Large-scale spatial variation in the breeding performance of song thrushesTurdus philomelos and blackbirds T-merula in Britain

1. Spatial variation in breeding performance is of critical importance in u nderstanding the large-scale distribution and abundance of living species, and in understanding species conservation. We studied the large-scale spati al variation in reproductive output of two species of declining British bir d, the song thrush Turdus philomelos and the blackbird Turdus merula. 2. We developed a method to predict spatial variation in reproductive outpu t. Brood size and nest failure rates during the incubation and nestling per iods were related to environmental factors using generalized linear models. Predicted values obtained from these models were combined to give values o f number of fledglings produced per nesting attempt for 10-km squares throu ghout Britain. 3. We observed substantial spatial variation in reproductive output for bot h species; the component that varied most was nest failure rate during incu bation. We were more successful in relating environmental factors to spatia l variation in reproductive output for song thrush than for blackbird. 4. Reproductive output in both species was affected mainly by factors that vary on a small spatial scale. Nest failure rate during incubation increase d significantly where corvids were more abundant, suggesting a role for avi an nest predators in determining spatial variation in reproductive output. 5. Our approach can be extended readily to other species of birds, to other taxonomic groups and to finer spatial scales. Such models could be used to evaluate the implications of current and proposed wider countryside manage ment for spatial variation in breeding performance. Evaluations based on br eeding success as well as numbers are likely to be more robust than those b ased solely on abundance.