Optimal stopover decisions of migrating birds under variable stopover quality: model predictions and the field data.

Dataset on departure fuel loads, stopover length and fuel deposition rate of the European robins Erithacus rubecula during their migration in the Baltic area is presented. We test these empirical data against the predictions of an optimal migration model assuming that robins minimize time spent on migration, and that fuel deposition rate varies stochastically. The latter assumption sets this model apart from the alternative ones and makes it more realistic. In particular, it is applicable in frequently observed situations when fuel deposition rate is negative. Our model assumes stochastic variation of the fuel deposition rate at sites along the migratory rout and thus is applicable when negative values of fuel deposition rate are recorded. The model predicts the relationship between fuel deposition rate and departure fuel load rather well. The agreement between the observed and the predicted values of optimal stopover duration is much poorer. Predictions of optimal migration theory are known to be dependent on the form of flight equation chosen. Our model fits the data best when the costs of transport are low. This supports the idea that transport costs of fuel stores may be low, especially when fuel stores are modest.