By Susan Milius
The largest comparison yet of his-and-hers bill lengths in hummingbirds shows that the more the difference in color between sexes, the longer the females’ bills tend to be.
The survey of 166 species also linked bill length and breeding systems, reports Robert E. Bleiweiss of the University of Wisconsin-Madison in the Dec. 22, 1999 Proceedings of the Royal Society of London B. In birds that cluster to breed, male bills tend to be proportionately equal to or longer than their mates’.