In the column Since You Asked in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here is a question from our December 2016 issue:
Why are so many male birds larger than female birds? – Jay Holmes, Enid, Oklahoma
Males are larger than females in most birds. The differences are pronounced in species in which males compete for females, and especially so in polygynous species, where males mate with more than one female. The situation is reversed in some birds, such as shorebirds, a family in which females mate with more than one male. In those cases, the females tend to be larger.
Another notable exception to male-biased size dimorphism is in raptors and owls, where a number of theories attempt to explain why females are larger than males. The most plausible explanation hinges on the relative rarity of live prey. If male and female raptors are different sizes, the disparity may limit competition between them. It is also thought that males evolved to be smaller because small live prey is more abundant than large prey, and the male does more hunting when the female is incubating. Additionally, females may be larger to support egg production and incubation. Among raptors, the size differences are especially conspicuous among accipiters and other species that specialize in fast, agile prey.
The largest disparity in size between the sexes is in the Great Bustard, Otis tarda, of Eurasia, where females are only a third the size of males. – Julie Craves
About Julie Craves
Julie is supervisor of avian research at the Rouge River Bird Observatory at the University of Michigan Dearborn and a research associate at the university’s Environmental Interpretive Center. She writes about her research on the blog Net Results, and she maintains the website Coffee & Conservation, a thorough resource on where coffee comes from and its impact on wild birds.
New to birdwatching?
Sign up for our free e-newsletter to receive news, photos of birds, attracting and ID tips, descriptions of birding hotspots, and more delivered to your inbox every other week. Sign up now.