Bird Flight Speeds in Nature: Optimized or a Compromise?

Abstract

A total of 6203 air speeds were reported for 12 spp. [Larus argentatus, L. californicus, L. delawarensis, L. atricula, Gelochelidon nilotica, Sterna hirundo, S. fuscata, S. albifrons, S. maxima, Chlidonias niger, Anous stolidus and Rynchops niger] of gulls, terns and skimmers under various wind conditions. The means and distributions of air speeds from these data are compared with those predicted by the equations of Tucker and Greenewalt for birds to achieve minimum metabolic rate in flight, minimum cost of transport through the air, or minimum cost of transport over the ground. The field data give strong support to the idea that the average air speeds achieved by birds flying in the vicinity of a colony are a compromise between those predicted for minimum metabolic rate and for minimum cost of transport over the ground. The latter appears to be of overriding importance. Relatively large variances in air speeds are wholly characteristic as expected given the relatively flat shape of prediction curves for metabolic rate and cost of transport during flight.