Migration within the annual cycle: species, sex and age differences

Abstract

The three main energy-demanding events in the yearly cycles of birds—breeding, moult and migration—tend to occur at different times of year from one another, with minimal overlap. The sequence in which these various events occur varies according to the ecological conditions (including those encountered during migration) in which particular populations live, and in general, moult is more variable in timing than other events. Some migratory birds moult in their breeding areas after nesting is over, others moult at a staging area on migration, while others moult in winter quarters. Yet others show a split moult, replacing part of their plumage in one place and part in another, moult usually being arrested during the intervening migration. Split moults are often associated with split migrations, as the birds pause for several weeks on their journeys to replace their feathers. Different variants in these patterns occur in different geographical populations of the same species. Some types of birds overlap breeding and moult, and some also overlap moult and migration, especially body moult, which can occur without reducing flight efficiency. A few bird species, with short breeding cycles, have been found to nest in two different areas in a single year, separated by migration. In species in which the young are raised by only one parent, the other parent is able to begin post-breeding moult or migration at a much earlier date. In species in which individuals do not breed in their early years of life, moults and migrations are shifted into the more favourable period which in older birds is occupied by breeding. The main message of this review is the huge variation that occurs in the annual cycles of birds, in the timing and duration of different processes, the sequence in which they occur, and the degree of overlap between them. Variations occur not only between species and populations but also between sex and age-groups within the same population. It is suggested that many species remain in a state of suspended readiness for breeding, moult or migration, and can undertake any of these processes over a greater part of the year than that in which they are normally expressed. Two or more of these processes can remain in this state of readiness simultaneously, enabling the bird to suppress, advance or delay a particular process, according to the conditions in which it finds itself.