The risk of nest predation may be related to nest size, in which case predation might cause selection for smaller nests, but body size forces larger species to build larger nests. This study attempts to find out whether nest size influences predation rate in two coexisting congeneric birds, the Reed Warbler Acrocephalus scirpaccus and the Great reed Warbler A. arundinaceus that differ in body size. Abandoned nests of both species, each containing one quail egg and one plasticine egg, were placed in the same type of habitat and at the same height. Nest size affected predation probability of the experimental nests since predation rate was lower in the smaller Reed Warbler, than in the Great Reed Warbler. Condition of eggs after predation did not differ between species which suggests that they share the same predators. Most marks on the plasticine eggs were produced by rodents. To test whether the scent of plasticine or quail eggs could attract predators, we performed a parallel experiment using empty Sherman traps and traps baited with a quail egg and a plasticine egg placed in the same area as nests. Capture success was similar in both types of trap and thus the scent of the eggs did not attract the predators. We discuss the implications of these findings for the interactions between these warbler species during the breeding season.
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