Table 3.

Two foraging currencies, net rate and efficiency of energy intake, estimated for different foraging environments and for the hypothetical case of birds with high mass in the poor environment

Net intake (W) Efficiency
EnvironmentActual massHigh massActual massHigh mass
  • Net intake is the difference between the rates of metabolisable energy gain and energy expenditure during a foraging cycle (W). Efficiency is the ratio of metabolisable energy gain over energy expenditure during a foraging cycle (dimensionless). A foraging cycle included the flying time needed to obtain one pellet, the perch time following these flights and the time for handling and eating a pellet (the latter two were estimated at 2 s, but within reasonable limits these estimates had negligible effect). Computations used data from Table 1, and assumed that MRflight=20.5 W in the rich and intermediate environment and MRflight=17.5 W in the poor environment.