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

Abstract

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 spe-cies 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 fre-quency to the level of potential threat to the nest. The proportion of mobbing events result-ing in physical contact (mobbing intensity) declined from the most mobbed 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 be-tween mobbing intensity and mobbing frequency in the study of nest-defence behaviour.