Flight performance: Frigatebirds ride high on thermals.

Abstract

Aspects of the morphology and life history of frigatebirds verge on the extreme, and how they spend their time at sea has been a mystery until now. Here we use data collected by altimeters and satellite transmitters attached to individual frigatebirds to show that these birds are continuously on the wing, day and night — they fly in a succession of climbs and descents, soaring in circles on thermals to heights of up to 2,500 m and gliding down in the direction of travel. The birds' curious morphology and flight patterns result in extremely low costs of foraging, but they also cause them to travel slowly over large distances, putting frigatebirds at an evolutionary extreme that enables them to exploit tropical waters in which prey is scarce.