Trends in the numbers and mortality patterns of sparrowhawks (Accipiter nisus) and kestrels (Falco tinnunculus) in Britain, as revealed by carcass analyses

Abstract

The numbers of sparrowhawks Accipiter nisus received for study at Monks Wood Research Station increased greatly between 1963 and 1992, then declined by 50% in 1992–97. The long-term increase reflected the national population recovery over this period and followed the reduction in the use of organochlorine pesticides. It was most marked in the eastern parts of Britain where the earlier population decline was greatest. The numbers of kestrels Falco tinnunculus received for study fluctuated greatly from year to year, and showed only a slight net increase over the study period. There was a marked increase in the late 1970s in eastern districts, the only areas where kestrels had obviously declined during the peak years of organochlorine use. The sparrowhawk carcasses revealed two peaks in recorded mortality: August–September, affecting mainly recently independent juveniles; March–April, affecting similar numbers of adults and juveniles. Kestrels showed no obvious seasonal peak in recorded mortality, but fewest carcasses were received in the early breeding season April–May. Among sparrowhawk carcasses, the sex ratio was significantly biased towards females (0.8 male to 1.0 female), but among kestrel carcasses the sex ratio was close to unity. Overall, 64% of sparrowhawks and 69% of kestrels were juvenile (= first year) birds, implying a mean first-year mortality in the two species of 64% and 69%. The proportion of juveniles among sparrowhawk carcasses increased during the study period (probably reflecting improved breeding success), but the proportion of juveniles among kestrel carcasses showed no long-term trend. The most important causes of recorded deaths in both species were collisions (65% for sparrowhawk and 35% for kestrel) and starvation (18% for sparrowhawk and 40% for kestrel). The frequencies of different mortality causes recorded among carcasses found were unlikely to reflect their frequencies in the population at large, but they did reveal changes over time. During 1962–97, deaths attributed to organochlorine poisoning and shooting declined in frequency, while accidental deaths increased in frequency, especially collisions with road vehicles in kestrels and with windows in sparrowhawks.