Hummingbirds' surprising insect-catching style.

High-speed videos of hummingbirds catching fruit flies show that the birds' lower bills are unexpectedly flexible, say researchers. In three species with straight, narrow bills, the lower half can bend downward part way along its length, even though it has no joint, says Gregor M. Yanega of the University of Connecticut in Storrs. The motion also flares open a bird's mouth to gape extra wide.

These moves come in the final fractions of a second in a hummingbird's attempt to snag an insect out of the air, Yanega and his Connecticut colleague Margaret A. Rubega report in the April 8 Nature. The researchers say that they appear to be the first to document these bug-nabbing refinements. Earlier studies had shown that the upper bill can bend, but the lower one had seemed rigid.

Hummingbirds are the textbook examples of physical adaptation. Their specially shaped bills enable the birds to probe deeply into flowers for nectar but seem unwieldy for catching insects. Nectar alone doesn't sustain the birds, especially during migration or the stresses of egg laying, Yanega explains.

He became curious about trade-offs between bill adaptations for nectar sipping and bug catching. To observe the birds in action, Yanega developed what he calls a "glorified fish tank" that keeps the birds in camera range as they hunt airborne fruit flies. With a 500-frame-per-second camera, he filmed three North American straight-billed species: ruby-throated, magnificent, and blue-throated hummingbirds.

These birds don't pinch insects out of the air with the tips of their bills, Yanega reports. Instead, they rush at the insect with their mouths wide open, making the 15[degrees] to 20[degrees] bend in the lower bill.

In the hummingbird videos, most of the successful captures snagged the insect close to the mouth end of the hummingbird's bill, says Yanega. After snatching an insect near the tip of their bills, the birds seemed to have trouble working it down to their mouths.

"I'm not surprised by their results," says Ethan Temeles of Amherst College in Massachusetts, who studies hummingbird evolution (SN: 7/22/00, p. 52). His videos of hummingbirds feeding on artificial flowers showed some fancy flexibility.

Temeles raises the question of whether the bill bending reported by Yanega and Rubega might be used during flower feeding as well as insect predation.

For decades of hummingbird studies, "the whole insect diet has been overlooked," says Douglas Altshuler of the California Institute of Technology in Pasadena, who has studied hummingbird ecology and biomechanics. He predicts that the new paper, along with a few others in the past 10 years, will fuel interest in hummingbirds as insect hunters.