Applications of large-scale studies of demographic rates to bird conservation

Abstract

Methods for diagnosing causes of bird population declines are described, paying special attention to methods that rely on comparisons among geographical regions and/or time periods with different population trends. Comparisons of breeding productivity and survival among regions or periods provide a valuable check on the diagnosis of external causes of a decline indicated by correlation with population trend. Ringing or tagging programmes can give information on population levels and are the main source of information on survival. Age ratios from counts or catches of juveniles and adults in standardized trapping programmes can provide data on breeding productivity. Nest recording schemes yield estimates of productivity if females make one breeding attempt per year or if the number of attempts per female can be estimated from supplementary data. Population models should be used to explore hypotheses about the external causes and demographic mechanisms of population declines. Models are described which have combinations of parameter values set to give rates of population decline similar to those seen in various passerines in Britain during the 1980s and 1990s. These models indicate that demographic changes associated with declines are subtle and difficult to detect if the demographic rate which changes is density-dependent. Analytical procedures should be devised to compare the fit of models with different underlying demographic mechanisms for the decline to data on survival and breeding success recorded before, during and after declines.