EN
Analysis of weight and size of the fledglings of pied flycatchers (Ficedula hypoleuca) and great tits (Parus major) was carried out during the breeding season in 1995. The presence of flea ectoparasites influenced fledglings’ weight but, contrary to expectations, birds were heavier in infested nests. The wing and tarsus size of the pied flycatchers’ fledglings were lower in nests where fleas were found, whereas great tits’ fledglings in parasitized nests had longer wings and comparable tarsi. These differences could be attributed to clutch size adjustment which appeared to take place in flea-infested nests of great tits.