Scaling of migration and the annual cycle of birds

In a seasonally fluctuating environment migratory birds face decisions relating to how to schedule their main life-history activities, i.e. breeding, moult and migration. The time required to complete any one of these depends on overall size. In this paper I derive scaling functions about how time required for breeding, moult and migration depends on body mass. The sum of these increases with increasing body mass with a critical mass where the duration of non-overlapping breeding, moult and migration is equal to one year. Beyond this mass one or more of the processes must be modified. Large species may refrain from annual breeding and skip years without breeding. The replacement of one set of feathers may also take more than one year, or in long-distance migrants the timing of moult is shifted from a post-breeding moult before autumn migration to a post-migration moult in the wintering area. Different adaptations for efficient migration are also discussed. Finally, I use a simple graphical model to derive a condition for when overloading fuel at the final stopover site is worthwhile, leading to arrival at the breeding site with surplus energy that may be used as capital for breeding.