Post-juvenile moult of the Brambling (Fringilla montifringilla, Passeriformes, Fringillidae): a study at four places

The duration of the moult and its different stages in young Bramblings, their variability under the influence of photoperiodic conditions are considered. The timing and duration of moult seasons in different places of the diastributiion area are also investigated, and their determining factors clarified. The moulting process is divided into 7 stages. The signs of the stages are the beginning or end of feather replacement in the selected tracts of the feather coat. An analysis of the experimental data revealed the influence of photoperiodic conditions on the duration of the moult of an individual. Under photoperiods corresponding to early moult timing, moult duration averaged 62.0 days, vs 52.3 days under the conditions of late moult timing. The moult duration is shown to drop as a result of a decrease in the duration of the beginning stages. Data derived from repeated catches in nature confirmed the results of the experiment. A negative relationship was found between the timing of the capture of Bramblings and the duration of their intervals between stages 2 and 3, 3 and 4. A comparison of the duration and timing of the seasons of post-juvenile moult at four different places of the distribution area showed their great geographic variability. The duration of the season is increased in the following sequence: the Lower Ob region (66°40 N, 66°40 E), the White Sea region (67°06 N, 32°41 E), the Middle Ob region (59°58 N, 74°22 E), and the Ladoga region (60°41 N, 32°57 E), amounting to 39, 64, 92, 116 days, respectively. In almost the same sequence, the dates of these seasons reliably shift from early to late. The median catch dates for moulting birds are 24.08 in the Lower Ob region, 28.08 in the White Sea region, 1.09 in the Ladoga region, and 10.09 in the Middle Ob region. At the early start of the moult season in the Ladoga region (2.07), its end turned out to be one of the latest (25.10), and the duration is twice as long as that of an individual moult. In the Lower Ob, at the late start of the season (1.08), it ends earlier than at other locations of the study area (8.09), and its duration is almost half as long as the moult of an individual. An analysis of the number of birds with different stages and a comparison of the timing of registration of successive stages at each research location with the duration of interstage intervals in moulting individuals has allowed us to come to the following conclusion. During the post-juvenile moult season, the Lower Ob region is inhabited mainly by local Bramblings with the beginning moult stages. They fly away from this subarctic area before the inset of intense feather replacement. In the study area of the White Sea region, the moult season is dominated by birds that appeared as a result of post-fledging (juvenile) migration. They have middle moult stages. In the Ladoga and Middle Ob regions, both local birds and Bramblings from remote territories are recorded. Among the latter, there are birds that live during middle moult stages, but individuals with the last moult stages, which follow in transit during the autumn passage, predominate. This determines the longer duration and the later dates of the moult seasons in the Ladoga and Middle Ob regions.