Handsome blue tit dads have more sons

A female blue tit with a particularly attractive mate is more likely to have sons than a female matched with a ho-hum guy is.

A new study showing this bias on a Swedish island marks the first evidence in the wild that a mate’s looks tilt the sex ratio of offspring, says Ben C. Sheldon of Uppsala University in Sweden.

Other work has shown that such factors as environmental resources can slant sex ratios. The idea that a mate’s good looks also could affect sex ratios received support from a study of zebra finches in the 1980s. It found that male finches artificially rendered more dashing—when researchers fitted red bands on birds’ legs—sired predominantly male offspring.

Good-looking sons offer extra grandchildren, Sheldon says, because a studly male fathers young in other nests besides his own. “The best males do better than the best females,” he says.

To test the idea in wild birds, he and his colleagues tracked males with various degrees of sex appeal. Female blue tits prefer males with more ultraviolet light (UV) reflecting from their crests (SN: 4/18/98, p. 252). In a group of 20 nests, sex ratios did indeed vary, Sheldon and his colleagues report in the Dec. 23/30, 1999 Nature. The high-UV suitors sired up to 70 percent sons, and the duds fathered clutches that were perhaps 30 percent male.

However, in 21 nests where researchers eliminated the UV cue by smearing the male’s crest with sunblock, the previously attractive birds did not have extra sons.

That bright-UV crest offers more than just eye candy, the researchers found. The more UV a male sports, the more likely he is to survive to a second season.

Just how the sex ratios shift is a mystery. Sheldon dismisses the possibility of selective abortion or resorption of wrong-sex eggs because he saw few gaps in the egg-a-day laying routine. Instead, he says he suspects that the ratio skew occurs at ovulation.

When the researchers considered male attractiveness and other factors, such as habitat quality, that also affect sex ratio, they could explain half the variation from the randomness predicted by classical genetics.

“Birds have a surprising ability to control what sex of offspring they produce,” Sheldon says. “They’re not prisoners of genetic laws.”