Empty nests in the great tit (Parus major) and the pied flycatcher (Ficedula hypoleuca) in a polluted area.

Great tits (Parus major) are sometimes found to incubate in their nests even though the nests contain no eggs. This phenomenon has been observed in different parts of Europe (Sweden, Finland, UK, Netherlands, Germany) and it has become more common during the 1980s. We analysed the occurrence of empty nests in P. major and in pied flycatcher (Ficedula hypoleuca) in SW Finland from 6-year data collected in a polluted area around a copper smelter which emits large quantities of sulphuric oxides and heavy metals. Among first broods 3.3% of P. major females incubated in empty nests. Incubation on empty nests was not observed in F. hypoleuca, but in this species 2.4% of the nests remained without eggs. In both species empty nests were more common close to the pollution source than farther away. Histopathological analyses in P. major suggest that there are diverse reasons for the incapability of a female to lay. One out of five P. major females captured from incubating in an empty nest had dark cysts in her oviduct. Two P. major females had very scanty medullary bone tissue in their tibiotarsus and tarsometatarsus and one female showed exceptionally high concentrations of lead in her bone tissue. We conclude that air pollutants enhance the number of empty nests especially in F. hypoleuca, but cannot wholly explain the phenomenon in P. major.