Movement ecology of the golden eagle Aquila chrysaetos and the semidomesticated reindeer Rangifer tarandus : synchronous movements in a boreal ecosystem

Abstract: The golden eagle Aquila chrysaetos in Sweden are believed to be rather sedentary. Hence, studies in Norway suggest the opposite with a diurnal pattern with seasonal migrations to e.g. south Sweden during the winter – distances of thousands of kilometers. Almost 90 percent of the eagles in Sweden are distributed in the northern parts and occur only in scattered patches in the southern parts of the country. Still, there are reports of eagles visiting feeding ground in these parts of the country during the winter although their origin so far has been unknown. In these boreal ecosystems there are reports that the semi-domesticated reindeer Rangifer tarandus L. constitutes the main prey base for native large carnivores. Here, studies on nesting eagle’s food preferences have reviled it to be an important scavenger on reindeer although its extent to prey is concealed. Still, interactions between the species effect ecosystem functions and thus, its resilience. Today, there are few studies on golden eagle movement behavior and there is no study in relation to the movement of the semidomesticated reindeer. On a landscape level, identification of such behaviors enables us to quantify the extent, duration and the timing of the movement dynamics. It gives us understanding to what level these movements are synchronized or not – movement behaviors depending on different life stages, e.g. age, environmental factors and food resources. In Northern Sweden, the ecosystem is characterized by high seasonality causing periods with limited access to food for both predators and prey causing population fluctuation and seasonal migrations. This seasonal movement behavior gives rise to large reindeer migrations twice a year: from the boreal forest in the winter (the winter feeding range) to the alpine tundra in the summer (calving- and summer feeding range). Due to eagles tracked with GPS –transmitters, I had the opportunity to test for spatio-temporal synchronization on the timing of the movement of golden eagle (n = 43) and reindeer migration during two consecutive years (2011 and 2012). My results suggests that both juvenile- and adult golden eagles migrate – often over 1000 km. Further, a behavioral change points analyses suggested that individuals likely synchronized the timing of migration to those of the reindeer’s in spring. Hence, the synchronism was less significant during the autumn indicating several factors participating. Hopefully, this study will help to increase our knowledge on movement patterns of golden eagles in Scandinavia, and also, rise questions for further studies of both eagles and reindeers and thereby develop the management of both species in the future.