We measured variation in body mass in male and female Kestrels in relation to age, food availability and reproduction. All body mass data were corrected for time of day by transforming them to morning weights, since there was a significant daily increase throughout most of the year in both sexes. Juveniles weighed less than adults, especially in low vole years. Both sexes had higher body mass in winter than in summer. Breeding females had maximum body mass during reproduction, whereas non-breeding females maintained a relatively low body mass throughout the reproductive season. Ad lib. fed Kestrels, held in captivity, showed the same difference in body mass between breeding and non-breeding females. This indicates a different body mass set point for breeders and non-breeders. Females rapidly increased in mass two weeks before laying. After laying they lost c. 30 g of body mass and maintained the same level until the young hatched. From hatching onwards female weights dropped sharply to a minimum level during moult. Males showed a gradual decrease in body mass of about 10 g during the whole reproductive phase. Ad lib. fed Kestrels held in captivity showed the same annual variations in body mass, but were heavier then free-living birds throughout the year except during laying (both sexes) and incubation (only females). Late-laying females produced smaller clutches and weighed less than early layers during the entire reproductive phase and the same trend in body mass showed up in males. Females layed at an earlier date in high vole years and attained higher body mass during incubation compared with low vole years. Analysis of covariance indicated that the dependence of body mass on laying date was the same in low and high vole years, and thus reflects variation within years related to individual nutritional conditions rather than spurious correlation between general effects of annual vole density on both mean laying date and mean body mass. Deposition of fewer body reserves by late females may let them gain time to advance laying date, maintaining more reserves may buffer early females against adverse weather.
|Number of pages||14|
|Publication status||Published - 1988|