Seasonal variation in naturally occurring mobbing behaviour of drongos (Dicruridae) towards two avian predators

Abstract

I tested the hypothesis that mobbing chiefly functions as a nest defence mechanism by studying the mobbing behaviour of two species of drongo (black drongo Dicrurus macrocercus and ashy drongo D. leucophaeus) towards two species of raptor in rain forest of western Java, Indonesia. It was predicted that firstly mobbing would occur more frequent and more intense during the drongo's breeding season and secondly that it would be more frequently and more intensely directed towards the nest and egg predating black eagle Ictinaetus malayensis compared to the relatively harmless Javan hawk-eagle Spizaetus bartelsi. I found a seasonal difference in mobbing frequency and intensity in the predicted direction for the Javan hawk-eagle but not for the black eagle. Black eagles were not more frequently or more intensely mobbed than Javan hawkeagles. In the tropics, mobbing to repel a predator may be adaptive at both seasons due to the year-round residency of drongos. Furthermore, if black eagles use the frequency or intensity of mobbing as a clue to locate nests, a lack of seasonal difference in mobbing behaviour by drongos may be an evolutionary adaptive strategy.