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 trend s, it is useful first to determine the demographic mechanisms through which they have acted. We present new analyses of survival rates, usin g British ring-recovery data from 1962 to 1995, for 31 farmland passer ine species. Separate estimates gf the survival rates of adults and fi rst-years and, where possible, males and females, are calculated speci fic to periods of increase, stability and decline identified objective ly from each species' Common Birds Census (CBC) index trend. The diffe rences between these estimates are assessed for statistical significan ce by comparing models allowing age-, sex- and trend-specific variatio n with models constraining these parameters. The variations in surviva l with respect to CBC trend direction show whether effects on survival have occurred which are consistent with their being an important mech anism mediating the effects of the environment on abundance. Adults ge nerally had significantly higher survival rates than first-years and t here was a common trend for male survival to be higher than that of fe males, although sex-specific variation was not significant for most sp ecies. Variations in survival appear to have contributed to the intera ctions between environmental change and abundance for at least 13 of t he species studied.