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Resumen:
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Vertical banks in mining sites can provide safe nesting sites for burrowing bird species. These burrows can then, in turn, provide nest sites for species unable to create burrows themselves, especially in areas where the abundance of safe nesting holes is a limiting factor. Thus, primary burrowing species can provide ecosystem services for secondary cavity-nesting species. Our objective was to study the availability of burrows of biotic origin in mining sites, and their role in improving colonization and local biodiversity of cliff-nesting birds (birds that rely primarily on rocky or sandy walls to breed). We selected the Sand Martin because it is a colonial ecological engineer species that nests frequently in mining sites. First, we estimated Sand Martin burrow abundance and occupation rates and identified secondary cavity-nesting birds in 8849 burrows from 30 colonies in ten mining sites in central Spain. Second, we studied the dynamics of the Sand Martin burrows, by estimating annual construction and disappearance rates. Finally, we studied factors that could favour secondary cavity-nesting bird occupation. We found that Sand Martins burrowed more than was previously estimated in mining sites, and that their burrows were used by five species of secondary cavity-nesting birds. The number of available burrows each year varied due to relatively high annual construction and disappearance rates. Numbers of Sand Martin pairs and burrows in the colonies were the main factors favouring secondary cavity occupation. Our results support promoting Sand Martins in mine restoration projects, not only to benefit this endangered bird, but also because their ecosystem services can benefit other cliff-nesting species of birds, thus increasing local biodiversity.
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