Predation-mediated bird nest diversity: an experimental test

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Abstract
Differences in nesting characteristics among species of a bird community may increase the mean reproductive success of all species by decreasing mean predation levels on nests (the predation/diversity hypothesis). I tested this hypothesis by (i) manipulating nest-type richness using natural translocated nests, and (ii) correlating predation levels on artificial nests with understory bird richness. I translocated abandoned natural nests (158 nests in 1992 and 199 nests in 1993) of five species of passerines to four (in 1992) and five (in 1993) 1.5 km long transects (ravines) in two-species or five-species assemblages. Experiments conducted in hardwood forests of southern Illinois, U.S.A., during two breeding seasons (1992 and 1993) showed significantly higher predation levels in low than high nest richness sites in 1992 and 1993, and in overall predation during both years. Predation levels, however, varied considerably within richness treatments. If high site-to-site variability in nest predation is not due to stochastic noise, nesting traits may be selected in opposite directions and strengths in different parts of a species' habitat or range. The nest predation/diversity hypothesis was supported at the landscape level but not at the local (ravine) level, stressing the importance of predation as a process structuring bird communities at a large spatial scale. The consequences of this high variation in nest-predation levels on populations, communities, and ecological processes should be well understood before conclusions are drawn.