Fossils suggest that the first raptors appeared 30 to 50 million years ago. These early raptors looked similar to extant buzzards, though they are apparently not the predecessors of modern buzzards. Raptors probably became widespread before or during the Miocene.
Raptors have long played an important role in human history. Birds or prey have had religious or symbolic significance in many cultures, beginning with ancient civilizations and continuing with present-day societies. In the United States, bald eagles serve as the national emblem.
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.
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.
Known Predators:
Anti-predator Adaptations: cryptic
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.
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.
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.
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.
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.
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.
Other Physical Features: endothermic ; bilateral symmetry ; polymorphic
Sexual Dimorphism: female larger; sexes colored or patterned differently; male more colorful
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.
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.
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.
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.
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.
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.
Habitat Regions: temperate ; tropical ; terrestrial
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Aquatic Biomes: coastal
Wetlands: marsh ; swamp ; bog
Other Habitat Features: urban ; suburban ; agricultural ; riparian ; estuarine
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.
Biogeographic Regions: nearctic (Native ); palearctic (Native ); oriental (Native ); ethiopian (Native ); neotropical (Native ); australian (Native ); oceanic islands (Native )
Other Geographic Terms: holarctic ; cosmopolitan
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.
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.
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.
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.
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.
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.
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.
Foraging Behavior: stores or caches food
Primary Diet: carnivore (Eats terrestrial vertebrates, Piscivore , Eats eggs, Insectivore , Eats non-insect arthropods, Molluscivore , Scavenger )
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.
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.
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.
Commensal/Parasitic Species:
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.
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.
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.
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.
Positive Impacts: pet trade ; food ; body parts are source of valuable material; ecotourism ; controls pest population
Raptors are known to prey on game species, such as quails, partridges, pheasants and rabbits. They also occasionally take domesticated animals such as lambs and poultry.
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.
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.
Though accurate worldwide population estimates for most accipitrid species are unavailable, data on well-known species suggests that overall raptor numbers are declining. Sixty of the 237 species in the Accipitridae are listed on The World Conservation Union’s IUCN Red List of Threatened Species. Nine of these are critically endangered, 4 are endangered, 23 are vulnerable, 23 are near threatened and 1 is data deficient.
Human persecution through shooting, trapping and poisoning has historically been the most significant source of raptor mortality. Though such activities are now illegal in most developed countries, and raptors are protected by legislation such as the Migratory Bird Treaty Act, shooting and poisoning continue to be a significant source of raptor mortality. Between the 1940's and 1960's, many accipitrids were poisoned by widely used organochlorine pesticides, such as DDT. These pesticides accumulated in the prey that raptors ingested, and caused population declines in many species. Use of these pesticides has declined sharply, though they continue to be used in many countries where raptors or their prey spend the winter. Populations of many species that were poisoned by organochlorine pesticides during the middle 1900’s are now experiencing dramatic increases.
Habitat loss is the largest threat facing raptor populations today. However, nest disturbance may also be a significant source of local reproductive failure and population decline in raptor species. Repeated disturbances at the nest force adults to spend more time on nest defense, and less time feeding and caring for their young. Disturbance can also cause nest abandonment in shy species, including bald eagles and white-tailed sea-eagles. The many potential sources of nest disturbance include forestry activities, roads, off-road vehicles, recreational activities such as cliff climbing and hang-gliding, low-flying aircraft and military exercises.
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.
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.
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.
Communication Channels: visual ; acoustic
Perception Channels: visual ; tactile ; acoustic ; chemical
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.
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.
Mating System: monogamous ; polyandrous ; polygynous ; cooperative breeder
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.
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.
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.
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.
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.
Key Reproductive Features: iteroparous ; seasonal breeding ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); oviparous
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.
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.
Parental Investment: precocial ; male parental care ; female parental care
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The Accipitridae (/ˌæksɪˈpɪtrɪdiː, -deɪ/) is one of the three families within the order Accipitriformes,[2] and is a family of small to large birds of prey with strongly hooked bills and variable morphology based on diet. They feed on a range of prey items from insects to medium-sized mammals, with a number feeding on carrion and a few feeding on fruit. The Accipitridae have a cosmopolitan distribution, being found on all the world's continents (except Antarctica) and a number of oceanic island groups. Some species are migratory. The family contains 255 species which are divided into 70 genera.
Many well-known birds such as hawks, eagles, kites, harriers and Old World vultures are included in this group. The osprey is usually placed in a separate family (Pandionidae), as is the secretary bird (Sagittariidae), and the New World vultures are also usually now regarded as a separate family or order. Karyotype data[3][4][5] indicate the accipitrids analysed are indeed a distinct monophyletic group.
The accipitrids have been variously divided into some five to ten subfamilies. Most share a very similar morphology, but many of these groups contain taxa that are more aberrant. These are placed in their respective position more for lack of better evidence than anything else. The phylogenetic layout of the accipitrids has thus always been a matter of dispute.
The accipitrids are recognizable by a peculiar rearrangement of their chromosomes.[6] Apart from this, morphology and mtDNA cytochrome b sequence data give a confusing picture of these birds' inter-relationships. The hawks, kites, eagles and Old World vultures as presently assigned in all likelihood do not form monophyletic groups.
According to the molecular data, the Buteoninae are most likely poly- or paraphyletic, with the true eagles, the sea eagles, and the buteonine hawks apparently representing distinct lineages. These appear to form a group with the Milvinae, Accipitrinae and Circinae, but the exact relationships between the lineages are not robustly resolvable. The Perninae and possibly the Elaninae are older lineages, as are the Old World vultures. The latter are likely also poly- or paraphyletic, with some aberrant species like the bearded and Egyptian vultures standing apart from the naked-necked "true" vultures.[7]
Below is the taxonomy after the phylogeny from the studies of Mindell et al. (2018),[8] Starikov & Wink (2020),[9] and Sangster et al. (2021).[10] The family contains 255 species and is divided into 70 genera.[11]
As with most other birds of prey, the fossil record of this group is fairly complete from the latter Eocene onwards (c.35 mya), with modern genera being well documented since the Early Oligocene, or around 30 mya.
Accipitrids are known since Early Eocene times, or about from 50 mya onwards, but these early remains are too fragmentary and/or basal to properly assign a place in the phylogeny. Likewise, molecular methods are of limited value in determining evolutionary relationships of and within the accipitrids. The group may have originated on either side of the Atlantic, which during that time was only 60–80% its present width. As evidenced by fossils like Pengana, some 25 mya, accipitrids in all likelihood rapidly acquired a global distribution, initially probably extending even to Antarctica.
Specimen AMNH FR 2941, a left coracoid from the Late Eocene Irdin Manha Formation of Chimney Butte (Inner Mongolia), was initially assessed as a basal mid-sized buteonine;[18] it is today considered to be more likely to belong in the Gruiformes genus Eogrus.[19] The Early Oligocene genus Cruschedula was formerly thought to belong to Spheniscidae, however reexamination of the holotype in 1943 resulted in the genus being placed in Accipitridae.[20] Further examination in 1980 resulted in placement as Aves incertae sedis.[21]
The Accipitridae are a diverse family with a great deal of variation in size and shape. They range in size from the tiny pearl kite (Gampsonyx swainsonii) and little sparrowhawk (Accipiter minullus), both of which are 23 cm (9 in) in length and weigh about 85 g (3 oz), to the cinereous vulture (Aegypius monachus), which measures up to 120 cm (47 in) and weighs up to 14 kg (31 lbs). Wingspan can vary from 39 cm (15 in) in the little sparrowhawk to more than 300 cm (120 in) in the cinereous and Himalayan vultures (Gyps himalayensis). In these extreme species, wing chord length can range from 113 to 890 mm (4.4 to 35.0 in) and culmen length from 11 to 88 mm (0.43 to 3.46 in). Until the 14th century, even these huge vultures were surpassed by the extinct Haast's eagle (Hieraaetus moorei) of New Zealand, which is estimated to have measured up to 140 cm (55 in) and to have weighed 15 to 16.5 kg (33 to 36 lb) in the largest females.[22][23] In terms of body mass, the Accipitridae are the most diverse family of birds and may also be in terms of some aspects of linear size diversity, although lag behind the true parrots and pheasant family in length diversity.[24] Most accipitrids exhibit sexual dimorphism in size, although, unusually for birds, it is the females that are larger than the males.[25] This sexual difference in size is most pronounced in active species that hunt birds, such as the Accipiter hawks, in which the size difference averages 25–50%. In a majority of species, such as generalist hunters and rodent-, reptile-, fish-, and insect-hunting specialists, the dimorphism is less, usually between a 5% to 30% size difference. In the carrion-eating Old World vultures and snail eating kites, the difference is largely non-existent, though sometimes the female may average slightly heavier.[24]
The beaks of accipitrids are strong and hooked (sometimes very hooked, as in the hook-billed kite or snail kite). In some species, there is a notch or 'tooth' in the upper mandible. In all accipitrids, the base of the upper mandible is covered by a fleshy membrane called the cere, which is usually yellow in colour. The tarsi of different species vary by diet; those of bird-hunting species, such as sparrowhawks, are long and thin, whilst species that hunt large mammals have much thicker, stronger tarsi, and the tarsi of the snake-eagles have thick scales to protect from bites.
The plumage of the Accipitridae can be striking, but rarely utilises bright colours; most birds use combinations of white, grey, buff, brown and black.[26] Overall they tend to be paler below, which helps them seem less conspicuous when seen from below. There is seldom sexual dimorphism in plumage, when it occurs the males are brighter or the females resemble juveniles. In many species juveniles have a distinctly different plumage. Some accipitrids mimic the plumage patterns of other hawks and eagles. Resembling a less dangerous species may fool prey; resembling a more dangerous species may reduce mobbing by other birds.[27] Several species of accipitrid have crests used in signalling, and even species without crests can raise the feathers of the crown when alarmed or excited. In contrast most of the Old World vultures possess bare heads without feathers; this is thought to prevent soiling on the feathers and aid in thermoregulation.[28]
The senses of the Accipitridae are adapted to hunting (or scavenging), and in particular their vision is legendary. The sight of some hawks and eagles is up to 8 times better than that of humans. Large eyes with two foveae provide binocular vision and a "hawk eye" for movement and distance judging. In addition the Accipitridae have the largest pectens of any birds. The eyes are tube shaped and cannot move much in their sockets. In addition to excellent vision many species have excellent hearing, but unlike in owls sight is generally the principal sense used for hunting. Hearing may be used to locate prey hidden in vegetation, but sight is still used to catch the prey. Like most birds the Accipitridae generally have a poor sense of smell; even the Old World vultures make no use of the sense, in contrast to the New World vultures in the family Cathartidae.
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Accipitrids are predominantly predators and most species actively hunt for their prey. Prey is usually captured and killed in the powerful talons of the raptor and then carried off to be torn apart with a hooked bill for eating or feeding to nestlings. A majority of accipitrids are opportunistic predators that will take any prey that they can kill. However, most have a preference for a certain type of prey, which in harriers and the numerous buteonine hawks (including more than 30 species in the genus Buteo) tends towards small mammals such as rodents.
Among the raptors that mainly favor small mammals, harriers generally hunt by hovering over openings until they detect their prey and descend upon them. Due to the specificity of their hunting style, prey preferences, and habitat preferences, usually only one harrier species tends to be found per region.[29]
Buteonine hawks usually watch for prey from a perch but most species will also readily hunt on the wing, including from a high soar. Many buteonines are amongst the most generalized feeders, often feeding on any active small animal they find, and will generally eat whatever diurnal rodent or lagomorph is most locally common. Some buteonines, however, are more specialized, such as certain species in the genus Buteogallus, which have evolved to specialize in feeding on crabs. Larger Buteogallus, namely the solitary eagles, and Geranoaetus are much larger than other buteonines and seem to have become avian apex predators of specific habitat niches—for example, savanna, cloud forest and páramo in South America—and are thus honorary "eagles".[30][31]
In Accipiter hawks (the most species-rich accipitrid genus with nearly 50 extant species), prey is mainly other birds. Accipiters are in general forest- and thicket-dwelling species. Accipiter hawks usually ambush birds in dense vegetation, a dangerous hunting method that requires great agility. Many smaller tropical species of Accipiter eat nearly equal portions of insects and reptiles and amphibians as they do of birds while some of the larger species have become more generalized and may feed extensively on rodents and lagomorphs, as well as other various non-avian animals.
Most accipitrids will supplement their diet with non-putrid carrion, but none are specialized for this as well as the 14–16 species of vultures, which have evolved very large bodies (which leave them equipped to fill their crop with carrion); weaker, less specialized feet than other accipitrids; large wingspans to spend long periods of time in flight over openings scanning for carcasses; and complex social behavior in order to establish a mixed species hierarchy at carrion. The New World vultures have attained several similar characteristics, but only through convergent evolution, and are seemingly not directly related to Old World vultures and other accipitrids. The lammergeier (Gypaetus barbatus) is an aberrant cousin of the Old World vultures that has maintained strong feet that it uses to carry and drop large bones in order to crack them open to feed on bone marrow, their primary food, a technique they also sometimes use for live prey items, like tortoises.[24]
A few species may opportunistically feed on fruit. In one species, the palm-nut vulture (Gypohierax angolensis) (possibly not closely related to other "vultures"), it may form more than half of the diet.[32] Most accipitrids will not eat plant material.
Insects are taken exclusively by around 12 species, in great numbers by 44 additional species, and opportunistically by a great many others.[26] The diet of the honey-buzzards includes not only the adults and young of social insects such as wasps and bees, but the honey and combs from their nests.[33]
The snail kite (Rostrhamus sociabilis), slender-billed kite (Helicolestes hamatus) and hook-billed kites (Chondrohierax uncinatus) are specialists in consuming snails, which usually constitute 50–95% of their diet. Other "kites"—a loose assemblance of smallish raptors, many of which are strong, buoyant fliers—are divided into two groups. One, exclusively in the Old World, the milvine or "large" kites, are often quite common, very generalized and often weakly predaceous feeders whereas the other kites, known as elanine or "small" kites and cosmopolitan in distribution, are supremely aerial, active hunters that generally alternate their primary food between insects and small mammals. One species allied with the latter kite group, the bat hawk (Macheiramphus alcinus), has come to specialize in hunting bats.[34]
"Eagles" are several raptors that are not necessarily closely related, but can be broadly defined by large body size (larger than other raptors, excluding vultures) and the taking of typically larger prey, including mid-sized mammals and larger birds. The most diverse group of eagles is the "booted eagles", a variable group of about 38 species defined by their feathering covering their legs (shared by only a couple of buteonine species).
Most accipitrids usually hunt prey smaller than themselves. However, many accipitrids of almost all sizes have been recorded as capturing and then flying with prey of equal weight or even slightly heavier than themselves in their talons, a feat that requires great strength. Occasionally, an eagle or other raptor that kills prey considerably heavier than itself (too heavy for the raptor to carry and fly with) will then have to leave prey at the site of the kill and later return repeatedly to feed or dismember and bring to a perch or nest piece by piece. This has the advantage of providing a surplus of food but has the disadvantage of potentially attracting scavengers or other predators which can steal the kill or even attack the feeding accipitrid. Using this method, accipitrids such as the golden eagle (Aquila chrysaetos), wedge-tailed eagle (Aquila audax), martial eagle (Polemaetus bellicosus) and crowned eagle (Stephanoaetus coronatus) have successfully hunted ungulates, such as deer and antelope, and other large animals (kangaroos and emus in the wedge-tailed) weighing more than 30 kg (66 lb), 7–8 times their own mass. More typical prey for these powerful booted eagle species weigh between 0.5 and 5 kg (1.1 and 11.0 lb).[24][35]
The Haliaeetus eagles and the osprey (Pandion haliaetus) mainly prefer to prey on fish, which comprising more than 90% of food for the osprey and some fish eagles. These large acciptrids may supplement their diets with aquatic animals other than fish, especially sea eagles, which also hunt large numbers of water birds and are expert kleptoparasites.
Reptiles and amphibians are hunted by almost all variety of acciptrids when the opportunity arises and may be favored over other prey by some eagles, i.e. Spizaetus hawk-eagles and the "eagles" in Buteogallus, and several species of buteonine hawks found in the tropics. Bazas and forest hawks in the genus Accipiter may take reptiles from trees whilst other species may hunt them on the ground. Snakes are the primary prey of the snake-eagles (Circaetus) and serpent-eagles (Spilornis and Dryotriorchis). The mammal-hunting, huge and endangered Philippine eagle (Pithecophaga jefferyi) is most closely related to the snake-eagles.[26][24] Another striking aberration of the snake-eagle lineage is the bateleur (Terathopius ecaudatus), which has evolved unusually bright plumage in adults, with a huge red cere, red feet, bright yellow bill, and boldly contrasting grey-and-white markings over black plumage. The bateleur feeds extensively on carrion and almost any other feeding opportunity that presents itself.[36][37]
In terms of their reproductive biology and socio-sexual behavior, accipitrids share many characteristics with other extant groups of birds that appear not be directly related, but all of which have evolved to become active predators of other warm-blooded creatures. Some of the characteristics shared with these other groups, including falcons, owls, skuas and shrikes, are sexual dimorphism in size, with the female typically larger than the male; extreme devotion of breeding pairs to each other or to a dedicated nesting site; strict and often ferocious territorial behavior; and, on hatching, occasional competition amongst nestlings, including regular siblicide in several species.
Before the onset of the nesting season, adult accipitrids often devote a majority of their time to excluding other members of their own species and even of other species from their nesting territories. In several species, this occurs by territorial display flights over the border of their breeding ranges. In several forest dwelling varieties, however, vocalizations are used to establish territories. Due to the density of the habitat, display flights are apparently impractical.
While a single devoted breeding pair is considered typical, research has revealed that in varied accipitrids, multiple birds engaging in nesting behavior is more commonly than previously thought. Some harriers have evolved to become polygynous, with a single smaller male breeding with and then helping multiple females raise young.[38] The most extreme known species of accipitrid in terms of sociality is the Harris's hawks (Parabuteo unicinctus), which up to seven fully-grown birds may hunt, nest and brood cooperatively, with the extra birds typically being prior years' offspring of the breeding pair.[39][40]
Unlike the other two larger groups of raptorial birds, the owls and most falcons, accipitrids typically build their own nest. Nest sites are typically in relatively secure places, such as the crook of a large tree or an ample cliff ledge, and can vary in elevation from the flat ground of prairies or steppe to near the peaks of the tallest mountains. Accipitrids will readily return to use a nest site repeatedly, which has resulted in several of the largest bird's nests known, as a single nest may see decades of use, with more material added each breeding season. The single largest known tree nest known for any animal, belonging to a bald eagle (Haliaeetus leucocephalus), was found to be 6.1 m (20 ft) deep and 2.9 meters (9.5 ft) across, and to weigh 3 short tons (2.7 metric tons).[41] Some species, especially eagles, will build multiple nests for use in alternating years. Although they usually use nests they build themselves, accipitrids sometimes use abandoned nests build by other animals or pirate nests from other birds, typically other types of accipitrid.
Compared to most other types of birds, the stretch from egg-laying to independence in young birds is prolonged. In accipitrids, the breeding season ranges from about two to three months to roughly a year and a half, the latter in some of the larger tropical eagles. Species inhabiting temperate ranges as a rule have shorter breeding seasons due to the shorter stretches of warm weather that facilitates ready capture of prey.
Usually from 2 to 6 eggs are laid in accipitrids, a relatively small clutch, and some species may lay only one egg. In almost all accipitrids, eggs are laid at intervals rather than all at once and in some larger species the intervals can be several days. This results in one of the hatchlings being larger and more advanced in development than its siblings. The benefits of siblicide, which is at least occasionally recorded in many species and almost always occurs in some, such as tropical members of the booted eagle group, is that the smaller siblings are a kind of insurance policy that if the oldest, strongest nestling dies, one of the smaller siblings may take its place. In most species that have displayed siblicide, times of food plenty may result in two or more the nestlings being successfully raised to fledging.
In most accipitrids, the smaller males typically obtain food both for the incubating and brooding female and the nestlings. Males, however, occasionally take a shift incubating or even more sporadically brooding the nestlings, which allows the female to hunt. Most accipitrids feed their nestlings strips of meat or whole prey items, but most vultures feed their nestlings via regurgitation.
Fledgling often takes considerable effort for young birds and may take several weeks as opposed to days in many other types of birds. Once independent of their parents, young accipitrids often most wander for considerable stretches of time, ranging from 1 to 5 years, before they attain maturity. Most accipitrids have distinct plumages in their immature stage, which presumably serves as a visual cue to others of their species and may allow them to avoid territorial fights. Shortly after attaining mature plumages, pairs form, with a male typically displaying, often in flight but sometimes vocally, to win over a female. Many accipitrids breed with the same mate for several years or for life, although this is not the case for all species and, if a mate dies, the widowed bird will typically try to find another mate the next breeding season.[24][42]
Roadside hawk (Rupornis magnirostris griseocauda) eating speckled racer (Drymobius margaritiferus), Belize The Accipitridae (/ˌæksɪˈpɪtrɪdiː, -deɪ/) is one of the three families within the order Accipitriformes, and is a family of small to large birds of prey with strongly hooked bills and variable morphology based on diet. They feed on a range of prey items from insects to medium-sized mammals, with a number feeding on carrion and a few feeding on fruit. The Accipitridae have a cosmopolitan distribution, being found on all the world's continents (except Antarctica) and a number of oceanic island groups. Some species are migratory. The family contains 255 species which are divided into 70 genera.
Many well-known birds such as hawks, eagles, kites, harriers and Old World vultures are included in this group. The osprey is usually placed in a separate family (Pandionidae), as is the secretary bird (Sagittariidae), and the New World vultures are also usually now regarded as a separate family or order. Karyotype data indicate the accipitrids analysed are indeed a distinct monophyletic group.
