Rich calcium availability diminishes heavy metal toxicity in Pied Flycatcher

1. Eggshell thickness, egg size, clutch size and egg hatchability of Pied Flycatcher (Ficedula hypoleuca Pallas) are markedly depressed and nestlings show delayed ossification of their leg and wing bones near the copper smelter in Harjavalta, SW Finland. Both defects suggest problems in calcium availability or metabolism. However, another species, the Great Tit (Parus major L.), lays relatively normal eggs and clutches even in the most polluted areas. The Ca content of nestling faeces was measured to estimate between-species differences in Ca availability. 2. Parus major nestlings showed four times higher Ca contents in their faeces than F. hypoleuca nestlings, suggesting differential abilities to acquire Ca between the two species. 3. The availability of Ca-rich food items was estimated by counting spilled snail shells in the nests of F. hypoleuca postfledging. Snail shells were frequently found except at nests closest to the factory where the amount of exchangeable Ca in organic soil is low and ground layer vegetation is scanty. 4. The number of snail shells in F. hypoleuca nests correlated positively with the Ca concentration of nestling faeces. Furthermore, snail shell numbers and Ca concentration in faeces were positively associated with breeding success. 5. There was a significant interaction between Ca and Cu levels of faeces, so that the negative effect of Cu on the number of fledglings was stronger when the Ca level was low, i.e. F. hypoleuca demonstrated poor breeding success especially at territories high in heavy metals and low in snail shells. Our results show that heavy metals have detrimental effects on F. hypoleuca nestlings especially when Ca availability is poor.