Weather-influenced water-crossing behaviour of black kites (Milvus migrans) during migration

From 2014 to 2020, 32 black kites from various European countries were tagged with telemetry devices and tracked to study their spatiotemporal behaviour. Eleven birds which crossed the Mediterranean Sea and the Black Sea directly over large water bodies out of traditional migration routes over the Strait of Gibraltar, the Dardanelles/the Bosporus area and the east coast of the Black Sea were selected for this study. Ten birds attempted to cross the Mediterranean Sea and one attempted to cross the Black Sea. All black kites crossed the sea successfully but in one case the bird failed. The maximum water crossing length was recorded at 542 km. The average speed of the birds crossing the sea included a broad range from 27.7 to 97 kph. There was a correlation between average speed and tail-wind speed during the crossing. We conclude that the most favourable conditions for black kites when crossing large water bodies such as the Mediterranean and Black seas were sunny to partly cloudy weather with temperatures over 20 °C, the speed of the tailwind of 1.1–10.8 m/s and the air pressure over the standard value of 1013 hPa.