Temporal variation in the annual survival rates of six granivorous birds with contrasting population trends

To identify the environmental changes responsible for the declines in abund ance shown by many granivorous bird species, the demographic mechanism thro ugh which the changes have acted must be determined. Ring-recovery data wer e used to estimate the annual survival rates (since 1962) of six seed-eatin g bird species with contrasting population trends to identify whether varia tions in survival could have been the mechanism behind population change. T he survival rates of Bullfinch Pyrrhula pyrrhula, Chaffinch Fringilla coele bs, Goldfinch Carduelis carduelis, Greenfinch C. chloris, Linnet C. cannabi na and House Sparrow Passer domesticus were estimated using models allowing age- and time-specificity in survival (reporting rates could be assumed to be constant). Three tests of the importance of variations in survival in d etermining population trend were conducted: (1) simple population models wi th constant productivity showed whether temporal changes in survival were s ufficient alone to explain observed trends in abundance, (2) survival model s incorporating changes in abundance as a covariate identified whether annu al survival rates were associated with population changes, and (3) mean sur vival rates found in objectively identified periods of increase, decline an d stability in each species' population trend were compared. These analyses suggested that environmental change has led to the observed population tre nds for Goldfinch and House Sparrow largely through effects on survival. We aker relationships between variations in survival and population trend were found for Bullfinch, Chaffinch and Linnet, but other factors such as breed ing success are likely to have been at least as important for these species , and also for Greenfinch. Checking analyses incorporating density-dependen ce did not alter these conclusions.