Morphological adaptations to migration in songbirds

Bird migration is associated with morphological adaptations, particularly with traits related to flight. Therefore, changes in migratory propensity could potentially be connected to changes in morphology. The blackbirds from the region of Lake Constance are a partially migratory population, with a decreasing trend in migration over the last decades. The aim of this study was to determine whether changes in migratory propensity were accompanied by temporal morphological changes. Historical morphological data of blackbirds (Turdus merula) from a partially migratory population from 2009 to 2026 were analysed for temporal trends in wing length, tarsus length and body mass. Furthermore, blackbird populations from Poland, Germany and France were compared regarding their morphological traits, as these populations have different migratory phenotypes. Finally, song thrushes were compared with partially migratory blackbirds regarding their wing morphology. As these song thrushes are obligate migrants, differences in wing aspect ratio and wing length between the two closely related species were investigated. When investigated across all blackbirds, wing length and tarsus length showed no significant temporal trend across years. However, wing length increased significantly in male birds, while tarsus length and body mass decreased significantly in female birds within the observed period. Body mass also showed a significant temporal decrease when investigated across all birds. Significant differences were found among Polish, German and French populations, but no consistent pattern was found across all observed morphological traits. Polish blackbirds had the longest wings, while the French population had the longest tarsi and the Polish population had the lowest body mass. The performed PCA combined an analysis of all three populations and the investigated morphological traits. A strong overlap across all populations and observed morphological traits was found. The interspecific analysis showed that song thrushes (Turdus philomelos) had a significantly higher wing aspect ratio compared to blackbirds, while having significantly shorter wings. In conclusion, no consistent morphological response to the temporal decrease in mi gratory propensity was observed. Rather, the detected patterns were specific to sex, trait or population. Clearer morphological differences were found in the interspecific comparison between the two species with different migratory phenotypes.