Net energy versus efficiency maximizing by foraging ring-billed gulls

Summary

Ring-billed gulls (Larus delawarensis) breeding at Dog Lake, Manitoba often feed by following tractors pulling cultivating implements around fields. Tractor-following gulls always land immediately behind the cultivating implement, where they feed on earthworms or grain. Afeer a feeding bout on the ground (patch residence time), gulls fly up, pursue the tractor and repeat the cycle. We use net energy maximizing (energy gained per unit time) and efficiency maximizing (energy gained per unit energy expended) models to make quantitative predictions of patch residence time, and compare these predictions to observations. If we assume flight speed to be constrained at the observed mean, the observations fall between the predictions of the two models, and the models explain approximately equal and highly significant proportions of the overall variation in patch residence time. If both flight speed and patch residence time are allowed to vary in the models, the efficiency maximizing model more closely predicts observed patch residence times, and the net energy maximizing model more closely predicts observed flight speeds. We discuss whether breeding ringbilled gulls may be truly intermediate between net energy and efficiency maximizing, and how measurements of flight speed may be useful in further investigations.