By Lee R. Franks
Birds and people are “sight animals.” For both, the eyes are the dominant sense organs, vastly more important than their inferior sense of smell. The reasons for our sensory similarity to birds can be found in human evolutionary history. At one point the ancestors of Homo sapiens were small, tree-dwelling primates. When leaping from limb to limb and snatching insect prey with the hands, sharp, binocular vision was very handy; those of our forebears that tried to smell the location of a branch on which to land were unlikely to survive to reproduce. And since in the breezy treetops odors quickly dissipate, they do not provide good clues for detecting food, enemies, or mates. Birds, flying higher and faster than primates leap, naturally also evolved sight as their major device for orienting to the world.
The term “hawk-eyed” accurately describes raptors like the Red-shouldered Hawk, Buteo lineatus. They have eyes designed for high “visual acuity.” There is, in fact, evidence that hawks can distinguish their prey at something like two or three times the distance that a human being can detect the same creature. One way they have attained such a high degree of acuity is by having relatively large eyes. But more than size alone appears to account for the astonishing performance of the eyes of hawks. 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. A large pupil and a highly curved cornea admit plenty of light to keep the image on the retina bright.
The Red-shouldered, a mid-sized hawk (19 in.) does most of its hunting from a perch. Sitting atop a fencepost, utility pole or tree, it waits patiently to detect snakes, mice, and frogs. They prefer riparian and oak woodlands, but also occupy eucalyptus groves and residential areas.
The pair that breeds at Edgewood nest in a large eucalyptus tree near the parking lot. The breeding season is about 150 days, beginning in February. They usually lay 3 eggs.
Distinguished by its “red” shoulder patches, black and white checkered flight feathers, heavily banded tail, and dark rufous chest with white, horizontal streaks. The sexes look alike, but the female is larger in size. Their tails, which are relatively long for a mid-sized hawk, are marked by several wide dark bars with intervening narrow white bars and a white tip. Wings appear 2-toned when viewed from below, with rufous inner feathers contrasting with black and white wing linings.
Flies with wings and tail outspread when circling above territory. When hunting in open, flies low and directly toward prey. Flies 6-15 meters high through trees below the canopy, often gliding or swooping up to nests. Leaves nest by dropping off to pick up speed and clear branches before flapping. The female roosts on nest until young are 3-4 weeks old. The male generally roosts several hundred meters away from nest. These birds are solitary or in pairs year-round. They are rarely found in flocks, even during migration.
The Red-shouldered Hawk is the most vocal raptor in our area. Adults and juveniles call at any time of year, but calling becomes more regular and frequent between November and May, peaking in the January to April period. Female vocalizations are noticeably lower-pitched than those of the male. While their vocal array consists of seven calls, the most common is the kee-aah, which has accent on the first syllable and a drawn-out second syllable with a downward inflection. This call is usually performed 5-10 times, followed by a 10-20 minute interval of silence. The call is used when announcing territory in early spring, up to start of incubation, after which adults become much more quiet. The call is also used as an alarm.
Red-shouldered Hawks maintain their territories and home ranges throughout the year and begin to pair in February. Courtship lasts about 3 weeks, with nest building/refurbishing starting before courtship ends. Egg laying takes place in March, with hatching occurring about 5 weeks later. The nesting period averages about 6-7 weeks with nest departure generally taking place in June. Their nests are composed mostly of live or dead sticks, dried leaves, strips of bark and lichens. The inner cavity is lined with finer shreds of inner bark, mosses and lichens. Pairs, not necessarily the same birds, may reuse the same nest for many years. The nest in the Park has been used for the past 4 years.
The Birds of North America, No 107, 1994; Scott T. Crocoll
The Birder’s Handbook; Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye
The pictures for this article depict the actual Red-shouldered Hawks that live at Edgewood. They were photographed by George Raiche and appear on his website, Digibird. There you will find a fascinating photo-history of their nesting activities from 2000 to 2003.
This article was originally published in the Edgewood Explorer, June 2004.