Flight altitudes and flight activities of adult Red Kites (Milvus milvus) in the breeding area as determined by GPS telemetry

Abstract

The Red Kite has come to the forefront of general consciousness in recent decades. A major contributing factor has been repeated collisions of these raptors with wind turbines and the important role they play in the approval process for new turbines. Efforts to improve their protection sometime encounter challenges due to limited knowledge about some life history aspects, e.g., their flight altitude and flight activity. We investigated these parameters from 2012 to 2018 in an approximately 1000-km2 study area in the vicinity of Weimar (Federal State of Thuringia, Federal Republic of Germany) in 29 breeding birds (19 males and 10 females) equipped with GPS loggers. In addition to more than 11 million GPS data records, accelerometer data from the loggers were evaluated. The start of morning activities correlated with sunrise. Most frequently, Red Kites initiated their first hunting flights immediately at sunrise; males started hunting at 9 min and females started hunting at 12 min after sunrise (medians). The Red Kites mostly foraged and hunted their prey during flight. The proportion of time spent flying during their activity phase was highest during midday hours, reaching 30–80% in males depending on the phase of the breeding season. For males, the time spent flying was approximately two and a half times that for females. The birds sporadically reached flight altitudes of up to 1600 m above ground in the breeding area, while they maintained lower altitudes between 5 and 60 m more frequently and predominantly while foraging (56% of location fixes). Higher altitudes were occupied much less frequently than lower altitudes and were often used to travel relatively long distances in an energy-saving manner. The end of activity was well before sunset, at 87 min prior to sunset for males and 154 min prior to sunset (medians) for females.