Partial Migration in a Central European Raptor Species: An Analysis of Ring Re-Encounter Data of Common KestrelsFalco tinnunculus

Abstract

In partially migratory bird species, some individuals of a population migrate while others stay in the breeding area. Although Common Kestrels Falco tinnunculus are defined as partial migrants, their migratory strategies are still not well described. We investigated ringing and re-encounter data of Kestrels marked as nestlings between 1924 and 2011 in Germany. We defined four populations corresponding to the natural regions of Germany. Although both migratory and resident individuals were found independently of sex or age class at the time of recovery, in general, females and juveniles travelled larger distances than males and adults, respectively. We illustrated the initiation of migratory movements in contrast to dispersal by combining distances and directions in two levels (< 100 km and ≥ 100 km), showing that migration is initiated mostly in September/October, while in August movements seem to mostly reflect dispersal. The NAO Index as well as age class, region and re-encounter period (1950–1970, 1971–1990 or > 1990 as indicator of responses to climate change) of birds in autumn and winter were integrated into Generalized Linear Models. We found that in autumn and winter a tendency to migrate is primarily shown by juveniles and it was significantly higher in years before 1971 than in recent decades. In addition, a higher NAO Index in summer is linked with decreased proportion of birds re-encountered far (> 100 km) from their natal sites in winter, whereas a higher NAO Index in autumn is linked with increased proportion of high-distance re-encounters (> 100 km).