Thermal Soaring Compared in Three Dissimilar Tropical Bird Species, Fregata Magnificens, Pelecanus Occidentals and Coragyps Atratus

Abstract

All three species were observed in straight flight, and circling in thermals, from Flamenco Island, Panama. Measurements were made by or-nithodolite, an instrument which records a series of timed, threedimensional position estimates, from which speeds, circle diameters and rates of climb can be calculated. Mean lift coefficients in straight glides ranged from 0·72 to 0·84, except in slope soaring, where a mean of 1·6 was recorded for both the frigatebird and black vulture. Mean circling radii were proportional to wing loading, and varied from 12·0 m for the frigatebird to 18·0 m for the pelican. Mean rates of climb ranged from 0·40ms−1 for the black vulture to 0·57ms−1 for the pelican. All species showed mean circling lift coefficients between 1·33 and 1·45, and angles of bank between 22·9° and 24·7°. It is argued that the frigatebird is adapted to stay airborne continuously, day and night, for extended periods, by exploiting thermals over the sea under trade wind cumulus clouds. The low wing loading is seen as an adaptation to circling in narrow thermals, and the low disc loading as an adaptation giving low minimum power, when flapping cannot be avoided. Take-off appears to be possible only by dropping from an elevated perch, not from the ground or water surface. The low aspect-ratio, tip-slotted wings of the black vulture are certainly less efficient for soaring, but appear to be better for upward take-off from the ground. The pelican also has tip-slotted wings. Although its aspect ratio is higher, it is still capable of taking off from a level water surface.