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.