POSTJUVENILE MOULT IN THE COMMON CROSSBILL (LOXIA CURVIROSTRA CURVIROSTRA): PHOTOPERIODIC REGULATION AND ITS ROLE IN ANNUAL CYCLE SYNCHRONIZATION

Common Crossbills are known to start breeding for the first time before they have completely moulted to adult plumage. These breeding crossbills may retain a considerable proportion of juvenile (streaked) feathers, and bear two or even three generations of feathers. The questions therefore arise: what is the reason for such high variation in plumage condition; how much is it related to a bird's timing of birth; what is the timing of a moult; and how does it influence the subsequent phases of the annual cycle? The study is based on the results of in vivo examinations of the birds captured at the Ladoga Ornithological Station and in St. Petersburg suburbs (1482 birds), as well as the results of experimenting with juveniles captured from the wild (48) and with pulli (6) of laboratory-bred crossbills. According to trapping data, the moult season lasts from the end of the first ten days of August until mid-November, whereas the majority of birds start moulting for 1.5 months between August 15 and October 1. The timing of postjuvenal moult proves to be far more constant than that of hatching, where variation may be up to 7.5 months. Experiments with a photoperiodic control of the postjuvenile moult demonstrate that species-specific features in the Common Crossbill are a quite definite age at which the moult begins (50 days at min and 120-180 days at max) and a rather narrow postjuvenal moult interval with a short day length (14.5-13 h to 10-9 h of daylight) within which the moult is controlled by a PP-response. This synchronization of the moult onset is a direct consequence of the very high maximum age and the very short day length at the upper threshold of the photoperiodic interval: as long as there is more than 14.5 h of daylight, no moult would begin until an age of 4-6 months. The individuals that moult within the PP-interval thresholds replace plumage at a high rate. After the day length gets below the lower threshold of the PP-interval, an inhibiting role of the photoperiodic response is activated, terminating the moult in the birds that started moulting too late, while their plumage mainly retain juvenile generation feathers. The annual cycle of seasonal events in the Common Crossbill has some distinctive features, although resting on the common structure of avian annual cycles. One of such features is that sexual activity begins under short daylight conditions if a bird finds itself in an area with abundantly fruiting spruce or larch trees. Another feature of the species' annual cycle is multiple migration activity. The third distinctive feature is a strictly autumnal postjuvenal moult season, with short daylights. It is in this period that an individual's annual cycle gets synchronized with the phenological conditions in the environment and in the population.