Which birds participate in mass concentrations of Bramblings Fringilla montifringilla?\u2014Ring recoveries, biometry, age and sex composition

Abstract

The Brambling Fringilla montifringilla combines several special features of migration and wintering: differential migration according to age and sex groups, large differences in winter densities as a response to food availability, and flocking behaviour as a response to local mass fructification of the beech Fagus sp. resulting in roosts of several million birds. This study examines (a) whether Bramblings participating in mass concentrations originate from different, or additional, breeding grounds compared with birds present in normal winters, and (b) how differential migration according to age and sex groups combines with irruptive migration and with congregations in mass roosts, i.e., whether the benefits from mass concentrations differ between age and sex groups. Wing biometry (wing length, lengths of outer primaries, and wing pointedness) did not vary geographically, as shown by museum skins, and was therefore not helpful to reveal differences in origin between birds of mass concentrations and normal winters. The extent of the post-juvenile moult was smaller in mass concentration birds and suggests that slightly later-born juveniles, i.e., born further east or north, participate in mass concentrations. Ringing recoveries could not demonstrate a difference in breeding range origin of Bramblings which participate in mass concentrations versus Bramblings in normal winters in Switzerland. However, this may be due to insufficient recoveries of ringed birds. A rough estimate suggests that the number of Bramblings migrating SW on a broad front across Europe could be sufficient to form mass concentrations of up to 10 million birds which are halted by a large area of beech mast. Across Europe, the proportion of juvenile males decreased towards south (i.e., with increasing migration distance) in autumn migrants and wintering birds, while adult females showed the opposite pattern, and adult males and juvenile females were intermediate. Mass concentrations in Switzerland were composed of significantly more adults, especially adult males, and fewer juveniles, particularly juvenile females, while the sex–age composition outside mass concentrations was similar to normal winters. Adults and females likely benefit from wintering further south where snow is less likely to cover food of this ground-feeding finch. On the other hand, the costs of migration are probably substantial in terms of direct costs or indirectly in terms of reaching the breeding grounds, so that juvenile males prefer to winter closer to the breeding area than adults. Feeding in large flocks in beech mast areas incurs the high risk of being cut off from this food by snowfall. Adult males are apparently best adapted to this risk, and hence predominate in mass concentrations.