Explaining annual fluctuations in breeding density of fieldfares Turdus pilaris– combined influences of factors operating during breeding, migration and wintering

In a subalpine birch forest in Central Norway, the breeding population of fieldfare Turdus pilaris varied from 3 to 63 pairs per km² during 1966–2000. For the period 1971–1995, the breeding density was negatively related to the number of days with >75% snow covered ground in April, presumably because snow cover reduce the availability of earthworms. In a multiple regression model for the period 1974–1995, snow cover in spring, the number of migrating fieldfares observed at two ornithological stations at the southern coast of Norway in April, the mean temperature in November in the preceding year, winter temperatures, rowanberry production in the previous autumn, and the interaction effect of the two latter variables (high value of one of these variables reduced the negative effect of a low value of the other) explained 80% of the variation in fieldfare breeding density. Autumn and winter temperatures, as well as rowanberry production, contributed through a positive effect, probably because these factors affected the number of fieldfare present in Norway throughout winter, which in some years may constitute a significant proportion of the overall fieldfare population.