The extent of the size difference between female raptors and male raptors, known as reversed size dimorphism, varies between species, being most extreme in the sparrowhawk.
Studies in the 1970s revealed that the degree of dimorphism was linked to diet: species that fed on slow-moving prey (eg scavengers such as vultures) showed little dimorphism, while those that fed on the fastest prey (eg hawks that hunt other birds) were the most dimorphic.
The demands of catching fast and agile prey favour a predator whose size and weight closely match those of its quarry. However, size and fat reserves in female raptors are linked to breeding success, so this additional selection pressure falling on females may lead to size dimorphism.