Age and Breeding in Sparrowhawks

Abstract

In a population of sparrowhawks studied for 10 yr, yearlings formed 17% of breeding cocks and 16% of breeding hens. On average, yearlings were less numerous as breeders in upland forest plantations than they were in woods in upland sheepwalk or on low farmland but no significant variation in these figures occurred between years. The oldest cock and hen were both in their 9th yr. The average age of breeding cocks (2.16 yr) in the sample was lower than that of breeding hens (2.54 yr), associated with which the average age of mate was higher for cocks than for hens. Some birds bred in their 1st yr, others not until their 2nd or later year. Some individuals aged 2 or more years did not breed in a given year, even though they had bred previously. These findings applied to both cocks and hens. Yearling sparrowhawks were mainly brown in color, while adults were blue-gray. Mating was selective with respect to age (and plumage color), with more yearling-yearling and more adult-adult pairs, and fewer adult-yearling pairs than expected by chance. Among adult (blue-gray) birds, no evidence was found for selective mating with respect to age. On average, adult-adult pairs produced most young and yearling-yearling pairs produced least, while mixed pairs were intermediate. Age of both cock and hen had some influence on the number of young produced. Among known-age hens, laying dates, clutch sizes and nest success all improved up to .apprx. age 4 yr and then deteriorated again. These trends in hens could not be explained by age-related variations in the organochlorine content of the eggs. They held in both upland and lowland habitats, and after correction to allow for variations in performance associated with year, and (where appropriate) with laying date. Despite a general improvement in performance between 1st-2nd yr birds, 2nd yr hens which had bred as yearlings performed no better than did other 2nd yr hens which were not known to have bred as yearlings.