The strength of selection in the context of migration speed.

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 theta = -(u*2Pgamma"/v*2gammaP"), where gamma is the rate of energy expenditure during flight and P is the rate at which energy is gained during foraging. The dimensionless ratio P/gamma 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 theta depends on this ratio, it also depends on the curvatures of the functions, as represented by gamma" and P". I use this simple example to make some general points about the strength of selection.