I use a model of avian migration based on maximization of overall migration speed to compare the strength of selection acting on foraging performance and flight speed. Let the optimal foraging behaviour be u* and the optimal flight speed be v*. It is shown that at this optimum, the ratio of the strength of selection on foraging to the strength of selection on flight speed is θ =–(u*2Pγ″/v*2γP″), where γ is the rate of energy expenditure during flight and P is the rate at which energy is gained during foraging. The dimensionless ratio P/γ is the ratio of time spent building up fuel to time spent flying which A. Hedenström and T. Alerstam showed was much greater than unity. Although θ depends on this ratio, it also depends on the curvatures of the functions, as represented by γ″ and P″. I use this simple example to make some general points about the strength of selection.