Long-lasting mobbing of the pied flycatcher increases the risk of nest predation

Mobbing behavior may provide real benefits because mobbing prey individuals often cause a predator to leave the vicinity. However, mobbing calls of prey can attract acoustically oriented predators thus increasing nest predation. Therefore, a real value of mobbing as a type of adaptive behavior may depend on its duration. In this experimental study, we tested whether mobbing duration by the pied flycatcher Ficedula hypoleuca, a small hole-nesting passerine, increases the risk of nest predation. From the top of one nest-box within each of 78 experimental plots, we played back long calls of pied flycatchers, whereas recordings of short mobbing calls were played back from the top of another nearby nest-box. The nest-boxes were arranged in pairs, and each of them contained a quail Coturnix coturnix egg. Long-call nest-boxes were depredated by martens Martes martes significantly more often than short-call nest-boxes. This predator usually hunts by night and may eavesdrop on the calls of their mobbing prey while resting nearby during the day. The results of the present study show that long-lasting conspicuous mobbing calls may carry a significant cost for the breeding birds.