Growth of nestling sparrowhawks (Accipiter nisus)

Abstract

Growth rates, mortality and parental care of nestling sparrowhawks were studied in southwest Scotland. Ae Forest was a conifer plantation 200–400 metres above sea‐level, while the Annan valley consisted of farmland, woods, and small plantations on low ground.Nestling sparrowhawks were measured daily from hatching for 21–24 days. Weight, tarsus length and outermost primary feather length were recorded. Nestlings could be sexed by the age of 16 days from the larger size of females, which were significantly heavier than males at one day, had longer tarsi at nine days, and longer primaries at 18 days. Growth rates were calculated using linear regression over standardized periods of about 10 days.Growth rates were independent of brood size, and were negatively correlated with hatching date in one area. Hatching order and growth rate were correlated within broods. The greatest differences in growth rates were found between zones of Ae Forest, and between forest and valley.Twenty‐one per cent of nestlings over two days old died. Causes of mortality were starvation, wet weather, predation and desertion. Most of the mortality occurred in parts of Ae Forest remote from valley woodlands.The presence or absence of the adult female was noted on nest visits. When habitually brooding, until the young were 11 days old, the hen was present on about 85% of visits. By fledging this figure fell to 66% in the valley, and to 32% in the forest.The development of sexual dimorphism is discussed; females gained weight and body size faster than males, which developed various skills sooner.It is suggested that differences in growth rates between parts of the study area were related to food supply. Poor growth rates, high mortality, and lack of parental care all occurred in areas which were remote from sources of abundant prey, as measured by song‐bird censuses.