Blissful ignorance? Departure rules for migrants in a spatially heterogeneous environment

Incomplete knowledge of the variation in site quality to be encountered dur ing migration may make it necessary for birds to base their departure decis ions on behavioural rules of thumb. Here I analyse four such rules for time -minimizers and show how they perform in spatially heterogeneous, autocorre lated environments. The first rule projects the currently experienced fuell ing rate onto all the future sites, the second rule is based on a fixed exp ected rate along the route, the third rule assumes that the expected rate a t sites nearby is close to the value currently experienced and the fourth r ule uses the experienced values to form an expectation during migration. Si mulations demonstrate that a global update rule which uses the current expe rience and interprets it as the global value to be expected at all the futu re sites is outperforming the second rule, which is based on fixed expectat ions, in nearly all circumstances and that it is very robust with respect t o spatial autocorrelation and the degree of variability in fuelling rates. This rule leads to the repeatedly observed pattern of a shallow relationshi p between departure fuel load and fuel deposition rate. A third rule shows its best performance under the assumption that the quality of sites only ch anges little with distance and therefore comes close to the global update r ule. The fourth departure rule, a modified version of the global update rul e, which allows some adjustments of the expected value through all the expe rienced values so far, leads only to small increases in performance. (C) 19 99 Academic Press.