THE ABUNDANCE OF RIVER CORRIDOR BIRDS IN RELATION TO THEIR HABITATS -A MODELING APPROACH

1. Abundance of territories on 837 500-m sections of river corridor di stributed throughout England and Scotland, collected as part of the Br itish Trust for Ornithology Waterways Bird Survey, were analysed in re lation to measured habitat characteristics at these sites. 2. The vari ation in habitat types and the abundance of birds across the range of river sections was analysed using detrended correspondence analysis. T he first axis of both ordinations was similar, showing a major trend f rom upland fast flowing to lowland slow flowing rivers in both cases. 3. The first axis of the habitat characteristics ordination was used a s a measure of habitat 'type' as an independent variable and the abund ance of territories of each species used as dependent variables in ser ies of log-linear regression models with a restricted data set of Engl ish and Welsh rivers. Twenty-three species showed significant relation ships with water quality, 20 to the first DCA habitat variable, 10 to the second DCA habitat variable, 22 to Northing, 21 to Easting and 17 to river altitude. 4. These single variable models were extended to jo int variable models using a parsimonious approach to variable selectio n. In analyses of deviance, water quality followed by the first DCA ha bitat variable, were the most important factors affecting the abundanc e of bird territories. For 13 species where water quality was included as a significant factor in the model, the parameter estimates were ne gative, indicating that rivers with poor water quality had fewer terri tories, with redshank, grey and pied wagtail being the species most se nsitive to water quality. 5. The numbers of territories of all species predicted from the above models, based on measured habitat and enviro nmental characteristics, were compared with observed numbers determine d by field survey of three river systems not included in the original analysis. Predicted territories of mallard, moorhen, coot and reed bun ting matched observed numbers reasonably. In the case of shelduck, lap wing, oystercatcher and whitethroat, more were observed than predicted , whereas for little grebe, pied wagtail and sedge warbler, more were predicted than observed. Those species for which territory data were o verdispersed were those for which model predictions were worst, relati ng to the poor fit assumed by the Poisson error term in the model. 6. The value of an integrated modelling approach to policy formulation an d river management prescriptions using these results is discussed.