Annual captures, autumn migration phenology, and morphological traits of the Common Blackbird (Turdus merula) over 24 years

In this long-term study, the timing of autumn migration, stopover duration, wing and third primary length, body mass, fat load, and pectoral muscle of the Common Blackbird (Turdus merula) were studied between 1998 and 2022 in Tomord, Hungary. We found a significant increase in Common Blackbirds passing through the study site. There was no substantial difference between the timings of autumn migration in the two sexes. Still, the adults started their migration slightly later than juveniles. This timing was unchanged between 1998 and 2022, indicating that juveniles and adults of both sexes use the same adaptation strategy to determine the departure time. We found that wing length slightly but significantly decreased in both age and sex groups, whereas there was an increase in the length of the third primary during the study period. In our view, these wing morphology changes cannot be explained by local adaptations but by a change in the population composition: most individuals captured are short-distance migrants. Our results indicated that body mass and condition of all groups increased from September to November within a year, which can be interpreted in two ways: on the one hand, it could be caused by an increase in the proportion of adults, and on the other hand, there could be an increase in the proportion of Common Blackbirds arriving from territories to the north of the Pannonian region as the autumn migration progresses. The similarity in changes in the phenology and morphological traits of the different age and sex groups during the study period emphasizes that environmental change caused no detectable intraspecific selection pressures.