Birds of prey, also known as raptors, hunt and feed on other animals. The term "raptor" is derived from the Latin word rapere (meaning to seize or take by force). These birds are characterized by keen vision that allows them to detect prey during flight and powerful talons and beaks.
Many species of birds may be considered partly or exclusively predatory. However, in ornithology, the term "bird of prey" applies only to birds of the families listed below. Taken literally, the term "bird of prey" has a wide meaning that includes many birds that hunt and feed on animals and also birds that eat very small insects. In ornithology, the definition for "bird of prey" has a narrower meaning: birds that have very good eyesightfor finding food, strong feet for holding food, and a strong curved beak for tearing flesh. Most birds of prey also have strong curved talons for catching or killing prey. An example of this difference in definition, the narrower definition excludes storks and gulls, which can eat quite large fish, partly because these birds catch and kill prey entirely with their beaks, and similarly bird-eating skuas, fish-eating penguins, and vertebrate-eating kookaburras are excluded. Birds of prey generally prey on vertebrates, which are usually quite large relative to the size of the bird. Most also eat carrion, at least occasionally, and vultures and condors eat carrion as their main food source. Many raptor species are considered apex predators.
The order Accipitriformes is inferred to have originated about 44 million years ago when it split from the common ancestor of the secretarybird (Sagittarius serpentarius) and the rest of the accipitrid species. The phylogeny of Accipitriformes is complex and difficult to unravel. Widespread paraphylies were observed in many phylogenetic studies. Unfortunately more recent and detailed studies show similar results. However, according to the findings of a 2014 study, the sister relationship between larger clades of Accipitriformes was well supported (e.g. relationship of Harpagus kites to buzzards and sea eagles and these latter two with Accipiter hawks are sister taxa of the clade containing Aquilinae and Harpiinae).
The diurnal birds of prey are formally classified into five families of two orders.
- Accipitridae: hawks, eagles, buzzards, harriers, kites and Old World vultures
- Pandionidae: the osprey
- Sagittariidae: the secretarybird
- Falconidae: falcons, caracaras and forest falcons
- Cathartidae: New World vultures
These families were traditionally grouped together in a single order Falconiformes, however are now split into two orders: Falconiformes and Accipitriformes. The Cathartidae are sometimes placed separately in an enlarged stork family (Ciconiiformes), and may be raised to an order of their own (Cathartiiformes).
The secretary bird and/or osprey are sometimes listed as subfamilies of Acciptridae: Sagittariinae and Pandioninae, respectively.
Australia's letter-winged kite is a member of the family Accipitridae, although it is a wholly nocturnal bird.
The nocturnal birds of prey – the owls – are classified separately as members of two extant families of the order Strigiformes:
- Strigidae: "typical owls"
- Tytonidae: barn and bay owls
The taxonomy of Carl Linnaeus grouped birds (class Aves) into orders, genera and species, with no formal ranks between genus and order. He placed all birds of prey into a single order, Accipitres, subdividing this into four genera: Vultur (vultures), Falco (eagles, hawks, falcons, etc.), Strix (owls), and Lanius (shrikes). This approach was followed by subsequent authors such as Gmelin, Latham, and Turnton.
Louis Pierre Veillot used additional ranks: order, tribe, family, genus, species. Birds of prey (order Accipitres) were divided into diurnal and nocturnal tribes; the owls remained monogeneric (family Ægolii, genus Strix), whilst the diurnal raptors were divided into three families: Vulturini, Gypaëti, and Accipitrini.
Thus Veillot's families were similar to the Linnaean genera, with the difference that shrikes were no longer included amongst the birds of prey. In addition to the original Vultur and Falco (now reduced in scope), Veillot adopted four genera from Savigny: Phene, Haliæetus, Pandion, and Elanus. He also introduced five new genera of vultures (Gypagus, Catharista, Daptrius, Ibycter, Polyborus) and eleven new genera of accipitrines (Aquila, Circaëtus, Circus, Buteo, Milvus, Ictinia, Physeta, Harpia, Spizaëtus, Asturina, Sparvius).
The common names for various birds of prey are based on structure, but many of the traditional names do not reflect the evolutionary relationships between the groups.
- Eagles tend to be large birds with long, broad wings and massive feet. Booted eagles have legs and feet feathered to the toes and build very large stick nests.
- Ospreys, a single species found worldwide that specializes in catching fish and builds large stick nests.
- Kites have long wings and relatively weak legs. They spend much of their time soaring. They will take live vertebrate prey, but mostly feed on insects or even carrion.
- The true hawks are medium-sized birds of prey that usually belong to the genus Accipiter (see below). They are mainly woodland birds that hunt by sudden dashes from a concealed perch. They usually have long tails for tight steering.
- Buzzards are medium-large raptors with robust bodies and broad wings, or, alternatively, any bird of the genus Buteo (also commonly known as "hawks" in North America).
- Harriers are large, slender hawk-like birds with long tails and long thin legs. Most use a combination of keen eyesight and hearing to hunt small vertebrates, gliding on their long broad wings and circling low over grasslands and marshes.
- Vultures are carrion-eating raptors of two distinct biological families: the Accipitridae, which only occurs in the Eastern Hemisphere; and the Cathartidae, which only occurs in the Western Hemisphere. Members of both groups have heads either partly or fully devoid of feathers.
- Falcons are medium-size birds of prey with long pointy wings. Unlike most other raptors, they belong to the Falconidae, rather than the Accipitridae. Many are particularly swift flyers.
- Caracaras are a distinct subgroup of the Falconidae unique to the New World, and most common in the Neotropics – their broad wings, naked faces and appetites of a generalist suggest some level of convergence with either the Buteos or the vulturine birds, or both.
- Owls are variable-sized, typically night-specialized hunting birds. They fly almost silently due to their special feather structure that reduces turbulence. They have particularly acute hearing.
Many of these English-language group names originally referred to particular species encountered in Britain. As English-speaking people travelled further, the familiar names were applied to new birds with similar characteristics. Names that have generalized this way include: kite (Milvus milvus), sparrow-hawk or sparhawk (Accipiter nisus), goshawk (Accipiter gentilis), kestrel (Falco tinninculus), hobby (Falco subbuteo), harrier (simplified from "hen-harrier", Circus cyaneus), buzzard (Buteo buteo).
The earliest event occurred nearly 14–12 million years ago. This result seems to be one of the oldest dates published so far in the case of birds of prey. For example, a previous reconstruction of migratory behaviour in one Buteo clade with a result of the origin of migration around 5 million years ago was also supported by that study.
Migratory species of raptors had a southern origin because it seems that all of the major lineages within Accipitridae had an origin to one of the biogeographic realms of the Southern Hemisphere. The appearance of migratory behaviour occurred in the tropics parallel with the range expansion of migratory species to temperate habitats. Similar results of southern origin in other taxonomic groups can be found in the literature.
Distribution and biogeographic history highly determine the origin of migration in birds of prey. Based on some comparative analyses, diet breadth also has an effect on the evolution of migratory behaviour in this group, but its relevance needs further investigations. The evolution of migration in animals seems to be a complex and difficult field with many unanswered questions.
Raptors are known to display patterns of sexual dimorphism. It is commonly believed that the dimorphisms found in raptors occur due to sexual selection or environmental factors. In general, hypotheses in favor of ecological factors being the cause for sexual dimorphism in raptors are rejected. This is due to the fact that the ecological model is less parsimonious, meaning that its explanation is more complex than that of the sexual selection model. Additionally, ecological models are much harder to test for due to the fact that a great deal of data is required.
Dimorphisms can also be the product of intrasexual selection between males and females. It appears that both genders of the species play a role in the sexual dimorphism within raptors; females tend to compete with other females to find good places to nest and attract males, and males competing with other males for adequate hunting ground so they appear as the most healthy mate. It has also been proposed that sexual dimorphism is merely the product of disruptive selection, and is merely a stepping stone in the process of speciation, especially if the traits that define gender are independent across a species. Sexual dimorphism can be viewed as something that can accelerate the rate of speciation.
In non-predatory birds, males are typically larger than females. However, in birds of prey, the opposite is the case. For instance, take into account the kestrel, a type of falcon in which males are the primary providers, and the females are responsible for nurturing the young. In this species, the smaller kestrels are, the less food is needed and thus, they can survive in environments that are harsher. This is particularly true in the male kestrels. It has become more energetically favorable for male kestrels to remain smaller than their female counterparts because of the fact that smaller males have an agility advantage when it comes to defending the nest and hunting. Larger females are favored because they can incubate larger numbers of offspring, while also being able to breed a larger clutch size.