| Publication Type: | Journal Article |
| Year of Publication: | 1984 |
| Authors: | Newton, I, Marquiss, M |
| Journal: | Journal of Animal Ecology |
| Volume: | 53 |
| Issue: | 3 |
| Date Published: | 1984 |
| ISBN Number: | 00218790 |
| Keywords: | Accipiter, Accipiter nisus, Accipitridae |
| Abstract: | (1) Each year the start of laying among sparrowhawks in south Scotland was spread over a 4-6 week period, between late April and early June. The earliest pairs to lay produced the most young and, on average, the later a pair laid, the fewer the young raised. Seasonal declines were noted in clutch sizes (and hence brood sizes), and in the proportions of nests which were successful, as clutch desertions increased through the season. These trends held in every year of an 11-year study, in all main habitats and in all female age groups. Differences in mean performance between years, habitats and age groups were small compared with the seasonal variation within each of these categories. (2) The seasonal decline in performance ran contrary to the seasonal trend in food supply, which increased during the period that eggs were laid and desertions occurred. (3) It was interpreted in terms of local variations in prey availability and individual variations in hunting success. Those individuals which had access to a good prey supply, or were efficient hunters, were able to produce the earliest and largest clutches, and most often continue with the breeding attempt. Those which had access to a poor prey supply, or were poor hunters, produced later, smaller clutches and often deserted their eggs during incubation. From several lines of evidence, the late layers were short of food despite better supplies in the environment at the date they laid and failed. (4) At all stages, breeding performance was correlated with the body weight of the female, and hence with the food supply to the female, which was provided by the male. The most productive females gained weight earliest in spring, reached a higher weight prior to egg laying, lost less weight during laying, and started incubation at a higher weight than did less productive (later) females. (5) Late fledged nestlings were most likely to be found dead in their first year and early fledged birds were more likely to be recruited to the breeding population. No relationship was found between laying dates of females and their subsequent survival. (6) Repeatability of laying dates was calculated at 0.83 for males and 0.64 for females, but these high values were due largely to individuals using the same territories in successive years. No significant regressions of laying dates of offspring on those of parents were found. It is suggested that all individuals inherit the same trait to lay as early and as large a clutch (up to six) as possible, but that most are compromised in this by other conditions, particularly food supply. |
| URL: | http://www.jstor.org/stable/4661 |
| Short Title: | Journal of Animal Ecology |
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