Stopover decisions under wind influence

Despite evidence that wind conditions are ail important factor in determini ng stopover decisions, models of time-minimizing bird migration have up to now emphasized the optimal response of the migrants to variations in fuel a cquisition rates. We present a simple model of a time-minimizing migrant fa ced with two potential wind conditions on each day, which occur with a fixe d probability. Wind assistance is modelled as a multiplicative Factor in th e flight range equation. We identify conditions under which birds leave the stopover site even with no tailwinds and renditions where the birds leave only with tailwinds in cases with global and local Variation of the fuel de position rate. The optimal policy depends on the probability and amount of wind assistance. In all cases there is an initial period at a stopover site when the bird should stay and build up its initially small fuel reserves i rrespective of wind. After this initial time, there is a period when the: o ptimal departure decision is to leave when tailwinds occur but stay and con tinue fuel deposition in other winds. If the probability of tailwinds is lo w the bird should at some later time change its policy to leave even in unf avourable winds. However, if a certain threshold value of the probability o f favourable winds is reached the birds should never leave without wind ass istance. These patterns lead to a complex relationship between departure lo ad and fuel deposition rate. We compare our predictions with a null-model w here the birds simply leave as soon as favourable winds occur. Wt: further show that the inclusion of wind assistance cannot explain the discrepancy b etween observed and predicted values of departure loads under local variati on in Fuelling rates.