Size differences between males and females are huge in the Great Bustard. Photo by Andrej Chudý, via Wikimedia Commons.

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.

Read other questions that Julie has answered in “Since You Asked.”

If you have a question about birds for Julie, send it to [email protected] or visit our Contact page.

Reader photos of Elegant Trogon show how to tell females from males.

How to distinguish male and female waxwings.

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