Diagnosing the causes of bird population declines using comparative methods: The value of data from ringing

Abstract

Methods for diagnosing the causes of bird population declines are described. Special attention is paid to methods which rely on comparisons across geographical regions and/or time periods with different population trends. Comparisons of breeding success and survival across regions or periods provide a valuable check on the diagnosis of the causes of a decline determined by correlation of population trends with candidate causal external factors. Ringing can provide information on population levels and is the main source of information on survival. Age ratios of trapped birds and catches of juveniles and adults in standardised trapping programmes can provide data on breeding success. The latter information complements residts from nest recording because it reflects season-long breeding success, rather than success per breeding attempt. Morphometric measurements of trapped birds also have potential uses for the assessment of breeding success. The computerisation of ringing information, as well as of recoveries, is essential for comparative analysis of demographic parameters. Population models should be used to test hypotheses about the external causes and demographic mechanisms of population declines. Models are developed with combinations of parameter values set to give rates of population decline similar to those seen in various passerines in the 1980s and 1990s. These models indicate the probable magnitude of demographic changes associated with declines under various sets of assumptions about density-dependence. Statistical methods could be developed to compare the fit of models with different underlying demographic mechanisms for the decline to data on survival and breeding success recorded before and during declines. A role is identified forco-operative comparative studies involving both amateur and professional ornithologists. These would compare external variables and estimates of demographic rates from ringing among selected study areas with contrasting population trends. © 1999 British Trust for Ornithology.