Summary 1. The stress caused by air pollutants was studied at biochemical, morphological and ecological levels in the Pied Flycatcher, Ficedula hypoleuca , and Great Tit, Parus major , nestlings in 10 study sites along the pollution gradient of a copper smelter. 2. First, stress was measured using four biomarkers from blood and liver: ethoxyresorufin O -deethylase (EROD) enzyme, haemoglobin, stress protein Hsp70 and deltaaminolevulinic acid dehydratase (ALA-d) enzyme. Second, the amount of fluctuating asymmetry (FA) in the length of the 3rd primary and the outermost rectrix (in P. major ) and the length and thickness of the tarsus were measured. These stress indicators were further examined in relation to the breeding performance of birds, i.e. the ecological response. 3. EROD activity was increased in F. hypoleuca nestlings near the pollution source. In P. major , it was not related to the pollution gradient but correlated well with the proportion of starved nestlings, i.e. the nutritional stress during the nestling period. The variations in haemoglobin, Hsp70 and ALA-d enzyme activity were not significantly related to pollution gradient. 4. The tarsus length of F. hypoleuca nestlings and primary length of P. major nestlings showed increased asymmetry in the vicinity of the pollution source. Breeding success decreased towards the pollution source in both species. 5. In F. hypoleuca , the pollution-related stress was verified at all three target levels. In P. major , pollution-related effects were found at morphological and ecological levels but not at the biochemical level, which suggests that direct toxic effect of heavy metals was not the main mechanism in this species
