Consequences of Habitat Loss at Migratory Stopover Sites: A Theoretical Investigation

Abstract

We use a dynamic optimization model to assess the consequences of habitat loss at migratory stopover sites. We emphasize costs birds face during stopover (e.g. costs of gaining energy), the timing of site use and the behavioural rules birds might use to implement migratory strategies. Behavioural rules may be flexible enough that birds can still produce optimal behaviour in the changed environment, or the rules may result in suboptimal behaviour. If birds behave optimally in the altered environment, habitat loss on the wintering ground has the highest impact, because this site, unlike the intermediate stopover sites, cannot be skipped if the quality drops below a threshold. If birds continue to use the old behavioural rules that now result in suboptimal behaviour, we can distinguish two cases. Birds can continue to use a constant foraging behaviour that was optimal in the unaltered environment under many circumstances. Then the effects of habitat loss are proportional to the length of stay before habitat loss and the departure fuel load from a site. However, the effects of habitat loss do not depend on the location of the site within the network. In some circumstances birds are expected to forage with intensities that are below maximum. If birds use the foraging behaviour that is appropriate for a given fuel load, time and site but inappropriate for the altered fuel gain, then changes at sites close to the breeding ground have a greater impact than more distant sites. Finally we discuss the importance of sites that are not used before habitat loss. If birds behave optimally these sites may be used in an altered environment and can buffer against habitat loss at other sites.