To walk or to fly? How birds choose among foraging modes

We test the predictive value of the main energetic currencies used in forag ing theory using starlings that choose between two foraging modes (walking versus flying). Walking is low-cost, low-yield, whereas flying is the oppos ite. We fixed experimentally, at 11 different values, the amount of flight required to get one food reward, and for each flight cost value, we titrate d the amount of walking until the birds showed indifference between foragin g modes. We then compared the indifference points to those predicted by gro ss rate of gain over time, net rate of gain over time, and the ratio of gai n to expenditure (efficiency). The results for the choice between modes sho w strong qualitative and quantitative support for net rate of gain over tim e over the alternatives. However, the birds foraged for only a fraction of the available time, indicating that the choice between foraging and resting could not be explained by any of these currencies. We suggest that this di screpancy could be accounted for functionally because nonenergetic factors such as predation risk may differ between resting and foraging in any mode but may not differ much between foraging modes, hence releasing the choice between foraging modes from the influence of such factors. Alternatively, t he discrepancy may be attributable to the use of predictable (rather than s tochastic) ratios of effort per prey in our experiment, and it may thus be better understood with mechanistic rather than functional arguments.