Large‐scale spatial variation in the breeding performance of song thrushesTurdus philomelosand blackbirdsT. merulain Britain

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Abstract
1. Spatial variation in breeding performance is of critical importance in understanding the large‐scale distribution and abundance of living species, and in understanding species conservation. We studied the large‐scale spatial variation in reproductive output of two species of declining British bird, the song thrushTurdus philomelosand the blackbirdTurdus merula.2. We developed a method to predict spatial variation in reproductive output. Brood size and nest failure rates during the incubation and nestling periods were related to environmental factors using generalized linear models. Predicted values obtained from these models were combined to give values of number of fledglings produced per nesting attempt for 10‐km squares throughout Britain.3. We observed substantial spatial variation in reproductive output for both species; the component that varied most was nest failure rate during incubation. We were more successful in relating environmental factors to spatial 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 increased significantly where corvids were more abundant, suggesting a role for avian 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 management for spatial variation in breeding performance. Evaluations based on breeding success as well as numbers are likely to be more robust than those based solely on abundance.