Estimation and limitation of numbers of floaters in a Eurasian Sparrowhawk population



Over a 20-year period, the numbers of Eurasian Sparrowhawk Accipiter nisus nests found in a 200 km2 area in south Scotland remained relatively stable (mean 33.3 pairs, CV = 10.6%). Nest numbers fluctuated from year to year in a manner expected of a population subject to density-dependent regulation. The numbers of non-breeders (floaters) could not be counted directly, but the number of female floaters was estimated, using known mortality rates, from the numbers of females recruited to the breeding population each year at different ages. Female floater numbers were estimated by two methods: Method A assumed that birds bred for the first time in their first, second or third year, in the same ratio as they were found breeding for the first time in the study area; and Method B assumed that all third-year birds found breeding for the first time in the study area had bred previously, unknown to us, outside the area. Under Method A, floaters consisted of some 1-year and some 2-year birds; while under Method B, floaters consisted only of some 1-year birds. Under both methods, the estimated number of female floaters fluctuated greatly from year to year, but under Method A they averaged 0.90 per female breeder, and under Method B they averaged 0.28 per female breeder. The Method B estimate was most consistent with other data and with the finding that some birds found breeding in the study area were likely to have bred previously outside the area (because of territory changes). Moreover, the mean and variance in female floater numbers estimated by Method B were similar in magnitude to the values obtained in a simulation model. In this model, breeding density was given a fixed ceiling, while breeding success was allowed to vary from year to year within the limits observed in the study area. It was concluded that (a) recruitment of floaters to the breeding sector was density dependent with respect to breeder numbers, i.e. broadly speaking, floaters filled gaps in the territorial system left by the deaths and movements of established breeders; and (b) floater numbers themselves were probably not regulated in a density dependent manner, but depended on whatever was the balance between annual additions (from reproduction and immigration) and subtractions (from mortality, emigration and entry to the breeding sector).