Accipitridaeeagles, hawks, and kites(Also: hawks, eagles, and relatives)

Diversity

The family Accipitridae encompasses many of the diurnal birds of prey, including the familiar hawks and eagles. It is one of the largest avian families, and the largest family in the order Falconiformes. The Howard and Moore Checklist of the Birds of the World recognizes 233 species in 67 genera in this family worldwide. Twenty-four of these species and 14 genera are native to North America. Many of the species in this family also include multiple subspecies. For example, up to 23 subspecies of the variable goshawk are recognized. ("", 2003; Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

A great deal of diversity exists among the members of the Accipitridae. Members of this family span the globe, living in habitats as wide ranging as tundra, alpine meadows and rainforests. They eat fish, mammals, birds, bats, invertebrates, carrion and some fruit. They nest on cliffs, in trees or sometimes on the ground and lay between one and nine eggs. Physical size is also quite variable within this group, with wingspans ranging from 50 cm to 3 m. (Snyder, 2001; Thiollay, 1994)

Geographic Range

Accipitrids are found world wide. They live on every continent except Antarctica, and on most oceanic islands. They inhabit every major habitat type except the northernmost arctic tundra and the driest deserts. The highest diversity of accipitrid species is found in tropical regions. (Kemp and Newton, 2003; Thiollay, 1994)

Habitat

Accipitrids are found in most terrestrial habitats, including tundra, alpine meadows, grasslands, desserts, sea coasts, rainforests, woodlands, agricultural, suburban and even some urban areas. They are also found at most elevations, from coastal areas at sea level to the tops of mountains. The highest numbers of accipitrid species are found in forests and woodlands, whereas less productive habitats such as desert steppes and tundra can typically support only one or two species. Many raptor species rely on trees for nesting, perching, roosting and hunting. Therefore, habitats with trees can generally support many more raptor species than those without trees. ("Hawks and Eagles (Accipitridae)", 2003; Snyder, 2001; Thiollay, 1994)

Accipitrids appear to select habitat based largely on food availability. A high quality accipitrid habitat is one with large quantities of high quality food that is accessible and predictable over time. Other characteristics that may be important in habitat selection are the frequency and intensity of human disturbance, risk of predation and availability of suitable nest sites, foraging perches and shelters. (Snyder, 2001; Thiollay, 1994)

Many Accipitrids migrate between breeding and wintering habitats. Individuals that migrate south for the winter usually choose a wintering habitat that is similar in structure to their breeding habitat. For example, species that breed in open woodlands generally choose wintering habitats in fairly open tropical forests. (Thiollay, 1994)

Physical Description

Accipitrids are diurnal birds of prey with broad wings, hooked beaks, strong legs and feet and sharp talons. All accipitrids have a cere, which is a waxy membrane that is often brightly colored, and covers the base of the upper mandible. They have large eyes that are shaded by a supraorbital ridge in most species, making the face appear fierce. Adult Accipitrids have wingspans ranging from 50 to 300 cm and total body lengths ranging from 25 to 150 cm. The body masses of Accipitrids range from 80 g to 12.5 kg. ("Hawks and Eagles (Accipitridae)", 2003; Snyder, 2001; Thiollay, 1994)

An assortment of body shapes and plumage can be seen among Accipitrids. For example, tails can be square, rounded, wedge-shaped or even forked. Accipitrid physical characteristics reflect adaptations to their habitat, mode of foraging and prey. For example, forest dwelling species generally have short wings and long tails, a combination which affords them maneuverability. Species that live in open habitats and hunt by soaring generally have long, broad wings and short tails, which allow them to soar easily over long distances. Many of the soaring species also have "fingers", which are long outer primary feathers that reduce drag on the wings and allow the birds to soar at slow speeds without stalling. ("Hawks and Eagles (Accipitridae)", 2003; Thiollay, 1994)

Accipitrids are colored to blend in with their environment, and are usually brown, black or gray with some streaking or barring. Many have paler underparts than upperparts and barred underwing and tail feathers, a patterning that may make them less visible to prey. Several species of raptors show plumage polymorphism (light and dark morphs) or variation in plumage across their geographic range. ("Hawks and Eagles (Accipitridae)", 2003; Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

In most Accipitridae species, females are larger than males. This characteristic, called reversed sexual size dimorphism, is especially dramatic in bird-eating species. Though there is no consensus among scientists about why reversed sexual size dimorphism occurs, it is clearly more pronounced in species that hunt quick, agile prey. Some Accipitrids also show sexual dimorphism in plumage coloration. In species where this occurs, the male is usually more brightly colored than the female, and the female looks similar to the juveniles. ("Hawks and Eagles (Accipitridae)", 2003; Snyder, 2001; Thiollay, 1994)

First-year accipitrid plumage is usually quite different from adult plumage, and very similar between species. Most juveniles have brown upperparts, sometimes mottled with light spots, and lighter underparts with brownish streaks. Juveniles also often have bigger (broader or longer) wing and tail feathers than adults, which may serve to make flying easier for the novice birds. After growing their first feathers, Accipitrids molt once per year. Many species develop adult plumage after the first year. In other species, including most eagles, the transformation from juvenile to adult plumage takes place over the course of several years, and the young bird passes through several sets of intermediate plumage. (Snyder, 2001; Thiollay, 1994)

Accipitrids share many traits with their falconid relatives, including strong beaks, feet and talons, and forward directed eyes. However, accipitrids are distinguished from falcons by their yellow, red or hazel, rather than brown eyes, nest building behavior (not exhibited in falcons), skeletal differences and forceful ejection of excreta. ("Hawks and Eagles (Accipitridae)", 2003; Sibley and Ahlquist, 1990; Snyder, 2001; Thiollay, 1994)

  • Sexual Dimorphism
  • female larger
  • sexes colored or patterned differently
  • male more colorful

Reproduction

Most accipitrids are monogamous, and many mate for life. Though polygyny and polyandry do occur regularly in some species, these species are exceptional. Most species of Accipitridae are solitary, and defend a territory around their nest. The size of a pair's territory and the extent to which they defend it from conspecifics depends on the species and the habitats. In general, open-country species chase-off intruders more than forest-dwelling species, probably because intruders are more difficult to detect in forested habitat. Colonial breeding does occur in some species at sites with high prey abundance and suitable nest-sites. North American examples of these species include northern harriers and snail kites. (Snyder, 2001; Thiollay, 1994)

All accipitrids build nests, which the male and female construct together. The male brings most of the nest material to the female, who arranges it at the nest site. In most species, the male also provides the majority of food to the female from the pre-laying stage up through much of the nestling stage. Provision of food by the male may be an important part of courtship during the pre-laying and laying stages. Copulation occurs before and during the time of egg laying. Several hundred copulations may take place during this period. Courtship behavior by males includes bringing food and nest materials to the female, and performing flight displays above the territory or nest. Males and females of some species also display together. The female may solicit copulation from the male by assuming solicitous postures. (Snyder, 2001; Thiollay, 1994; Snyder, 2001; Thiollay, 1994)

Accipitrids reach sexual maturity between ages one and nine. Generally, small species become sexually mature in their first year whereas larger species take longer to mature. In species that do not reach sexual maturity until they are more than one year old, many young birds spend the first year or more on the wintering grounds rather than returning north for the summer. When they do return north to breed, raptors show high fidelity to their natal breeding site, often settling in the same area or within 100 km of it. Breeding begins in the spring in temperate regions, and during the dry season in tropical regions. In regions where the weather is less predictable, raptors may breed throughout the year, or after weather events that signal an upcoming abundance of food, such as after irregular heavy rains. Accipitrids nest once per breeding season. Some species, mostly Buteos, will re-nest if they experience nest failure soon after egg laying. ("Hawks and Eagles (Accipitridae)", 2003; Snyder, 2001; Thiollay, 1994)

Female accipitrids lay between one and nine eggs per breeding season, though clutches bigger than six eggs are exceptional for any species. The number of eggs differs between and within species with food availability and latitude. Larger species usually lay clutches of one to two eggs, where as smaller accipiters and harriers normally lay clutches of five to six eggs. Birds that nest farther from the equator generally lay larger clutches than equatorial individuals of the same species. In many species, especially rodent specialists, clutch size also tends to track prey abundance. (Snyder, 2001; Thiollay, 1994)

One egg is laid every two to five days, and incubation begins with the first or second egg. Length of the incubation period differs considerably between species, and lasts from 28 to 60 days (longer for larger species). Both the male and female incubate the eggs in most species. Because incubation begins before all eggs have been laid, the eggs hatch asynchronously, and the older chicks are noticeably larger than they younger chicks. The older chicks are able to out-compete the smaller chicks, which often die of starvation or from aggression by the older chicks. This process of brood reduction is obligate in some species, but occurs only in times of food shortage in other species. (Snyder, 2001; Thiollay, 1994)

Males of most accipitrid species provide the majority of the food to the females and nestlings from the pre-laying stage to about half-way through the nesting period. The female usually feeds the chicks by tearing the food into bite-sized pieces. Young accipitrids begin to leave the nest when they are about a month old. They begin by hopping out of the nest along branches or ledges, and then making short flapping jumps between the nest and nearby perches. The length of these jumps increases until the chick can make short flights to nearby perches. The chicks then begin to spend most of their time away from the nest, perching nearby and returning to the nest to feed. Young birds fledge after a length of time that varies widely between species, but is roughly similar to the length of the incubation period. Fledglings continue to return to the nest to receive food from the parents even after they can fly. After up to several weeks of supplemental feeding from the parents, young birds disperse from their parents territory (though in some species, juveniles will remain in their parents’ territory for up to a year before being chased off by the adults). Juveniles may disperse in any direction, some even heading north, before beginning a southward migration to wintering grounds or finding a place to settle for the winter. ("Hawks and Eagles (Accipitridae)", 2003; Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

Nest building is one of the characteristics that distinguish accipitrids from falcons. Accipitrids build nests of sticks or twigs and line them with a softer material, such as the inner or outer bark of trees, frayed palm or agave leaves or seaweed. Some species also decorate their nests with fresh green leaves or conifer needles. Nests are built in a tree, on a cliff, or occasionally on the ground. Construction of a nest can take weeks to months, which may explain why nests are often re-used from year to year. Active accipitrid nests are usually easy to spot by the "whitewash" (a layer of excrement) below the nest. (Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

Accipitrid parents feed their young from the time of hatching to several weeks after fledging. When the chicks are very small, the female tears food into small pieces, which she feeds to the chicks. In some vultures, the adults instead regurgitate food into the mouths of the chicks. During the nestling period, the male brings food to the nest while the female spends most of her time at the nest protecting, feeding and caring for the chicks. The female begins hunting again about halfway through the nestling stage. The parents continue to bring food to the chicks for several weeks after they have fledged, which allows the chicks to practice flying and hunting while they have a reliable food source. Eventually, the chicks begin to successfully hunt for themselves, and leave the nest soon after this. (Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

While accipitrid chicks are young, the female spends the majority of her time at the nest. In addition to feeding the chicks, she defends them from predators and weather extremes. Young chicks are unable to regulate their body temperature and are dependant on adults to shade them from the sun and cover them in the rain. Females defend the nest by actively chasing off predators and by shielding the contents of the nest from view when predators fly over. Both males and females perform territorial display flights over the hunting/breeding territory to discourage intruders from entering. (Snyder, 2001; Thiollay, 1994)

Lifespan/Longevity

The average lifespan for most raptors is one to two years, though it is likely to be longer for some larger species. The oldest known free-living accipitrid lived to age 38. In captivity, large eagles and vultures have lived up to 60 years. (Snyder, 2001; Thiollay, 1994)

The mortality rate of accipitrid nestlings (during the period between hatching and fledging) is between 20 and 40 percent. The vast majority of this mortality is due to food shortage and/or siblicide, though predation can also be a significant source of mortality. The period immediately following fledging is the time of highest mortality during a raptor’s life. This is the time when the young bird is still mastering the arts of flying and hunting. Estimates of mortality for the interval between fledging and acquisition of adult plumage range as high as 90 percent for some species. Likely sources of mortality during this period include starvation, predation, disease and accidents. (Thiollay, 1994)

Estimates of annual adult mortality for accipitrids range from 65 to 90 percent. Generally, annual survival is higher in larger species and lower in smaller species. Sources of mortality for accipitrids have historically included deliberate shootings, poisoning and trapping. More recently, these sources of mortality have declined. However, Accipitrids still die from electrocution (power lines), collisions with vehicles and wind turbines, and poisoning from pesticides or from poisoned carcasses intended for other predators. (Thiollay, 1994)

Behavior

Most raptors are migratory, traveling annually between non-overlapping southern wintering grounds and northern breeding grounds. Others are non-migratory, essentially spending the entire year in the same locality. In some species, some populations are migratory while others are sedentary. Fall migration typically takes place between mid-August and November. Spring migration takes place between February and June in most species. (Snyder, 2001; Thiollay, 1994)

Unlike most other birds, almost all raptors migrate during the day. This probably allows them to take advantage of weather patterns, like thermal updrafts and winds that speed their flight. Most hawks avoid migrating over large bodies of water, where thermal updrafts are infrequent and weak. As a result of their dependence on thermal updrafts, most hawks tend to follow geographic features, such as mountain ridges and peninsulas, and become concentrated along these geographic features during the fall and spring migrations. Smaller species of hawks use flapping flight as well as thermals for migration. These species are less dependent on thermal updrafts and can therefore follow a broader migration route, cross larger expanses of water more easily, and travel alone more frequently. (Snyder, 2001; Thiollay, 1994)

Most Accipitrids are solitary or occasionally semi-colonial. In those species that are territorial, it is likely that population density is controlled by the quality and quantity of prey, which determine the minimum territory size. There is usually a population of non-breeding individuals that floats between or within occupied territories. These individuals are usually immature birds and adults that were unable to establish a territory. These individuals frequently take the place of breeding individuals that are killed during the breeding season, and may even raise the brood of a killed bird. (Snyder, 2001; Thiollay, 1994)

Most Accipitrids are diurnal (active during daylight hours). There are a few exceptional species, which include the bat hawk, which is crepuscular, and the letter-winged kite which hunts at dusk and at night. (Snyder, 2001; Thiollay, 1994)

Communication and Perception

Accipitrids use visual displays, often combined with vocalizations, to convey messages to one another. Many male accipitrids perform diving flight displays high above their territory to advertise that the territory is occupied. Some pairs of Accipitrids perform displays together. For example, red-tailed hawk pairs (Buteo jamaicensis) sometimes perform a display in which they glide together, with their legs dangling below them, after they have chased off an intruder from their territory. This display may be involved in both territoriality and courtship, as it is often followed by copulation. If an intruder enters a territory despite the owner’s flight displays, accipitrids use threat displays to discourage them. Threat displays usually involve raising the crest or head feathers, stretching the head and neck forward, and/or opening the wings as well as vocalizations. At the nest, a variety of greeting, solicitation and begging postures and/or vocalizations are used to communicate between the male and female and the parents and chicks. (Snyder, 2001)

Although accipitrids are largely silent outside of the breeding season, during breeding they sometimes use calls to communicate with each other. Calls can be used to signal hunger, alarm or location, to solicit copulation or while defending a territory or engaged in an aggressive interaction. Calls are usually only heard during the breeding season, and rarely at other times of the year. ("Hawks and Eagles (Accipitridae)", 2003; Snyder, 2001; Thiollay, 1994)

Members of the Accipitridae have eyesight that is four to eight times better than that of humans. This visual acuity allows them to spot prey from far away. For example, eagles are able to spot a vole or lizard from more than 400 m away. Accipitrids rely heavily on their acute eyesight for catching prey. It is by far, the most important sense they use for hunting. However, they also use hearing to detect prey, and are attracted to the calls of their prey. There is little evidence that smell is an important way that raptors sense their environment. Even the carrion-specializing vultures do not have a well-developed sense of smell. ("Hawks and Eagles (Accipitridae)", 2003; Snyder, 2001)

Food Habits

Members of the family Accipitridae are generally opportunistic predators that eat whatever prey is most abundant, accessible and easy to catch. Individual species may specialize on a certain group of prey, but most will also eat a wide variety of prey if it is available. Prey items include birds (adults, chicks and eggs), mammals (from bats to lambs), reptiles (including lizards and snakes), amphibians, fish, carrion and many different invertebrates. Though most Accipitrids are exclusively carnivorous, several species occasionally eat fruit, including the fruits of the oil palm (Elaeis guineensis), which is important in the diet of the palm-nut vulture. ("Hawks and Eagles (Accipitridae)", 2003; Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

Individual species of accipitrids do often specialize on specific groups of prey. For example, most buteos eat primarily small mammals and most accipiters prey on birds. The bat hawk specializes on bats, and the snail kite specializes on apple snails. Other species are generalist predators that eat a variety of prey. For example, the common black-hawk counts birds, fish, crayfish and aquatic insect larvae among its prey. Carrion is the main staple in the diet of some species, namely vultures, but is incidental in the diets of many other species. Animal and human excrement are also significant parts of the diets of a few species, including Egyptian vultures and hooded vultures. (Snyder, 2001; Thiollay, 1994)

Accipitrids use a range of hunting techniques, which depend on their prey, their habitat and their morphology. The most common method of hunting is perch-hunting. This method is the least energy-demanding and allows the hunter to detect inconspicuous prey, and to avoid being detected by their prey until they are descending upon it. Hovering and soaring are techniques employed by the large-winged Accipitrids that hunt in open habitats. Like perch hunting, this technique allows Accipitrids to detect inconspicuous prey. Slow soaring is also used by vultures, which use their excellent eyesight to locate carcasses and to watch other soaring vultures in case they have located a carcass. Still other species use ambush hunting, the technique of hiding quietly and ambushing prey as it passes by. Finally, active flight is a hunting technique used by many of the insectivorous species and the bat hawk. Many species use a behavior called "stooping" to surprise their prey. From very high, the bird folds its wings back and plummets toward the prey, opening it's wings and swinging its feet forward just before striking the prey. Vultures and eagles descending to prey can reach speeds of 90 km/hr or more. ("Hawks and Eagles (Accipitridae)", 2003; Kemp and Newton, 2003; Snyder, 2001; Thiollay, 1994)

A few species of accipitrids may use cryptic coloration to get close to their prey. For example, the plumage of zone-tailed hawks (Buteo albonotatus) looks similar to turkey vultures (Cathartes aura). By imitating turkey vultures both in plumage and in manner of flying, this hawk may be able to approach prey without being noticed by prey that are habituated to the harmless turkey vultures. (Snyder, 2001; Thiollay, 1994)

Most accipitrids hunt solitarily. However, cooperative hunting does occasionally occur in several buzzard and eagle species as well as some other accipitrid species. Generally, cooperation occurs between mated pairs or groups of related individuals. Cooperation generally increases hunting success rate and allows for the capture of larger prey than could be accomplished by a single individual. For example, groups of closely related Harris's hawks sometimes work together to flush jackrabbits from cover and catch them as they emerge. Working together, these birds catch more prey than they would be able to catch alone, and are often able to catch enough prey to satisfy the energetic requirements of the entire group. (Snyder, 2001; Thiollay, 1994)

Once they have captured prey, accipitrids kill their prey by repeatedly puncturing it with their talons. Prey are swallowed whole or ripped apart with the sharp, hooked beak. Indigestible materials like fur, feathers and insect exoskeletons, if swallowed, are regurgitated in a pellet. If a prey item is not consumed in entirety, it may be cached for later consumption, particularly during the breeding season. (Snyder, 2001; Thiollay, 1994)

Accipitrids (not in captivity) must ingest 10 to 25 percent of their body weight in prey each day. This amount varies with climate and body size. Larger species require a smaller proportion of their body weight. Consumption requirements increase in winter compared to summer and in temperate compared to tropical climates. Accipiters are almost never seen drinking. They presumably ingest sufficient water from their food. (Thiollay, 1994)

Predation

Accipitrid eggs and chicks are vulnerable to climbing and aerial predators. Parents, therefore, expend considerable effort protecting their offspring from predation. Parents actively defend a territory around the nest from conspecifics and potential predators. They may also attempt to camouflage the nest by decorating it with both live and dead vegetation. (Snyder, 2001; Thiollay, 1994)

Chicks exhibit behaviors to evade predation from a very early age. When they detect a predator, chicks either lay low in the nest and remain still, or lie on their backs and strike at the predator with their talons. As soon as they are able to leave the nest, chicks spend most of their time perched near the nest, rather than in it, which may make them less visible to predators. (Thiollay, 1994)

  • Anti-predator Adaptations
  • cryptic
  • Known Predators

Ecosystem Roles

Accipitrids impact populations of their prey at a local scale. For example, research has shown that predation by rough-legged buzzards is probably responsible for the 3- to 4-year population cycles of lemmings in the Arctic tundra. Predation pressure by accipitrids also influences the evolution of behavior, habitat selection, and life history of their prey species. Accipitrids are also host to a number of skin and feather mites, some of which are harmful and others of which are beneficial. (Philips, 2000; Thiollay, 1994)

Accipitrids regularly use the activities of other species to improve their hunting efficiency. Insect- and snake-eaters follow troops of monkeys, catching the insects and tree snakes that the monkeys disturb. Some Accipitrids also engage in kleptoparasitism, stealing food from other raptors. (Thiollay, 1994)

Nests built by accipitrids are regularly used by other bird species (including other accipitrids). For example, great horned owls, barred owls, and merlins all use the nests of accipitrid species. (Snyder, 2001; Thiollay, 1994)

Commensal/Parasitic Species
  • feather and skin mites

Economic Importance for Humans: Positive

Humans have used raptors (both Accipitrids and falcons) for hunting and recreation in the form of falconry since as early as 2000 BC. Though this practice has largely died, it continues in some areas. Falconers in the United States use northern goshawks, Eurasian sparrowhawks and golden eagles to hunt prey including quails, partridges and pheasants and rabbits and hares. (Thiollay, 1994)

Wild raptors include game animals and some domesticated animals, such as lambs and poultry among their prey. This led to a long period of widespread discrimination against raptors, beginning in the late 1700’s, and continuing to the 1970’s, when it generally ended in most developed countries. At the height of this period of destruction, roughly between 1860 and 1960, many governments gave rewards for the slaughter of millions of raptors. (Thiollay, 1994)

Raptors can play an important role as bioindicators of habitat quality and pollution. In fact, they are already used in the tropics to monitor forest degradation. They may also help maintain the dynamics and diversity of ecosystems by lowering numbers of dominant prey, thereby allowing less common prey species to survive. (Thiollay, 1994)

Eagle feathers are used by various indigenous societies in religious celebrations. Eagle feathers are used in arrows by some of the forest-dwelling tribes in South America. Body parts of many species are used in traditional medicine by healers in many parts of Asia, Africa, and the Caribbean. Raptors have been included in the cuisine of many regions, and are still eaten in Taiwan and the Philippines, as well as China. (Thiollay, 1994)

  • Positive Impacts
  • pet trade
  • food
  • body parts are source of valuable material