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Julie Craves explains why male birds usually outnumber females

Northern Cardinal, female, in Topsfield, Massachusetts, January 3, 2014, by Katherine Davis.
Northern Cardinal, female, in Topsfield, Massachusetts, January 3, 2014, by Katherine Davis.

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 October 2014 issue:

I’ve noticed that, among cardinals, more males than females show up at my feeder. Are the females shyer, or is there another reason they avoid the feeder? — Chase Proctor, Columbus, Ohio

You may be observing the phenomenon that the adult sex ratio in many bird species skews toward males. In a large study, adult males outnumbered females in about 60 percent of songbird species. Generally, the ratio of male to female chicks in the nest is about equal; the major cause of the male bias in adults is female mortality.

Female birds tend to have higher mortality rates for several reasons: Egg production is physically demanding, and in most songbirds, the female is the incubating parent. Incubation not only puts her at a higher risk from predators but also is an energetic strain.

Migration is always perilous, and in migratory species, the smaller sex tends to fly farther; in many species, this is the female. Differences in where and how each sex forages can also make females of some species more vulnerable to predation. — Julie Craves

About Julie Craves

Julie-Craves-120Julie 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 pageA version of this article was published in the October 2014 issue of BirdWatching. 

 

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