We have investigated whether differences in nestling diet found between locally sympatric Redstarts and Black Redstarts are caused by species-specific preferences or by a different food supply in their territories. The diet of nestlings in a mosaic-like urban environment was studied using the neck-collar method. We found no significant difference in the length of Redstart and Black Redstart prey items. However, the two species did bring to their nestlings invertebrates of different taxa. We used the variance partitioning method based on multivariate Redundancy Analysis to test the influence of habitat, timing of breeding, and the species of redstart itself on nestling-diet composition. Most of the variance in the nestling diet (all the canonical axes explained 70.6% of the variance) could be attributed to habitat variables (34%) and the timing of breeding (8.9%), but only 8.1% to the species of redstart. We suggest that the diet of the two redstart species is influenced largely by current prey availability and, consequently, that interspecific competition is avoided primarily by territory exclusion rather than by food-niche separation. We consider the variance partitioning method to be a powerful tool for identifying the effects of various explanatory variables that could influence food composition in birds.
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