Recent changes in migration dynamics, body mass, and wing morphology of the Eurasian Blue Tit (Cyanistes caeruleus) and Great Tit (Parus major) during the post-fledging season in Hungary

Abstract

Temporal trends in the timing of autumn migration, changes in body mass, wing and 3^rd primary lengths were analysed based on 23,141 Eurasian Blue Tits (Cyanistes caeruleus) and 20,757 Great Tits (Parus major) captured in three Hungarian ringing stations in the post-fledging period, between 2004 to 2024. The results showed that the timing of the autumn migration of tits in Hungary was stable in the long term for both age and sex groups. Usually, migration of the Eurasian Blue Tit starts in early September followed by some lower peaks in the number of migrating birds in the course of this month and early October, and a very clear maximum after the 20^th of October. The Great Tit migration began at the beginning of September, followed by two peaks in the first and second half of October. In both species the average body mass and wing length decreased during the dispersion season (August) between 2004 and 2024. We hypothesized that the warming climate of the Carpathian Basin has increased the fitness of individuals with smaller body sizes in the Hungarian nesting populations of both species. The results of the migration period (September–October) showed a more complex picture, which did not allow a clear interpretation of the mass and size changes according to Bergmann's rule. The decrease in average wing length and the increase in the average length of the 3^rd primaries, i.e. change in the wing morphology to a rounder shape might suggest an increased number of shorter- and rounder-winged individuals, and possibly, a shortening of migration distances in both species.