I examined both natural and artificial nests to investigate the relationship between nest-site characteristics and the risk of nest predation in a grassland-nesting passerine, the Black-browed Reed Warbler Acrocephalus bistrigiceps. Although an early-season artificial nest experiment did not detect significant effects because of infrequent predation, a late-season experiment indicated that nests were more likely to be depredated at sites where Phragmites reeds were highly dominant. Stems of the common reed P. communis are extremely thick and strong compared to those of other grasses. The results suggest that strong stems aid the effective search and attack on bird nests by predators. Among natural nests, however, predation was not explained by any nest-site characteristics, suggesting that the activity of parents and nestlings masked the effect of plant strength on nest predation.

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