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Changing balance between survival and recruitment explains population trends in red knots Calidris canutus islandica wintering in Britain, 1969-1995

By Theunis Piersma and Hugh Boyd


The demography of Red Knots Calidris canutus islandica wintering in Britain from 1969 to 1995 was examined using published data on winter numbers, unpublished ringing data, and information on the percentages first year birds in late autumn and winter (defined here as 'recruitment'). The maximum time between ringing and reporting was 24 years for a Red Knot ringed as an adult. The mortality rate of juveniles during their first winter was four times that of adults, but did not differ later. For three periods, recruitment minus mortality corresponded qualitatively with changes in population size. In 1969-77, when the numbers wintering in Britain were declining by 8.7% annually, the mean percentage juveniles in the wintering population was estimated at 12.1% and the mean adult mortality rate was estimated at 23.6% per year (balance -11.5%). In 1977-85, when numbers were increasing by 4.6% per annum, the average juvenile percentage was high (28.8%), but the adult mortality rate was lower than in the previous period (19.6%; balance +9.2%). In 1985-95, when wintering numbers showed no clear trend (average +0.9% per annum) but were still below those in 1969-72, juvenile percentages were again quite low (mean of 13.8%), but the adult mortality rate was also low (14.2%; balance -0.4%). Cold summers reduced both recruitment and adult survival. British Knot populations appear to fluctuate as a consequence of factors affecting survival as well as reproduction. There is evidence for density-dependent processes that affect overall reproductive success. Although probably acting sometime from late winter to early autumn, the mechanisms causing such density-dependence remain mysterious.

Year: 2001
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