The term hawk refers to birds of prey in any of three senses:
- Strictly, to mean any of the species in the bird sub-family Accipitrinae in the genera Accipiter, Micronisus, Melierax, Urotriorchis, and Megatriorchis. The large and widespread Accipiter genus includes goshawks, sparrowhawks, the Sharp-shinned Hawk and others. They are mainly woodland birds that hunt by sudden dashes from a concealed perch. They usually have long tails and high visual acuity.
- More generally, to mean small to medium-sized birds that are members of the Accipitridae, the family which includes the true hawks (Accipiters) and also eagles, kites, harriers, buzzards, and Old World vultures.
- Loosely, to mean almost any bird of prey.
The common names of birds in various parts of the world often use hawk loosely. For example, in North America, the Buteos are often called “hawks”.
In February 2005 the Canadian scientist Dr Louis Lefebvre announced a method of measuring avian IQ in terms of their innovation in feeding habits. Hawks were named among the most intelligent birds based on this scale.
Hawks are believed to have vision as good as 20/2, about eight times more acute than humans with good eyesight. This is because of many photoreceptors in the retina (Up to 1,000,000 per square mm, against 200,000 for humans), a very high number of nerves connecting the receptors to the brain, a second set of eye muscles not found in other animals, and an indented fovea which magnifies the central part of the visual field.