Polluted environment and cold weather induce laying gaps in great tit and pied flycatcher.

We studied the occurrence of laying gaps in free-living populations of the pied flycatcher, Ficedula hypoleuca, and the great tit, Parus major, in a pollution gradient of a copper smelter in south-west Finland. Laying gaps were 2.8 times more common in F. hypoleuca than in P. major. The probability of laying gaps was highest in the heavily polluted zone and lowest in the unpolluted zone for both bird species. Cold weather at the time of laying increased the number of laying gaps in both species, but in P. major this effect was most pronounced in the heavily polluted environment. In the most heavily polluted environment the laying gaps were more likely to occur near the beginning of the laying sequence in both species. The laying gap probability increased with increasing laying date in P. major but not in F. hypoleuca. We suggest that the increased number of laying gaps in the polluted environment results from limited Ca availability and the interference of heavy metals with Ca metabolism in laying females.