Falconiformes (Diurnal Birds of Prey)
FalconiformesFamily: Hawks and Eagles
Family: Secretary Birds
Family: Falcons and Caracaras
(Diurnal birds of prey)
Number of families 3
Number of genera, species 76 genera; 306 species
Evolution and systematics
Raptors are potentially as old as more "primitive" families, such as the loons (Gaviidae), with the oldest claimed fossil record, a tiny falcon-like bird from the British Isles, dating from the lower Eocene some 55 million years ago. Well-documented finds from the late Eocene and early Oligocene, some 30–50 million years ago, are all from Europe, with raptors showing up from the New World only in the Miocene, about 23 million years ago. However, there is no clue as to the order's geographic origin, with modern-day representatives found on every continent except Antarctica and the greatest diversity found in the Neotropics.
The oldest fossils are of forms unrelated to any modern-day species, though the osprey (Pandion haliaetus) has been around for 10 million years. There is also nothing to show that the different families of the Falconiformes share a common ancestor and, although this is a comparatively well-studied order, understanding of its systematics is limited. The raptors have been traditionally grouped in respect of their similar behavior, external morphology, moult patterns, and internal anatomy. Comparisons of feather proteins and DNA seem to confirm the relationships between the families, including the closer relationship between the monotypic Sagittariidae and the rest of the family than with the cranes (Gruidae) or bustards (Otididae), with which it shares some behavioral features.
The Falconiformes are divided into three families: the Accipitridae (hawks, eagles, and allies), the Falconidae (falcons, caracaras, and allies), and the unique Sagittariidae (secretary bird). Of the three, only the Falconidae is further divided, into the Polyborinae (caracaras and forest-falcons) and the Falconinae (the "true" falcons and falconets). The Accipitridae is numerically dominant and one of the largest avian families with more than 200 species, though the changing nature of taxonomy means that there may be up to 250, and the dividing lines between the genera and species are not especially precise.
Raptors have a strong, compact body and a large, generally rounded, head, joined by a strong neck that is very short in most species. The smallest species is the black-thighed falconet (Microhierax fringillarius), with a 12 in (30 cm) wingspan and weighing as little as 1.1 oz (28 g). At the other end of the scale, the Himalayan griffon vulture (Gyps himalayensis) has a wingspan of over 9 ft (3 m) and can weigh up to 26 lb (12 kg). In most species, there is significant sexual dimorphism, with males significantly smaller than females, enabling a pair to exploit a greater size range of prey. Although body length is a less useful measure in falconiformes than in other orders, it is notable that the secretary bird (Sagittarius serpentarius) stands at 4 ft (1.2 m) tall.
With a few exceptions, raptors excel in the air, and each family is well adapted to particular hunting techniques: the flight and tail feathers are large, with 10 primaries and 12–16 secondaries on each wing, and the bill and feet are designed for catching or ripping open the skin of prey. Even in those species that are not primarily carnivorous, such as the honey-buzzards, this sharp, hooked bill is essential to its lifestyle. Cutting edges of the upper mandible project over those of the lower mandible to form a scissors-like instrument. The legs are generally short, with long toes and bent, sharp claws, used to grasp prey. Often overlooked are the bristles at the base of
the bill of most species, which may protect the eyes when feeding, but may also provide sensory information on wriggling prey. In honey-buzzards, these are replaced by flattened, scale-like feathers, which may provide protection from bee and wasp stings, or may simply be to prevent honey from soiling the head plumage, in the same way that some kites and vultures have bare facial skin around the bill.
Most raptor species look relatively dull, with shades of brown, gray, and buff dominating the plumage. None is brightly colored and relatively few are predominantly black or chestnut and white as adults. In most orders, plumage color, especially of females, is camouflage to evade predators. This is less important for many adult raptors, which have few natural enemies, but the plumage has evolved to reduce detection as they hunt. For example, species that catch live prey, such as harriers (Circus) and falcons (Falco), tend to sport paler underparts, which make them less visible from below.
The Falconiformes are a global order, with the Accipitridae and the Falconidae found in all continents except Antarctica. Only the monotypic Sagittariidae is limited to a single zoogeographical region, the Afrotropics (though secretary bird-like fossils have been found in Europe and North America). At least one or two breeding species can be found in every major habitat type around the globe, from urban—where scavenging vultures and kites can thrive—to the high arctic tundra, where the gyr falcon (Falco rusticolus) raises its chicks on the abundant seabirds, waders, and lemmings.
The greatest number of species is found at lower latitudes and altitudes, especially among the Accipitridae, many of which require thermals to hunt. The Falconidae are more adaptable and able to exploit some of the harsher environments. The peregrine falcon (Falco peregrinus) may have the widest distribution of any breeding bird, and is now found in the center of some of the busiest cities, including London and New York.
At the top of the food chain in many habitats, birds of prey are good indicators of habitat quality—without sufficient food or nest sites, these long-lived birds cannot survive. Many species occur at low densities, requiring large home ranges for feeding, especially in higher latitudes. They are terrestrial birds, although Steller's sea-eagles (Haliaeetus pelagicus) will fish from drifting icebergs several miles from shore.
Tropical rainforests contain the greatest abundance of Falconiformes (especially the Accipitridae, caracaras, and forest-falcons), many of which nest or roost in trees, even though some forage in open, agricultural landscapes. The true falcons tend to reside in more open habitats, while some species—such as harriers—are more adapted to grasslands or even reedbeds.
Pairs of most species live a solitary lifestyle, especially those at higher latitudes, where resources are often scarcer and home range can be several dozen square miles. Some species are more social, particularly those that are less predatory, feeding on invertebrates. The adults of a few species, such as Eleonora's falcons (Falco eleonorae) and some vultures, nest colonially, while many roost and feed together outside the breeding season. Many migratory species also make their transcontinental journeys en masse. Social groups of immature birds are probably important in developing the skills for later breeding success.
Raptors have simple calls, usually repeated notes, often high pitched and harsh. Calls are used for many social situations, including maintaining contact between a pair or family. Kites and buzzards are the most vocal, with a variety of mewing and screeching calls, which peak during courtship.
Raptor migration is among the greatest spectacles of the avian world. Raptors with a low wing-loading are unable to fly for a long distance over water, so require thermals to make the distance. Thus, narrow peninsulas and isthmuses—such as Panama, Gibraltar, and Sinai—are the meeting points for the raptors from a whole region and, in the right conditions, thousands can pass every hour, for days or weeks on end. The principal long distance migrants are the Accipitridae species that breed in the northern hemisphere, with relatively small numbers of the Falconidae making such journeys, though a few make seasonal altitudinal movements.
Feeding ecology and diet
Most species are exclusively carnivorous, feeding on every major group of vertebrates and most of the invertebrates. Some, especially the larger Accipitridae, are generalist scavengers of carrion, but many have specialized diets, such as the eponymous bat falcon (Falco rufigularis) and the snail kite (Rostrhamussociabilis). Visual acuity is critical to success, especially among the high-speed falcons, and is up to eight times better than human eyesight.
In general, the Falconidae fly rapidly, taking prey on the wing at speed, catching it in their talons, and killing it with a bite to the back of the neck. By contrast, small Accipitridae tend to hunt from a perch, making a short flight to catch small prey on the ground, squeezing it to death with their strong feet, and often taking it a short distance for plucking or ripping. Larger hawks and eagles search for prey from thermals, so do not hunt until the air has warmed up, several hours after sunrise. Some genera have developed specialized methods, such as the hovering utilized by kestrels and the kicking adopted by the secretary bird. The soft organs, with the highest protein levels, are extracted first. Aerial feeders, such as Eurasian hobby (Falco subbuteo) swallow invertebrates alive and whole, while flying. Indigestible material is regurgitated in a pellet through the bill, some 16–18 hours later.
Most raptors are monogamous, though polygyny is known in three harrier species when there is an abundant food supply, and polyandry (where one female mates with several males, which help to rear the brood) is recorded occasionally from a few species. Small falcons breed at one year old, whereas large vultures and forest eagles do not mature until six to nine years, though it can be significantly sooner where the population is well below carrying capacity for the habitat.
Courtship is usually a simple, soaring display flight by the male to advertise his ownership of a territory, though some species undertake a dramatic "rollercoaster" flight. Most of the Falconidae use a shallow scrape on a cliff face or a hole in a tree, whereas the Accipitridae and the caracaras build a nest platform that can attain a height of several feet over several years. Territories are evenly spaced and often traditional—those of some larger species are used by different pairs over many decades.
As long-lived birds, raptors have more breeding opportunities than many bird families, but rearing such large young is costly on resources, so most species successfully rear just one young each year, and some of the larger species do not breed annually. Typically, males hunt on behalf of females during incubation and while the chicks are young. The sexual dimorphism comes into its own as the nestlings grow, with the male and female able to hunt for a wide range of prey. The time taken to fledge is related to the ultimate size of the adult, with young sea-eagles and vultures remaining in the nest for several months after hatching.
Raptor populations are, by nature, stable, with population density remaining remarkably constant over many decades. Their history during the last 400 years is indicative of the pressures facing birds of prey and their habitats, though—perhaps surprisingly—none has become extinct (though one falcon subspecies has been lost from central America). Thirty-four
species are listed as Threatened or Near Threatened by BirdLife International and IUCN, though the global populations are not known for most species.
Many species are believed to be less abundant than in the recent past as a result of habitat changes that have altered the prey base. In particular, deforestation and agricultural monocultures have had a dramatic effect on the densities of many species, with most finding it harder to survive, though a few—such as kestrels—have probably benefited from the expansion of low intensity farming into former forests. An indication in reverse comes from the post-Soviet abandonment of collective farms which resulted in reduced grazing and thus dramatic falls in the populations of susliks, followed rapidly by that of saker falcons (Falco cherrug).
As well as loss of wooded and wetland habitat, modern agriculture also brought organochlorine pesticides that significantly reduce breeding success, by preventing eggshells from thickening. During the 1950s and 1960s, populations of several falcon and hawk species fell dramatically in Europe and North America, ultimately resulting in the prohibition of the compounds and the subsequent recovery of most species. However, compounds such as DDT remain in widespread use in many parts of the world, with little knowledge of the deleterious effects on raptors.
Direct persecution remains a serious problem for some species, especially those that come into conflict with the land-uses adopted in their favored habitats. Depredation by raptors of livestock, particularly sheep, and small gamebirds can elicit a lethal response from farmers and gamekeepers. Carrion feeders are especially vulnerable to poison baits, targeted either at them or mammalian predators. In addition, some falcon species are targeted by collectors for sale or falconry, while in Britain raptors have long been the target of eggcollectors. The longevity and low breeding success of raptors means that recovery from any decline is slow.
The most dramatic decline occurred among the Gyps vultures of the Indian subcontinent in the late 1990s, in which an epidemic probably killed hundreds of thousands of birds in the region. Disease appears to be the cause, but by 2002, it remained unclear whether an environmental factor had made the birds more prone than previously.
The pressures on raptors have been recognized in many parts of the world by protective legislation. Indeed, in many parts of the world, members of the order have the highest levels of protection, enabling the populations of many species to recover to former levels. Their readiness to breed in captivity has enabled the reintroduction of several species into former parts of their range.
Significance to humans
Although most species do not live close to people, raptors have had a strong role in popular culture since prehistoric times, some being worshipped by earlier religions. Their image is used to symbolize power, freedom, and agility on the flags or arms of many nations and on many logos in the corporate world. Birds of prey have a special place in the hearts of conservation biologists for their role as an environmental indicator brought to the fore as the flagship for the campaign to ban certain pesticides in the 1960s. Popular with birdwatchers, raptors are a draw worth millions of dollars to many tropical areas.
Some species have always had a close relationship with people. At least 30 species can be found in large cities, and many carrion-feeders make regular use of conurbations. In Elizabethan London, red kites (Milvus milvus) scavenged on the streets, while vultures and kites are a regular sight on the refuse tips surrounding Asian and African cities. Their role as nature's cleaners has long been respected by society, especially by the Parsi sect in India.
Others have a more ambivalent relationship, seeing raptors as vermin that threatens their livelihood or sport, be it livestock farming, hunting, or pigeon-racing, though the perception of the damage caused is often much greater than the reality. The hooked bill is enough to warrant the blame. It is, therefore, ironic that in some parts of the world, hunters use raptors in falconry and hawking to hunt small game.
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BirdLife International. Wellbrook Court, Girton Road, Cambridge, Cambridgeshire CB3 0NA United Kingdom. Phone: +44 1 223 277 318. Fax: +44-1-223-277-200. E-mail: [email protected] Web site: <http://www.birdlife.net>
The Hawk and Owl Trust. 11 St Marys Close, Abbotskerswell, Newton Abbot, Devon TQ12 5QF United Kingdom. Phone: +44 (0)1626 334864. Fax: +44 (0)1626 334864. E-mail: [email protected] Web site: <http://www.hawkandowltrust.org>
Raptor Research Foundation. 1752 Robin Hood Road, Mt. Bethel, PA 18343 USA. Phone: (570) 897-6863. E-mail: [email protected] Web site: <http://biology.boisestate.edu/raptor/>
World Center for Birds of Prey, The Peregrine Fund. 566 West Flying Hawk Lane, Boise, Idaho 83709 USA. Phone: (208) 362-3716. Fax: (208) 362-2376. E-mail: [email protected] Web site: <http://www.peregrinefund.org>
USGS Raptor Information System.<http://ris.wr.usgs.gov/>