Breeding performance of great tits (Parus major) along a gradient of heavy metal pollution.

The possible effects of heavy metal pollution on various breeding parameters of the great tit (Parus major) were studied at four study sites in a pollution gradient near a large nonferrous smelter in Belgium during three consecutive breeding seasons. Significantly more females interrupted their laying period near the pollution source than did females farther away. At the two most polluted sites, hatching success was significantly reduced compared to two lesser-polluted study sites. Overall breeding success was also significantly lower toward the factory complex. This lowered breeding success was caused not only by the decreased hatching success toward the pollution source but also by the significant difference in fledging success among sites. Although a clear relationship was observed between the exposure to heavy metal pollution and an impaired reproduction, not all reproductive parameters were affected. We found no significant differences in the onset of laying, clutch size, length of the incubation period, and the proportion of nests with low-quality eggs among sites. Although nest box occupancy was every year slightly lower toward the smelter compared to farther away, differences among study sites were never significant.