Carrying capacity in overwintering migratory birds

The carrying capacity of the food supply in a non-breeding site for migrating birds can be measured either as the maximum number of bird-days that it can support or as the maximum numbers that can survive the non-breeding season. We identify the circumstances when it is appropriate to measure carrying capacity in either way, but we then show that birds may often emigrate or starve well before carrying capacity is reached, however it is defined. It is therefore unsafe to argue that, because the carrying capacity would not be reached, a change in site management would not affect birds. The key question in environmental impact studies of this kind is whether a change in site management would decrease the survival rate or the proportion of birds accumulating the body reserves required to migrate to the breeding grounds.