DO PARASITIC MITES DECREASE GROWTH OF NESTLING PIED FLYCATCHERS FICEDULA-HYPOLEUCA

The effect of a parasitic mite Dermanyssus hirundinis on the growth an d survival of nestling Pied Flycatcher Ficedula hypoleuca was tested b y an experiment where the parasite level was reduced in half of the ne st boxes using an insecticide. Although the abundance of mites was sig nificantly reduced in the experimental group, there was no difference in body mass, tarsus length or fledging rate between nestlings in the experimental and the unmanipulated group. There was however a negative correlation in the unmanipulated group between body mass and mite abu ndance, as had also been observed in an earlier, non-experimental stud y (Merino & Potti 1995). I suggest that this correlation is a conseque nce of variation in some aspect of parental quality and not a direct e ffect of the parasitic mites.