Postjuvenile Moult in the Common Crossbill (Loxia curvirostra curvirostra): Photoperiodic Regulation and Its Role in Synchronization of the Annual Cycle

Captures of Common Crossbills on the southeastern shore of Lake Ladoga and in the St. Petersburg suburbs (1482 individuals from 1968 to 2015) show that the postjuvenile moult is preceded by migration and the overwhelming majority of migrating young birds have plumage that consists solely of juvenile feathers. The postjuvenile moult is recorded from the end of the first ten days of August to the middle of November, the timing of the onset of moult varies within one and a half months with different individuals being observed to have significant distinctions in the number of replaced feathers. The experiments with photoperiodic regulation of the postjuvenile moult were made on 48 individuals captured in nature in the second half of summer and six young birds from one pair of Crossbills, which bred in captivity. The experimental groups were kept in the conditions of a natural decrease in the day (60° N), decrease in the daylength after the transfer from a long day to a short daylength, increasing daylength, constant long daylength, and constant short daylength. The results of the experiments show that the species-specific features of moult in the Common Crossbill are its start being regulated in a wide age range (from 50 to 180 days) and a rather narrow photoperiodic interval of moult with short daylengths (14.5–9 h of daylight). As long as there is more than 14.5 h of daylight, moult begins under the influence of the endogenous rhythm, but not earlier than the age of 4–6 months. The individuals that moult within the photoperiodic interval replace plumage at high rates. After the daylength gets below the lower threshold of the photoperiodic interval, an inhibiting role of the photoperiodic response is activated, terminating moult in the birds that started moult too late, and their plumage mainly retains juvenile generation feathers. Due to the regulation of moult, intrapopulation differences in the timing of annual cycles that are more than seven months at the time of birth of young Common Crossbills decrease almost five times by its start and become even smaller as it ends. It is assumed that this not only makes it possible to implement moult under optimal conditions but also makes first-year birds ready to start breeding in a timely manner if they find themselves in an area with abundantly fruiting spruce trees.