Breeding density of Sparrowhawk Accipiter nisus in relation to nest site availability, hatching success and winter weather

Abstract

A 360-km2 study area in southern Norway was searched for Sparrowhawk (Accipiter nisus) nests each year during 1985-96. The number of nests found increased from 21 in 1987 to 42 in 1992, and then decreased to 23 in 1996. Nest site spacing became more regular throughout the study period, with a higher frequency of the preferred young forest . The percentage of one-year-old breeding birds increased with greater snow depth during December-March, possibly because of the higher mortality of older winter-resident individuals . Both the proportion of hatched clutches and the number of breeding pairs were negatively correlated with snow depth and positively correlated with mean temperature in March. When snow depth was controlled for, there was also a positive correlation between the number of pairs and the proportion of hatched clutches in the previous year. The study confirms that the breeding density of the Sparrowhawk is sensitive to environmental conditions that influence the body condition, survival or breeding performance of the birds . It could, however, not be concluded that surplus non-breeding birds were absent in years of low breeding density.