Red-backed Shrikes {Lanius collurio) adjust the mobbing intensity, but not mobbing frequency, by assessing the potential threat to themselves from different predators

摘要

We studied the ability of Red-backed Shrikes to adjust their nest defence to the potential threat posed to defending adults and their nests. We presented mounts of two raptor species which prey on adult birds (Eurasian Sparrowhawk, Common Kestrel; differing in the proportion of adult passerines in their diets), and two species of nest predators (Common Magpie, Eurasian Jay; differing in the proportion of bird eggs and nestlings in their diets). A mounted Feral Pigeon was used as a control. Shrikes regularly mobbed the Sparrow-hawk, Kestrel and Jay, but not Magpie or Pigeon. The mobbing frequency, in terms of the number of mobbing events per 20 min, did not differ among the three regularly-mobbed predators. If shrikes tried to chase the predator away, they did not adjust the mobbing frequency to the level of potential threat to the nest. The proportion of mobbing events resulting in physical contact (mobbing intensity) declined from the mostmobbed species, Jay to the Kestrel, to the Sparrowhawk, which was considered most dangerous to adult shirkes. The Red-backed Shrikes appeared to adjusted their mobbing intensity by assessing the potential threat to themselves. Our results show the importance of a differentiation between mobbing intensity and mobbing frequency in the study of nest-defence behaviour.