Hummingbird Sexes Differ to Share Food

Aug. 2 -- Men and women may battle over mundane matters like in-laws or housekeeping, but male and female hummingbirds must spar over something a little more critical: food.

Food is especially contentious for hummingbirds since the creatures move their wings invisibly fast (up to 80 flaps a second for some species) and operate at such high metabolic rates that they can starve within one hour. In fact, these hovering birds burn more calories relative to their size than any other animal except flying insects.

“They’re like motorcycles with small fuel tanks,” says Bill Calder, an ecologist and longtime hummingbird expert at the University of Arizona. “They’re light and fast, but they’re always running on nearly empty.”

Fortunately, nature has found a way to divvy up food resources between the sexes of some hummingbird species.

Designed for Access

On the island of St. Lucia in West Indies, biologists have noted physical differences among the local male and female purple-throated carib humming bird. And their observations show that two kinds of nectar-bearing flowers are the most likely reason for the birds’ different designs.

“Usually sexual dimorphism [a feature that differs between males and females] is attributed to mating purposes, like males having brighter feathers to attract females,” says Ethan Temeles, a biologist at Amherst College in Massachusetts who has spent the past two summers on St. Lucia. “Here, ecology appears to have a role, which is very rare.”

The evidence, Temeles explains, is in the beak.

The males sport a straight bill (as Temeles says, “imagine if you stuck a yardstick on your nose.”) The female’s bill, meanwhile, is a third longer and twice as curved. It turns out both shapes are uniquely fitted to probe the stalks of two separate species of the nectar-filled Heliconia flower.

Observing the iridescent birds in the cool, dark rain forests of St. Lucia, Temeles and his students noted that 15 of 15 males fed on patches of H. caribaea while 11 of 18 females chose H. bihai instead. The flowers the males select have short tubes to their nectar, while the females’ choice have long curved stalks leading to their sugary source.