Why does Hawk Mountain have more migrating hawks in autumn than in spring?

Each autumn, thousands of birds of prey fly past Hawk Mountain Sanctuary, the world's first refuge for birds of prey. Since the Sanctuary's establishment in 1934, observers have identified and counted more than 1 million hawks, falcons, eagles and vultures. As anyone who has had the thrill of watching a Peregrine Falcon zip past North Lookout will undoubtedly agree, autumn raptor migration represents the most spectacular concentrated movement of land-based predators on earth. However, for several reasons, springtime movements of raptors are decidedly less spectacular (Heintzelman 1979, Kerlinger 1995).

Raptors occur in concentrations more often in autumn than during spring migration because fall migrants complete their journeys at a more leisurely pace than they follow in spring. As such, hawks, falcons, and eagles migrating in autumn have more time to concentrate along traditional migratory corridors and geographic bottlenecks. Several explanations have been offered for why raptors take longer to migrate in autumn, including seasonal differences in prey availability, variable conditions for soaring, the occurrence of molt, and the absence of spring's urge to breed.

Some ornithologists believe raptors migrate more slowly in autumn than in spring because prey is more available at the end of the prey's summer reproductive season; this makes it easier for migrating raptors to feed during fall migration. Others say that conditions for soaring are better along established migratory corridors in late summer and early fall than they are in spring. Still other experts suggest that feather molt plays a role. Many species of raptors interrupt their molt during autumn migration; as a result, migrants are often in "better feather" in spring--i.e., with a complete set of newly formed and fully grown plumage--and probably are better able to migrate. The most popular explanation for faster spring migration, however, is the birds' urge to reproduce. In spring, raptors may just be in a hurry to get back to their breeding grounds to claim the best territories for raising young (Berthold 1993, Lesham 1994).

Whatever the reason(s) for seasonal differences in the overall speed of migration, raptors tend to migrate over broader fronts, to flap more, and to glide and soar less during spring migration. They are less likely to wait for and take advantage of thermals and updrafts. Indeed, even at continental-scale bottlenecks such as the narrow strip of land at coastal Veracruz, Mexico, migrants put on a better show in fall than in spring.

This is not to say that raptors do not concentrate during spring migration. Long-distance buteo migrants which depend upon soaring, frequently concentrate during spring AND autumn migration. Swainson's and Broad-winged Hawks, for example, are seen migrating in large numbers each spring in coastal southernmost Texas. Spectacular flights of Broad-winged Hawks also can be observed at several watchsites further north, particularly along southern shores of Lake Erie and Lake Ontario, where northbound movements are diverted by these large bodies of water. At such times, migrating raptors take advantage of onshore breezes created when spring sunshine warms the region's land surfaces faster than the lakes' waters. As a result, large numbers of northbound raptors can be seen at raptor migration watchsites at Braddock Bay State Park and Derby Hill in New York, along the southern shores of Lake Ontario, and along the southern shores of Lake Erie at Conneaut and Cleveland, Ohio (Heintzelman 1979).

One other obvious factor that affects numbers of birds witnessed at Hawk Mountain in fall is the success of the breeding season. In spring, adult birds speed north to their breeding grounds and reproduce young birds that, in turn, will migrate south in autumn. Thus, the total population of hawks probably peaks just at the beginning of migration when adults and recently fledged young birds of the year all start moving southward; together the "old" and "young" raptors make for large concentrations of birds along their fall migratory pathways and provide the autumn spectacle for which Hawk Mountain Sanctuary is internationally known.

REFERENCES
Berthold, P. 1993. Bird migration: a general survey. Oxford University Press, Oxford, England.

Heintzelman, D. S. 1979. A guide to hawk watching in North America. Pennsylvania State University Press, University Park, Pennsylvania.

Kerlinger, P. 1995. How birds migrate. Stackpole Books, Mechanicsburg, Pennsylvania.

Lesham, Y. 1994. Global raptor migration "bottlenecks" as a parameter of longterm variations in raptor populations. Pages 49-53 in Raptor Conservation Today, edited by B.-U. Meyburg and R. D. Chancellor. World Working Group for Birds of Prey and Owls, Berlin, Germany.

How many feathers do hawks, eagles and falcons have?

Feathers, not flight, are the distinguishing characteristic of birds. If an animal has feathers, it is a bird. If an animal does not have feathers, it is not. Indeed, the tell-tale impressions of feathers--not the associated reptile-like bones--helped scientists identify the more than 150-million-year-old fossilized remains of Archaeopteryx lithographica, as those of an ancient bird.

In addition to providing a rigid airfoil for flight, feathers insulate a bird from summer heat and winter cold. Along with body oils, feathers help waterproof birds, and shield their skin from dust and debris. The various colors of feathers may serve to hide or advertise a bird's presence.

In general, birds have lots of feathers, especially around heat-sensitive areas such as the head and neck, where upwards of 40% of a bird's feathers can be found. Larger species tend to have more feathers than smaller species. For example, a 3- to 4-gram Ruby-throated Hummingbird (Archilochus colubris) has about 950 feathers, compared with 25,000 for a 15- to 18-lb Whistling Swan (Cygnus columbianus). Outside the tropics, birds tend to lose some of their feathers over the course of each breeding season, replacing them only in fall, just in time for winter. The number of feathers varies little among members of the same species.

Except for detailed studies of wing- and tail-feather molt in several raptors, the number of feathers that each species of raptor has not been well studied. Even so, we do know that Barred Owls (Strix varia) have about 9,200 feathers and that Bald Eagles (Haliaeetus leucocephalus) have about 7,100. Again proving its misnomer, 30% of a Bald Eagle's feathers are on its head! With a combined weight of almost 700 grams, the plumage of a Bald Eagle comprises about one-sixth of the bird's mass--almost three times as much as its skeleton!

Although almost all raptors are born with a relatively thin coating of down feathers, most develop a second thicker coat of down within days of hatching. Depending upon the species, true, fully-shafted feathers begin to appear within 10 days to 5-6 weeks, with smaller species "feathering out" more quickly than larger ones. By the time they are ready to fledge, young birds have acquired most of their first year's plumage.

In most raptors, feathers cover all but small portions of the faces, feet, and toes of adult birds, but there are exceptions. Old and New World vultures, for example, tend to have near-naked heads and necks, while only a few species such as the Lappet-faced Vulture (Aegypius tracheliotus) of Africa and the Middle East and the Red-headed or Pondicherry Vulture (A. calvus) of India, southern China, and Malaysia, have heads that are entirely devoid of feathers. For many vultures, feathers of the head and upper neck are limited to a small number of downy tufts, or so-called "cotton wool" feathers. Bald-headedness in vultures appears to be associated with their propensity for poking their heads into rotting carcasses of animals upon which they feed. Regularly banging one's head against decaying flesh would almost certainly soil and contaminate "unpreenable" head feathers, so sanitation is enhanced by the lack of head plumage.

By comparison, most owls are fully feathered--head, feet, and toes--and in some instances so much so that only the bird's eyes, beaks, and talons remain uncovered. Such full-feathering helps muffle an owl's flight as its flaps and glides through the still night air in search of prey. Feathered feet and toes also protect the owl against potential bites from prey. Snowy Owls (Nyctea scandiaca) have particularly well feathered feet and toes--useful insulation against cold arctic weather. The Rough-legged Hawk (Buteo lagopus), a tundra-dwelling diurnal raptor, shares this adaptation. Old World fishing owls, which prey on fish in water, have unfeathered feet and toes.

Like most birds, diurnal birds of prey have ten primary flight feathers on each wing, along with 14 to 25 secondaries (falcons typically have 16), and 12 or 14 tail feathers. Larger species tend to have greater numbers of secondaries. Northern Harriers (Circus cyaneus), for example, have 14; Cooper's Hawks (Accipiter cooperii) 15; Red-tailed Hawks (Buteo jamaicensis) 17; Great Horned Owls (Bubo virginianus) 14; Black Vultures (Coragyps atratus) 19; Osprey (Pandion haliaetus) 20; King Vultures (Sarcoramphus papa) 21; California Condors (Gymnogyps californianus) 22; and Andean Condors (Vultur gryphus) 25. Bobwhite Quail (Colinus virginianus)--which spend most of their time walking--have only 10 secondaries, while the extremely long-winged and long-flighted Wandering Albatross (Diomedea exulans) has 32.

Feathers--which are an astonishingly complex extension of a bird's skin--consist mainly of keratin, the fibrous protein that makes up human hair and nails. Although avian anatomists have classified feathers into dozens of different types, the typical or classic type is the "contour feather," the kind that provides the outermost of most of the bird's body (including wing and tail feathers). Contour feathers consist of a hollow, cylindrical shaft (the quill), from which a long series of sidebranches (barbs) that form the "vane" of the feather. Barbs, in turn, have their own series of interlocking sidebranches called "barbules," which serve to align and hold the barbs in place along the vane. The structural complexity each feather is considerable. The tail feathers of Northern Harrier, for example, are estimated to have at least 1.25 million barbules each. No wonder raptors spend so much time preening!

Many birds of prey have specialized down feathers called "powder down." Scattered across their body, these somewhat matted feathers grow continually, with parts breaking off into a fine powder that apparently helps absorb the grime and debris that raptors contact.

REFERENCES
Brodkorb, P. 1955. Numbers of feathers and weights of various systems in a Bald Eagle. Wilson Bulletin 67:142.

Burton, J. A. 1973. Owls of the world. E. P. Dutton, New York, New York, USA.

Mundy, P., D. Butchart, J. Ledger, and S. Piper. 1992. The vultures of Africa. Academic Press, London, England

Van Tyne, J., and A. J. Berger. 1976. Fundamentals of ornithology, 2nd ed. J. Wiley & Sons, New York, New York, USA.

Wetmore, A. 1936. The number of contour feathers in passerine and related birds. Auk 53:159-169

What makes a hawk a hawk and an eagle an eagle?

The English, or common, names of birds are often confusing. Consider, for example, "hawk" and "eagle." Although one might suppose that the two terms are useful in distinguishing lineages of raptors, in reality, the two have little, if any, taxonomic meaning.

Eagles include 60 members of four groups of raptors that have little in common except their last name (Table 1). Hawks, too, comprise a rather diverse collection of about 120 raptors that are most easily characterized by what they are not: hawks are not ospreys, vultures, kites, harriers, eagles, kestrels, falcons, nor falconets. (Hawks, however, can be buzzards, at least in Europe, where the name "buzzard" is typically used to identify many species of buteonine hawks.) In fact, eagles often are more closely related to hawks than to other eagles, and vice versa.

If hawks and eagles are not taxonomically distinct, what does make a hawk a hawk and an eagle an eagle? The linguistic histories of the two terms suggest an explanation.

"Eagle" is an English word derived from the French word aigle, which, in turn, is derived from the Latin word aquila, meaning "eagle." Aquila is derived from the word- root aquil, meaning "dark." Etymologists suggest that the name "aquila" was initially used to describe the largest raptors, because the expansive silhouettes of these birds appeared to "darken" the sky. Whether such birds were actually eagles or large vultures is another question in itself. Nesher, the Hebrew word for eagle, for example, has been used by different Biblical authors to describe both griffons (several species of large vultures in the genus Gyps) and eagles. Today the word "eagle" is used to describe especially large diurnal birds of prey, many of which weigh more than five pounds, and several of which weigh more than 15 pounds (Figure 1).

"Hawk" is a Middle English word derived from the Old English hafoc, a cognate of "have," in the sense of grasping or seizing. Unlike the origin of the word "eagle," there is nothing in the history of the word "hawk" that suggests a reference to size. Many hawks are, in fact, quite small. Male Sharp-shinned Hawks (Accipiter striatus), for example, often weigh less than four ounces, and about half of all species of hawks weigh less than a pound (see accompanying figure).

Thus, eagles are eagles and hawks are hawks mainly because of differences in size. Eagles are scaled-up versions of hawks and are fierce, diurnal birds of prey with especially large wingspans, beaks, and talons. Hawk Mountain's two eagles, the Bald Eagle (Haliaeetus leucocephalus) and Golden Eagle (Aquila chrysaetos) for example, each has a wingspan in excess of six feet, while those of its seven species of hawks range from two to four feet.

Nevertheless--and this is where the confusion begins--there is some overlap in the size ranges of hawks and eagles. Some large hawks are bigger than some small eagles. Weighing in at about 2.5 pounds, the Red-tailed Hawk (Buteo jamaicensis), for example, is considerably heavier than either the Wahlberg's Eagle (Aquila wahlbergi) of Africa (1.5 pounds), or the Rufous-bellied Eagle (Hieraaetus kienerii) of southern Asia (1.6 pounds), as well as several other species of eagles. The record for the lightest eagle may belong to the diminutive New Guinean race of the Australian Little Eagle (H. morphnoides)--individuals of which can weigh little more than a pound.

The fact that more than a dozen species of medium-size eagles are called "hawk eagles," and that one South American eagle is called a "buzzard eagle," only adds to the confusion (Table 2). The Changeable Hawk Eagle (Spizaetus cirrhatus) of India, southern Asia, and the Pacific Islands may hold the record for the most curiously named raptor. This species, which is sometimes called the Crested Hawk Eagle, derives it name from the fact that races of the species differ considerably in plumage coloration and pattern, with several island forms even lacking the distinctive crest of the predominant mainland form.

In addition to "hawk eagles," there also are several "hawk owls" and "eagle owls," all of which are true owls. Hawk owls tend to have longer and narrower wings and longer tails than many owls, as well as less conspicuous facial discs--features that render them more hawk-like in appearance than other owls. Eagle owls are some of the largest and most powerful owls.

Table 1:
TYPES OF EAGLES

SEA EAGLES
The so-called sea, or fish eagles comprise a group of 10 species of largely aquatic eagles, including the Bald Eagle (Haliaeetus leucocephalus). Found throughout the world except in Central and South America, sea eagles are most closely related to the kites.

SNAKE EAGLES
The snake, or serpent eagles comprise a group of 11 species of short-toed Old World eagles, most of which feed largely on snakes, and most of which occur in Africa, and feed largely on snakes. This group, too, appears to be closely related to the kites, as well as to another confusingly named group of raptors, the harrier hawks.

BUTEONINE EAGLES
The buteonine, or buteo-like eagles comprise a group of six species of Pacific Island and South American birds of prey. As their name suggests, buteo-like eagles are most closely related to the buteos, a group of broad-winged, short tailed diurnal raptors that includes three of the Sanctuary's most common migrants: the Red-tailed (Buteo jamaicensis), the Red-shouldered (B. lineatus), and the Broad-winged (B. platypterus) hawk.

BOOTED EAGLES
The 30 or so species of booted, or true eagles are so called because their legs and feet--unlike those of those of all other eagles--are fully feathered to the toes. Booted eagles, which include the Sanctuary's Golden Eagles (Aquila chrysaetos), also are closely related to buteos.

HAWK EAGLES
African Hawk Eagle (Hieraaetus spilogaster); 2.0; Africa
Ayre's Hawk Eagle (H. ayresii); 0.8; Africa
Black and White Hawk Eagle (Spizastur melanoleucus); 0.9; Tropical America
Black Hawk Eagle (Spizaetus tyrannus); 1.0; Tropical America
Blyth's Hawk Eagle (Spizaetus alboniger); ?; Asia and Pacific Islands
Cassin's Hawk Eagle; (Spizaetus africanus); 1.0; Africa
Celebes Hawk Eagle; (Spizaetus lanceolatus); ?; Pacific Islands
Crested (or Changeable) Hawk Eagle (Spizaetus cirrhatus); 1.5; Asia and Pacific Islands
Javan Hawk Eagle (Spizaetus bartelsi); ?; Java
Mountain Hawk Eagle (Spizaetus nipalensis); 3.0; Asia
Ornate Hawk Eagle (Spizaetus ornatus); 1.2; Tropical America
Philippine Hawk Eagle (Spizaetus philippensis); 1.1; Philippines
Wallace's Hawk Eagle; (Spizaetus nanus); ?; Asia and Pacific Islands

BUZZARD EAGLES
Black-chested Buzzard Eagle (Geranoaetus melanoleucus); 2.2; South America

HARRIER HAWKS
African Harrier Hawk (Polyboroides typus); 0.6; Africa
Madagascar Harrier Hawk (Polyboroides radiatus); ?; Madagascar

REFERENCES

Borror, D. J. 1960. Dictionary of word roots and combining forms. Mayfield Publishing Co., Palo Alto, California, USA.

Brown, L. 1976. David & Charles, Newton Abbot, England.

Brown, L., and D. Amadon. 1968. Eagles, hawks, and falcons of the world. McGraw-Hill, New York, New York, USA.

Burton, J. A. 1973. Owls of the world. E. P. Dutton, New York, New York, USA.

Choate, E. A. 1973. The dictionary of American bird names. Gambit, Boston, Massachusetts, USA.

Dunning, J. B., Jr. 1993. CRC handbook of avian body masses. CRC Press, Boca Raton, Florida, USA.

Holmgren, V. C. 1972. Bird walk through the Bible. Seabury Press, New York, New York, USA.

Jobling, J. A. 1991. A dictionary of scientific bird names. Oxford University Press, Oxford, England.

Sibley, C. G., and J. E. Ahlquist. 1990. Phylogeny and classification of birds. Yale University Press, New Haven, Connecticut, USA