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RAPTORS

     The word “raptor” is used to describe hawks and eagles of the Family Accipitridae, falcons of the family Falconidae, ospreys of the family Pandionidae, and even vultures of the family Cathartidae (although vultures now appear to be most closely related to storks). 

The first fossils of raptors consisted of sea eagles, booted eagles, and kites from the Eocene. Paracathartes was a stilt-legged vulture known from the Early Eocene which supports the conclusion that New World vultures evolved from storks. Buteo hawks are known from the Oligocene, falcons, osprey, and Old World vultures are known from the Miocene. The Daggert eagle existed in the Pleistocene in North America. Its long legs indicate that it might have been terrestrial (Weidensaul, 2000).

Haast’s eagle once inhabited New Zealand where it preyed on the giant moas. It could reach 30 pounds, much larger than the 15-20 pounds of the modern harpy eagle or 9 pounds of the golden eagle. One type of goshawk once reached a size similar to eagles, with a weight of about 7 pounds. The tiny hawk measures only 8 inches and specializes in preying on hummingbirds (Weidensaul, 2000)

The bird family Accipitridae is composed of more than 230 species and members of the family can be found throughout the world from tundra to tropical environments (Lerner, 2005). Buteo hawks are closely related to sea eagles with milvine kites forming their sister group. Snake eagles are most closely related to the Old World vultures of the subfamily Aegypiinae. Old World vultures of the subfamily Gypaetinae are most closely related to harrier hawks and kites of the subfamily Perninae (Lerner, 2005; Wink, 1996). Eagles are divided into four subfamilies which are not closely related, Old World vultures are divided into two subfamilies which are not closely related, harriers are divided into two subfamilies which are not closely related, and kites are divided into 3 subfamilies which are not closely related (Lerner, 2005; Wink, 1996). Genetic analyses indicate that falcons, New World vultures, and secretary birds are classified in families outside Acciptridae (Wink, 1996). Genetic analyses indicate that many of the traditionally accepted genera of raptors (such as those of eagles) are actually paraphyletic and new taxonomic relationships should be devised (Helbig, 2005).

     Most raptors have a visual acuity which is 2 ½-3 times as great as human eyes and some (such as Old World vultures) have an acuity 8 times that of humans.  Raptors also possess highly developed hearing and some species, such as harriers, possess facial disks which help them pinpoint sounds. 

     All raptors spend at least some of their time soaring on pockets of rising air.   Mountain ranges, such as the Appalachians, serve as a natural guides for winds and which can be used during migration.  Many raptors have separations between the feathers on their wingtips which are an adaptation for soaring.  Small falcons and kites are the raptors which are most able to hover, although larger raptors may also be capable of some degree of hovering.  Some hawks dive at speeds of over 100 mph and peregrine falcons can reach speeds of 175 to 200 mph during dives.  During these dives (called stoops), the wings are tucked close to the body.  In flapping flight, peregrine falcons can attain speeds of 62 mph, merlins can reach 45 mph, and goshawks 38 mph (Weidensaul, 1996).  

     Α pair of raptors may maintain multiple nests within a territory and use them every other year.  Frequently, both genders help to build the nest.

Female raptors are larger than males, which is unusual among birds.   Although most raptors are solitary, a few species can aggregate in flocks during at least part of the year.  Some raptors reuse nests from year to year; bald eagles may use the same nest for more than 35 years.  The sizes of the average clutch vary in different species of raptor.  Bald eagle clutches number 2-3 eggs, red tailed hawk clutches 2-4 eggs, and sharp shinned and northern harrier hawk clutches 4-5 eggs.

 

Incubation of Eggs (days)

Time spent by Nestlings in Nest (days)

Turkey Vulture

38-41

66-88

Osprey

32-43

44-59

Bald Eagle

34-37

70-98

Sharp-Shinned Hawk

30-35

20-27

Red Tailed Hawk

30-35

43-48

American Kestrel

27-31

28-31

(after Weidensaul, 1996).  

     The percent of juveniles which die in their first year vary in diverse raptor species: 53% of ospreys, 73% red-tailed hawks, 69% of kestrels, 57% peregrine falcons, 70% bald eagles, and 63% red shouldered hawks (Weidensaul, 1996).   A Swainson’s hawk which travels from Montana to Argentina travels a distance of almost 11,000 km (Johnsgard, 1990).

     A large territory may be required to support a single bird.  Bald eagles typically have a territory of 10.4 square miles while that of kestrels is .8 square miles.  Red tailed hawk territories vary in size in different environments from .5 to 1.5 square miles. Small raptors such as sharp shinned hawks must consume 25% of their body weight per day. Kestrels consume about 21% of their weight per day. Larger birds consumer a smaller percentage: 12% in peregrines, red-tailed hawks 7-10%, and turkey vultures 10% (Weidensaul, 2000).

     Since hawks, falcons, and eagles are at the tops of their food chains, they are vulnerable to the factors which affect their prey items.  Many raptor species were devastated by the ingestion of the pesticide DDT which had accumulated in their prey items.  In South America, the same deforestation which is endangering populations of monkeys is endangering the populations of the harpy eagle which hunts them.  Haast’s eagle was the world’s largest eagle (at 30 pounds) which lived in New Zealand and preyed on the flightless moas (which could stand at 12-15 feet tall).  It became extinct once the moas themselves were hunted to extinction (Weidensaul, 1996).

     The Egyptian vulture can throw rocks at ostrich eggs in order to break them and this behavior is the only instance known in raptors involving tool use (Weidensaul, 1996).   Bones make up the majority of the diet of bearded vultures ( Houston, 2001).A Harris hawk and a sakir falcon are depicted in the following images.

harris hawk harris hawk
sakir falcon  

A number of interspecific hybridizations are known to occur within raptors, including peregrine and prairie falcons, peregrine and lanner falcons, merlins and Eurasian kestrels, black and red kites, rough-legged hawks and common buzzards, buzzards and goshawks (although infertile), and a red-backed and Swainson’s hawk (Weidensaul, 2000).