EFFECTS OF ECTOPARASITES ON BREEDING SUCCESS OF GREAT TITS (PARUS-MAJOR) AND PIED FLYCATCHERS (FICEDULA-HYPOLEUCA) IN AN AIR-POLLUTION GRADIENT

Numbers of ectoparasitic blow fly (Protocalliphora azurea) larvae (Dip tera: Calliphoridae), adult and larval hen fleas, Ceratophyllus gallin ae (Siphonaptera: Ceratophyllidae), and other nest-dwelling insects we re counted from the nests of the great tit, Pal-us major, and the pied flycatcher, Ficedula hypoleuca, in southwestern Finland around a fact ory complex emitting large quantities of sulphuric oxides and heavy me tals. Protocalliphora azurea larvae were more frequently found in the nests of P. major than in those of F. hypoleuca. The prevalence of P. azurea larvae tended to be lower in polluted areas. The number of P. a zurea larvae correlated positively with nest size and brood size of P. major. This may be caused by difficulties in sanitation of large and crowded nests. The numbers of fleas and their larvae were negatively a ffected by moisture in the nests. Nests of F. hypoleuca contained more adult fleas in polluted areas. This may indicate reduced resistance o f birds exposed to heavy metals, but other possibilities are also disc ussed. We found a detrimental effect of P. azurea larvae on the growth of P. major nestlings, and an effect of fleas on mortality of F. hypo leuca nestlings, but these effects were not enhanced by air pollution. We conclude that the ectoparasites studied, at densities observed in our study area, are of minor importance in determining the breeding su ccess of these two bird species.