Age Structure and Survival in a Sparrowhawk Population

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Abstract
Using birds ringed in a local study, 3 sets of estimates were made of sparrow hawk survival based on the following criteria: age ratios among birds reported dead by members of the public; age ratios among birds trapped alive at nests in the study areas, and the year-year persistence of individual hens in the local breeding population. Breeding numbers declined slightly during the study period. Fron birds found dead, the annual survival of cocks aged 2-9 yr was estimated at 67%, and of hens aged 2-9 yr at 71%. First year survival was estimated at 31% for cocks and 49% for hens. From birds found breeding, the annual survival of a small sample of cocks aged 2-9 yr was estimated at 46%, and for a large sample of hens aged 2-9 yr at 64%. From the persistance of individual hens from year-year in the breeding population, annual survival was estimated at about 66%. These various survival estimates for the study population were generally higher than those obtained in previous studies, which were based on birds reported dead in widespread ringing schemes of long duration. An estimated 8% of hens born in the study areas subsequently bred there, and formed about 23% of the local breeding population. About 24% of breeding cocks had also been born in the areas. Of hens found for the 1st time breeding in the study population, 26% were in their 1st yr of life, 33% in their 2nd yr and 41% in a later year. Annual recruitment of hens to the breeding population was estimated at 40% on average.

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