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’.
“Think of it as who’s first at table,” Bleiweiss says. Dominant birds get first crack at flowers. “If you have a short straw and you get there first, you can sip off the top,” as he puts it. In contrast, the underclass tends to have longer bills for probing the dregs.
A difference in plumage color hints that one sex has more flower power, Bleiweiss says. If scrappy males monopolize nectar, females develop longer bills to reach the leftovers, he theorizes.
The same logic could also explain why clustering dampens the trend toward long female bills, Bleiweiss points out. Females in colonies, for instance, defend nearby flowers, so males scrabble for nectar, too, and don’t get by with short bills.