Post-breeding flocks of White Storks Ciconia ciconia in southern Poland: size, age composition and the geographical origin of birds

Capsule White Storks Ciconia ciconia forming migrating flocks in southern Poland in autumn came from central Poland and eastern Germany, and used ploughed fields and cut meadows to forage before they moved south. Aims The aim of this study was to collect information on the flocking behaviour of the White Stork in southern Poland, with a special focus on the origins of birds forming flocks, flock size, age characteristics of birds, and habitat selection. Methods During a study conducted in 2012-2019 in southern Poland, we recorded the size, age composition and geographic origin of White Storks forming autumn flocks. The collection of the large number of records was made possible by the use of a small aeroplane during the first two years of the study, which enabled us to locate flocks over a large study area. Results In total, 5685 storks were observed, with a median flock size of eight individuals, although flock size varied during the migration period and by time of day. For the 2980 birds for which age could be determined, there was a higher proportion of adults (55.7%) than juveniles (44.3%). Analysis of data from ringed birds showed that the mean (+/- sd) distance travelled from natal sites was 277.3 +/- 245.3 km, with birds coming mainly from southern Poland and eastern Germany. Storks foraged mainly in ploughed fields and cut meadows, and such habitats appear crucial for fuelling before migration. Conclusions Southern Poland provides an important stop-over site for migrating White Storks, which originate from a potentially large area. Hence, ensuring the continued availability of autumn foraging for White Storks in southern Poland benefits not only local birds but also a much larger population.