HAWKS, EAGLES

AND ALLIES

(Family ACCIPITRIDAE)

 

Members of this family, collectively known along with falcons as diurnal raptors, generally take vertebrate prey. Their superlative eyesight, hooked bills and sharp talons are obvious adaptations for hunting and eating such prey. More subtle adaptations reflect each species’ specialization: an accipiter’s short, rounded wings propel it through dense forest at high speed, steered by its long tail; longer, broader wings allow buteos to soar and glide endlessly while hunting or migrating; more slender wings and a light body let the Northern Harrier drift back and forth at low altitude over marshes and fields. Superior size and power enable eagles to take large prey that are beyond the capability of smaller raptors. Female birds of prey are generally larger than males; this difference is most pronounced in the accipiters. Smaller males, the primary providers of food for their mates and offspring in the nesting season, are thought to have greater hunting agility. Many species have spectacular aerial courtship displays, sometimes known as sky-dancing.

 

Few bird families experienced such dramatic changes of fortune during the 20th century as did the raptors. Many species declined under the pressure of direct persecution between the late 19th and mid-20th centuries. This persecution was based partly on the misperception that all hawks prey relentlessly on game birds and domestic fowl, and partly on the desire to obtain a trophy or simply to shoot a large, “tempting” target. Although attitudes started to moderate as early as the turn of the century, most raptors did not receive full legal protection in Canada and the U.S.A. until the 1960s. Raptor shooting is now greatly reduced in much of North America and northern Europe, but continues unabated in some other parts of the world, as illustrated by banding recoveries of Swainson’s Hawks from Middle and South America.

 

Their position on top of food chains also makes raptors vulnerable to environmental contaminants, especially those with a tendency to accumulate in fatty tissues. Thus, several species suffered catastrophic losses due to persistent pesticides, especially DDT, in the quarter-century following World War II. The primary effect was breeding failure due to eggshell thinning. Happily, most of these declines have been reversed since the use of DDT was banned in Canada and the U.S.A., though it continues to be used elsewhere. Nevertheless, direct poisoning of raptors by pesticides (especially those targeting grasshoppers and rodents), heavy metals, and other environmental contaminants is a recurring, worldwide problem that calls for continued vigilance.

 

Habitat loss is perhaps a less important factor than persecution and poisoning for Manitoba raptor populations, but modern agriculture has certainly contributed to the reduced abundance of many prairie birds, including the Ferruginous Hawk and Swainson’s Hawk. Forest-dwelling raptors are perhaps more affected by habitat losses in their wintering areas than in their breeding range, but we do not know the effects of the current, accelerating timber exploitation in the boreal forest on such species as the Northern Goshawk, Sharp-shinned Hawk, and Broad-winged Hawk.

 

Impressive numbers of raptors funnel along the Red and Pembina River valleys in spring, and are monitored annually by hawk-watchers near St. Adolphe and Windygates. Migration is especially concentrated at these sites in belated springs when deep snow remains on the surrounding farmland well into April. Numbers fluctuate from day to day with changing weather, and peak movements usually occur on mild, sunny days with southerly breezes, sometimes just ahead of an advancing storm system.

 

While Manitoba birders traditionally hold a one-day “hawk watch” in early or mid-September, raptor numbers in fall are more difficult to monitor than in spring. Fall migration tends to be dispersed, though there is some concentration of birds near the shores of major lakes and the southern edge of the boreal forest. Abundant grasshoppers may prompt Swainson’s Hawks to flock in early September, while burgeoning populations of meadow voles are sure to attract Rough-legged Hawks in the later fall months.

 

Though some raptor species are highly distinctive, others display a bewildering assortment of plumages according to age, sex and race, not to mention the existence of dark, light and intermediate-coloured “morphs”. This variety, coupled with the often distant views or brief glimpses obtained, makes raptor identification a lifelong challenge. Subtle differences in shape and flight characteristics are often more helpful clues to identification than plumage details.

 

Eleven species in this family breed in Manitoba, and two more are visitors whose breeding status is uncertain.