Using the optimal flight speed theory to predict net energy intake rate from flight speed of birds

Birds adjust flight speed according to their circumstances, and as such avian flight is a behavioural attribute rich with ecological implications. Flight mechanical theory enables the power expenditure of flight to be reliably estimated and was incorporated into foraging theory to predict the flight speed of foragers in a patchy habitat. The basic prediction is that at the net energy intake rate maximizing flight speed, the marginal cost of a further increase in flight speed matches the net rate of intake. This equality could theoretically be used in reverse, providing a relatively simple method to estimate the net energy intake rate. This could be useful for habitat quality assessments and conservation planning because intake rate estimates are otherwise often time-consuming, labor-intensive, and difficult. In this thesis I measured the flight speed of foragers in two natural foraging scenarios, and in an experiment in which I manipulated the attainable rate of energy intake. I demonstrate step-by-step how to predict net energy intake rate from flight speed including corrections for wind, flight distance (i.e., inter-patch distance) and flock size, all of which affect the power requirements of flight. The data support the theoretical predictions of the effects of wind and flight distance, as well as the effect of flock size (though interpretation of the latter remains uncertain because another published model makes reverse predictions). The data give limited support for the prediction that foragers adjust flight speed in relation to habitat foraging quality. Dunlin (Calidris alpina) flight speed slowed as predicted in the course of the winter on one mudflat, but not on another. As predicted, migrant western sandpipers (Calidris mauri) fly faster on small than on large mudflats, but the differences are slight and flight speed appears slow relative to predictions. Steller's jays (Cyanocitta cristatus) did not fly faster between patches in response to a manipulation of patch quality. Overall, about half of the flight speeds I measured were at maximum range speed or slower. I suggest that this might be explained if foragers were maximizing not the intake rate but the foraging gain ratio.