Monitoring the changes in the size of a population does not in itself provide sufficient information on which to base an effective conservation strategy (Goss-Custard 1993). Concurrent monitoring of breeding performance and survival rates is necessary to allow changes in population size to be properly interpreted (Temple & Wiens 1989) and, for long-lived species, can provide early warning of impending conservation problems (Pienkowski 1991).

Where good long-term data sets for breeding performance and survival are lacking, conservation action may have to be taken without an adequate understanding of the mechanisms involved or need to wait for detailed research to be undertaken. For many species, however, BTO already has the necessary data, collected by its volunteers over periods of several decades.

For a long-lived species, a decline in population size may only begin after a long period of low survival or reduced reproductive output. The classic example is that of the Peregrine, which in the UK suffered from poor breeding performance during the 1940s and 1950s due to sub-lethal DDT contamination. This decreased the capacity of the non-breeding population to buffer the severe mortality of breeding adults that occurred due to cyclodiene poisoning from the mid 1950s onwards (Ratcliffe 1993). Monitoring of breeding performance gave an early warning of subsequent numerical decline (Pienkowski 1991). Another example of a decline in breeding performance that presaged population decline is the catastrophic breeding failures of seabirds, particularly Arctic Terns, in Shetland (Monaghan et al. 1989, Walsh et al. 1995).

Farmland birds

During the mid 1980s, the BTO identified rapid declines in the population sizes of several farmland bird species (O'Connor & Shrubb 1986, Fuller et al. 1995). The BTO has since been able to investigate demographic mechanisms of these declines, using its long-term historical databases (Siriwardena et al. 1998a, 2000a).

This investigation, which was Government-funded and undertaken jointly with Oxford University, looked at changes in population size, breeding performance and survival rates of a variety of species in relation to changing farming practice. It showed that species responded to different aspects of the agricultural environment, but that typically these aspects were linked to intensification or regional specialisation. Declines in survival rates were found to be the main factor driving population decline in these species, with the exception of Linnet, for which the main factor appears to have been a decline in nesting success at the egg stage. The study was therefore able to eliminate some possible causes of change, and identify areas for future research, thus helping conservation bodies to use their scarce resources productively. This work made an important contribution to the wide programme of work on farmland birds undetaken by many research and conservation organisations (Aebischer et al. 2000, Vickery et al. 2004).

This report describes a number of other cases where the combined analysis of BTO data sets has helped to identify the causes of population declines, for example on the pages for Lapwing (Peach et al. 1994), Song Thrush (Baillie 1990, Thomson et al. 1997, Robinson et al. 2004), Sedge Warbler (Peach et al. 1991), Spotted Flycatcher (Freeman & Crick 2003), Starling (Freeman et al. 2002), and House Sparrow (Freeman & Crick 2002). A fully integrated approach, estimating trends in numbers and demographic parameters through a single model containing data from various BTO surveys, is introduced by Besbeas et al. (2002).

Next Section - 1.3 The aims of this report

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