Prey selection and prey handling in two raptors during the breeding season as revealed by the use of video monitoring
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- Master's theses (INA) 
I video monitored one nest of each of two avian predators, namely the rough-legged buzzard (Buteo lagopus) and the golden eagle (Aquila chrysaetos), in Oppland county, southern Norway, during May-August 2014, to analyse the composition of the diet and the handling of prey items. Of the 120 prey items recorded delivered at the golden eagle nest, mountain hare (Lepus timidus) was the most important prey species in terms of mass, with 65 % of the gross body mass of the prey delivered. Grouse (Tetraonidae sp.) was also important, the three species recorded together made up 22 % of the gross body mass in the golden eagle diet. This was in accordance with previous studies. However, a surprisingly large proportion of the diet of the golden eagle consisted of microtine rodents (voles and lemmings). In fact, 51 % of the delivered prey items were microtine rodents. In the region of my study area 2014 was a year with extremely high densities of microtine rodent, and my results may indicate that the golden eagle responds functionally to microtine rodents. The absence of ungulates among the items delivered at the golden eagle nest fits with the results from another golden eagle nest video monitored in Norway, but contrasts from most previous studies made on the diet of the golden eagle by traditional analyses of prey remains in the nest. Of the 253 prey items recorded delivered at the rough-legged buzzard nest, the most important prey in terms of both body mass and numbers was Microtus voles, which made up 71 % of the items delivered at the nest. Various other small prey were also utilized, in particular thrushes (Turdus sp.), but to a much smaller degree. The diet of the rough-legged buzzard suggests that this raptor responds functionally to Microtus voles, which is consistent with results from previous studies. In both raptors studied the male delivered most prey at the nest, while the female brooded and fed the nestlings. The female stayed at the nest feeding the nestlings a longer part of the nestling period than expected. This may be due to the fact that prey were abundant and that the prolonged stay was a strategy to enhance the female`s own fitness and thereby future survival. The probability that the nestlings would feed unassisted increased with their age, and with decreasing size of the prey item that was delivered at the nest. The rough-legged buzzard nestlings was also more likely to feed unassisted at low ambient temperature, and when the prey delivered was a mammal rather than a bird. The extent to which the various prey are included in the diet of the golden eagle and the rough-legged buzzard seems to be a result of their varying availability in time and space.