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

Abstract

We test the predictive value of the main energetic currencies used in foraging theory using starlings that choose between two foraging modes (walking versus flying). Walking is low-cost, low-yield, whereas flying is the opposite. We fixed experimentally, at 11 different values, the amount of flight required to get one food reward, and for each flight cost value, we titrated the amount of walking until the birds showed indifference between foraging modes. We then compared the indifference points to those predicted by gross rate of gain over time, net rate of gain over time, and the ratio of gain to expenditure (efficiency). The results for the choice between modes show strong qualitative and quantitative support for net rate of gain over time 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 discrepancy 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, the discrepancy may be attributable to the use of predictable (rather than stochastic) ratios of effort per prey in our experiment, and it may thus be better understood with mechanistic rather than functional arguments.

Figure 1

Indifference between foraging modes with different rates of expenditure (walking and flying) according to three currencies. The slopes of the dashed lines c_f and c_w represent energy expenditure rates during flying and walking, respectively. Reward size (R) is assumed to be fixed. Point F shows the energy expenditure and travel time flying to obtain one reward. In the experiments, F was fixed but the subjects' choices determined the travel time walking at the point of indifference. To equalize gross rate (G), time walking [w_t (G)] should equal time flying, (line g). To equalize net rate (line n), time and energy walking correspond to point N. Finally, to equalize efficiency, energy expenditure walking and flying are equal (E, line e).

Figure 2

Temporal relationship between the mean (±SE) number of walks per cycle resulting from the titration (r_w, left) and number of flights per cycle (r_f, right) in each treatment.

Figure 3

Total daily economy. Mean daily time foraging in walking (A, open symbols) and flying (A, solid symbols) modes, intake (B), and body mass (C).

Figure 4

Observed (solid circles, mean ± SD) and predicted (lines) number of walks per cycle (all birds plot shows mean ± SE). Net rate (middle lines) was the best predictor. Efficiency (top lines) overestimated r_w and gross intake (bottom lines) underestimated it.

Figure 5

Sensitivity analysis. Effect on predictions after modifying the energetic parameters to 75%, 100%, and 125% of the best estimates. We calculated all scores of relative deviation (100 × [(predicted − observed)/observed]) per bird and treatment. These values were averaged and the means are shown as lines. Shaded areas show one standard deviation of the mean difference.