Female hawks are generally larger than males, a phenomenon known as Reverse Sexual Dimorphism (RSD). This size difference is an exception to the rule observed across most animal species, where males tend to be the larger sex. This size disparity is a characteristic of nearly all birds of prey, including hawks, falcons, and owls. The evolution of this pattern has been the subject of study for biologists, leading to several theories about its cause.
Defining Reverse Sexual Dimorphism
Reverse Sexual Dimorphism describes a consistent physical difference between the sexes of a species where the female is notably larger than the male. In hawks, this difference can range from minimal to extreme, depending on the species. Females are often measured by weight, wingspan, or tarsus length, and can be anywhere from 5% to over 50% heavier than their male counterparts.
For example, a female Sharp-shinned Hawk can weigh nearly 83% more than the male. This size disparity provides one of the few reliable ways to distinguish the sexes in many hawk species that have similar plumage.
Evolutionary Theories for Size Difference
The most discussed theory for the evolution of RSD is the Niche Partitioning Hypothesis, which suggests that the size difference reduces competition for food between the male and female. By being smaller, the male can hunt smaller, more agile prey, while the larger female can focus on bigger prey items. This specialization allows the pair to utilize a broader range of the available food resources within their shared territory.
Another prominent idea is the Clutch Defense or Nest Defense Hypothesis, which posits that a larger female is better equipped to protect the eggs and nestlings from predators. Since the female does most of the incubating and brooding, her increased size acts as a deterrent, allowing her to more successfully defend her offspring. This selective pressure would favor genes for larger size in females, contributing to the observed dimorphism.
A third, less supported theory relates to handling aggression during the breeding season. This hypothesis suggests that a smaller male is less likely to injure the larger female during copulation or courtship feedings, or that the female needs to be larger to survive potential male aggression. However, some research suggests that the evolution of RSD is primarily driven by natural selection favoring smaller, more efficient foraging males.
How Size Variation Differs Among Species
The degree of reverse sexual dimorphism is not uniform across all hawk species, and the extent of the difference is often linked to the hawk’s hunting style and prey. Species that primarily hunt slow-moving or terrestrial prey, such as many of the large, soaring buteos like the Red-tailed Hawk, exhibit a relatively modest size difference. A female Red-tailed Hawk may average only about half a pound heavier than the male.
In contrast, the size disparity is most pronounced in accipiters and falcons, which are species that hunt fast, agile prey, particularly other birds. The Sharp-shinned Hawk and the Northern Goshawk are examples of accipiters where the female can be significantly heavier than the male. This extreme RSD is thought to be an adaptation that allows the smaller, more maneuverable male to be a highly effective hunter of swift avian prey, ensuring a steady food supply for the larger female and the young.