Male canaries hone their songs to get girls, but their sexy tweets might do more than that.

A new study reveals that female songbirds alter the size of their eggs, and possibly their chicks' sex, in response to hearing a sexy song from a male.

Researchers played an assortment of male canary songs—ranging from what birds consider sexy to less attractive—to female domesticated canaries. More attractive songs induced larger eggs.

In the wild, larger eggs are more likely to contain male chicks, but the researchers found no difference in brood sex ratio between the different songs played to the females. Since the sexy song generated a large egg but not male chicks, the researchers suggest mothers can influence their offspring's sex in more than one way.

The study is detailed in the June 2006 issue of the journal Ethology.

Scientists have long known the power of a male canary's song. A good-sounding song tells females that its composer is healthy and has good genes, and males with attractive songs get more girls.