Should I stay or should I go? Testing optimality models of stopover decisions in migrating birds

Time-minimizing migrants should leave a stopover site if the instantaneous speed of migration drops to the average speed of migration in the environme nt. This argument has been used in two different ways: either there is loca l variation in the fuel deposition rate and a fixed expected speed or there is global variation in the fuel deposition rate, i.e. locally experienced variation represents global variation along the route. The first case leads to a far steeper relationship between departure load and fuel deposition r ate than the second case. So far, data on departure loads have mainly been analysed within the concept of local variation of the fuel deposition rate and the result that the observed slopes are much lower than predicted has b een explained by changes in the expected speed along the route or by indivi dual differences in the expected speed. We show here that the observed rela tionships generally fall close to the predictions for global variation. We propose that migrants use a behavioural rule which projects the current exp erience into the future and therefore interprets local variation as global variation.