Variation in the survival rates of some British passerines with respect to their population trends on farmland

To identify the environmental changes responsible for population trends, it is useful first to determine the demographic mechanisms through which they have acted. We present new analyses of survival rates, using British ring-recovery data from 1962 to 1995, for 31 farmland passerine species. Separate estimates gf the survival rates of adults and first-years and, where possible, males and females, are calculated specific to periods of increase, stability and decline identified objectively from each species' Common Birds Census (CBC) index trend. The differences between these estimates are assessed for statistical significance by comparing models allowing age-, sex- and trend-specific variation with models constraining these parameters. The variations in survival with respect to CBC trend direction show whether effects on survival have occurred which are consistent with their being an important mechanism mediating the effects of the environment on abundance. Adults generally had significantly higher survival rates than first-years and there was a common trend for male survival to be higher than that of females, although sex-specific variation was not significant for most species. Variations in survival appear to have contributed to the interactions between environmental change and abundance for at least 13 of the species studied.