The timing of migration within the context of an annual routine

We illustrate how a general technique for finding optimal cyclic routines c an be applied to annual routines of migration. We present a schematic model in which there are two locations that differ in terms of food availability . We demonstrate how the model can be used to investigate the factors that determine the timing of migration. Reproductive values emerge from the mode l rather than being assumed. The form of the relationships between reproduc tive value and a bird's energy reserves and condition on the completion of the autumn migration is very different from that on the completion of the s pring migration. The difference suggests that the timing of migration is mo re critical in spring than in autumn.