Large-scale variation in flight feather molt as a mechanism enabling biennial breeding in albatrosses

Laysan and black-footed albatrosses, Phoebastria immutabilis and P. nigripes, exhibit both annual and biennial breeding frequencies, and annually replace flight feathers in patterns that can be described as large, small or medium in extent. Large molts are temporally incompatible with successful breeding. Small molts are temporally compatible with the longest breeding seasons. Medium molts are compatible with shorter, but still successful breeding seasons. On average, large and small molts combined replace the same feathers with the same frequencies as two medium molts combined. Thus, large and small annual molt patterns combined provide a mechanism for transferring time from one year to another enabling extended breeding seasons every other year, and thus biennial breeding. Medium-sized molts are compatible with annual breeding. Among multiple albatross species, large-scale, annual molt patterns can shift in response to shifting breeding frequencies, but there may be a time lag in the response. A newly identified period of rapid fattening following molt termination and preceding colony arrival suggests albatrosses maintain low fat stores throughout active molt to reduce wing-loading, intensifying temporal trade-offs between flight feather molt and breeding.