if he can be given hunting training with supportive care. Hopefully he can one day again be released into the wild and make a contribution to the survival of his species.
The Birds of North America, R.N. Rosenfield and J. Bielefeldt: The Birders Handbook, P.R. Ehrlich, D.S. Dobkin, D. Wheye.
This page at digibird has some terrific photos of Cooper’s Hawk adults and nestlings that were photographed at Edgewood in 2001.