Photoperiodic Control of the Molt Cycle in the Chaffinch (Fringilla coelebs)

Abstract

Abstract The molt cycle in Chaffinches (Fringilla coelebs) has a circannual periodicity under constant photoperiodic conditions (12:12LD, 20:4LD). The molt cycle and/or rate, however, can be modified by artificial alteration of daylength. The time of onset and completion of the post-nuptial molt in Chaffinches under natural conditions is a remote expression of vernal photostimulation. The timing of the molt is initiated at the end of the unifactorial phase of photoperiodic control. The onset of molt is induced after a latent period and its termination after another period, i.e. the molt begins and ends spontaneously. Under natural conditions, daylength immediately before and during the molt does not control either the time of onset or of termination of molt. The additional control system increases the rate of the molt in its early phase under short days, whereas under long days it decreases the rate of molt in its late phase. This system may play an adaptive role in synchronizing the end of the molt among birds that begin molt at different times. The postjuvenal molt begins and finishes spontaneously under control of a program of the individual development of juveniles. Short days increase the rate of molt and initiate earlier onset and completion, whereas long days decrease the rate of the molt and delay the times of onset and completion. Juvenile birds from late broods molt more rapidly than those from early broods. A combination of different programs and photoperiodic control of the molt synchronizes the termination of the molt in juveniles from early and late broods under natural conditions in summer and autumn.