GPS tracking of Red Kites (Milvus milvus) reveals fledgling number is negatively correlated with home range size

Abstract

To date, reliable studies on the spatial area use and home range size of the Red Kite (Milvus milvus) during the breeding season are lacking. Between 2007 and 2014, 43 adult individuals were fitted with GPS transmitters in Germany. The home range sizes of 27 males, which successfully reared 47 broods, ranged between 4.8 and 507.1 km2 based on the 95 % kernel utilization distribution. The median during the nestling and post-fledging dependent periods was 63.6 km2. The home ranges of 12 females, with a total of 21 successful broods, ranged between 1.1 and 307.3 km2. Within a single breeding season, there were considerable differences among home range sizes. There was also considerable variation in the home range size of adults during the course of a season. Across years, the median home range size of all males ranged between 21 and 186 km2, depending on prey availability. For individual males at the same nest site, the home range size varied up to a factor of 28 across years. Kites with very large home ranges had only one fledgling, which indicates that resources were scarce. Individuals with more nestlings had intermediate-sized to small home ranges. The relationship between the number of fledged young and home range size was modelled using a cumulative logit model. Fifty-six, 37, and 26 % of male kite fixes were beyond a 1, 1.5, and 2 km radius around the nest, respectively. Birds with very small or very large home ranges differed considerably from these average figures. Adults sometimes travel very long distances to visit distant grasslands during and shortly after mowing (up to more than 34 km) from the nest, due to the increased likelihood of prey availability at these sites. In conclusion, home rage size serves as a useful indicator of Red Kite habitat quality, which may provide key conservation information at the wider ecosystem level.