Falconiformes comprises a vast group of species, commonly known as birds of prey or diurnal raptors. It now seems certain that they constitute a polyphy-letic group and they must be divided into two orders, Accipitriformes and Falconiformes, to obtain monophyletic groupings. Nevertheless, many systematic uncertainties remain, because fossils able to confirm the existence of a common ancestor have not been found, even at the level of single families. Five families have been described: Cathartidae, the American vultures; Pandionidae, with a single cosmopolitan species; Accipitridae, with the greatest number of genera and species; Sagittaridae, with a single species exclusive to Africa; Falconidae, probably a separate order.
Because of the strong adaptive radiation (even within families), there are few traits shared by all species. Body size is extremely variable, even among the Italian species, ranging from 25-30 cm total length and 150-200 g weight of the Merlin (Falco columbarius) to 100 cm and 10-12 kg of the Black Vulture (Aegypius monachus). The shape of the wing is extremely variable, according to the type of predation and habitat. There are generally 10 long primaries, used for flight, and one short primary. In the genus Accipiter, the latter is absent. There are usually 12 retrices in the small-medium species, 14 in the big vultures of the genus Gyps. The bill is curved and hooked, adapted for snatching and tearing flesh. The horny structure of the bill terminates at the base with a naked area of skin forming a characteristic cere, where the nostrils open. The structure of the tarsi, legs and talons varies according to the type of prey: long toes and sharp curved claws in the ornithophagous species, short toes and strongly curved talons in those that capture reptiles, slightly curved talons in the genus Pernis and in vultures. The claw of the posterior toe is generally the longest. Unlike in other groups of birds, the right ovary and oviduct are developed and functional. The senses of hearing and sight are very well developed. The density of cones in the retina is the highest of all vertebrates. Some species present sexual dimorphism, with females larger than males. Small species have complete annual moults, while large species have continuous moults. Therefore, adults of the large species present plumage formed by feathers of different generations. The chicks are semi-altricial and nidicolous.
In Italy, 40 species have been recorded, belonging to 3 families: Pandionidae, Accipitridae and Falconidae. The following accidental species in Italy are not considered in this volume: Black-winged Kite Elanus caeruleus (3 records), Levant Sparrowhawk Accipiter brevipes (2 records), Tawny Eagle Aquila rapax (2 records), Steppe Eagle Aquila nipalensis (8 records), Imperial Eagle Aquila heliaca (15 records), Amur Falcon Falco amurensis (1 record), Barbary Falcon Falco pelegrinoides (9 records).