In most birds, males are larger than females, but in some birds, such as many shorebirds and birds of prey, the reverse is true. No one is certain why there is this "reversed sexual size dimorphism" in raptors, but a number of interesting hypotheses have been advanced. All are based on a well-established correlation this size difference between the sexes is less pronounced in species that pursue sluggish prey than in those that pursue birds. Vultures, whose prey are least agile of all, show little sexual size difference. Mammal-hunting buteos, such as the Red-tailed Hawk, evolved males that are somewhat smaller than females, whereas in bird-hunting accipiters and falcons, females may be half again as heavy as males.
One explanation for the females' larger size suggests that it protects them from aggressive males that are well equipped with sharp talons and beaks, and the killer instincts to go with them. According to this theory, over evolutionary time, females have preferred to mate with smaller, safer males – in fact, the female may have to be able to dominate the male for proper pair bonding to occur and for the male to remain in his key role as food provider to both female and young. Such a system would involve sexual selection for smaller size in males. Bird-hunting raptors are assumed to show aggression most suddenly, and to represent the greatest threat to their mates, and they are the ones exhibiting the greatest size difference.
In experimental pairings set up so that male American Kestrels were the larger of the pair, Cornell ornithologist Tom Cade found that the females did not suffer from an avian version of wife abuse. The size difference in kestrels was not very great, however, so this may not be a definitive test of the hypothesis. It seems that while sexual selection may play a role, there probably is more to it.
Another hypothesis proposes that the size differences allow the two sexes to hunt different prey and thus reduce competition for food. Competition is thought to be more severe among bird hunters than among other hawks, since their small agile prey are able to flee in three dimensions and are thus effectively scarcer than, say, carrion or ground squirrels. Indeed, there are data indicating that the hunting success of bird-chasing raptors is only about half that of raptors preying on mammals, and only a sixth that of raptors eating insects. Tom Cade has suggested that, for bird eaters, available food supply in the nesting territory can become limiting, making it adaptive for the male to specialize on small prey and for the female to specialize on large prey. The male feeds the female and young at the beginning of the nesting season; the female becomes an active hunter when the nestlings are larger, and the adults then tend to partition the prey resource in their territory.
But if reducing intersexual competition for food is the reason for the size difference in raptors, why aren't males sometimes the larger sex? One possible reason is that females need to be larger because they must accumulate reserves in order to produce their eggs. Another is that females do not forage for a substantial period while incubating eggs and brooding young. They avoid the risks of the hunt during that time, but they must rely on the small male to feed the entire family. Small fleet prey, aerial or terrestrial, are more abundant than large sluggish prey, so that over time smaller male bird-eating raptors would be favored over larger, less agile ones, because they would be better providers. For species that take more sluggish prey, however, small males would not be so advantageous, which might explain the relationship between prey speed and the male-female size discrepancy.