Avian molting

Feathers are the characteristic feature of birds and plumage provides several vital functions, including, uniquely, flight. Feathers are replaced annually in a strictly controlled bilaterally symmetrical sequence so that the roles of plumage are not compromised. Molt is a key life history stage for all birds and its timing is important. Birds normally molt after breeding, and there is a close relationship between the end of breeding and the start of molt. Birds avoid molting during migration and do not molt during winter. The timing and rate of molt are controlled by the annual cycle in photoperiod and modulated by non-photoperiodic factors such as the duration of breeding activity. Despite decades of research, little is known about the physiological control of molt. Thyroid hormones, gonadal steroid hormones and prolactin have all been shown to affect molt, but the latter is more likely to have a direct causal role.