Survival and dispersal of male CorncrakesCrex crexin a threatened population

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Abstract
Survival and between-year movements of Corncrakes ringed as chicks and adult males were investigated in Scotland and Ireland. The probability of movement to sites at various distances from the ringing place was described by a modified random walk model. Most males returned to within a few kilometres of where they were ringed but there were indications that birds ringed as chicks dispersed further than adults. The dispersal model was combined with census data to estimate the probability of subsequent capture for each ringed individual and mean annual survival probabilities. First-year survival was estimated at 0.236, but with a wide confidence interval. Adult survival was estimated more precisely at 0.186. Independent data from studies of breeding success and population trends, taken with the estimated adult survival from recaptures, indicated a higher first-year survival rate (0.339), but the confidence intervals of the two estimates overlapped. Because adult survival was low, the rate of population growth was estimated to be very sensitive to changes in productivity (chicks reared to independence per female). As a result, conservation measures which reduce losses of eggs and chicks during mowing, although they lead to modest increases in productivity, were estimated to be sufficient to reverse the long-term decline of the Corncrake population in Britain. Recent changes in population trend after the implementation of these measures are consistent with this expectation.