We compared predation rates of artificial nests baited with different egg types with the predation rates on nests of the Brown-and-yellow Marshbird (Pseudoleistes virescens), a Neotropical passerine common in the same area, in Argentina. We used deserted natural nests as experimental nests, baited with either Japanese Quail (Coturnix coturnix) or Shiny Cowbird (Molothrus bonariensis) eggs. Each experimental nest was baited with one natural and one paraffin-filled egg. Experimental nests baited with quail eggs suffered lower predation and egg losses than those baited with cowbird eggs. The main cause of egg loss and nest mortality in “cowbird nests” was pecking behavior of cowbirds. However, when we removed the effect of cowbirds, differences still held. In this area, the Shiny Cowbird is common and is the main cause of reduced breeding success of several host species. “Quail nests” had higher nest and egg survival than active Brown-and-yellow Marshbird nests. In contrast, “cowbird nests” and active Brown-and-yellow Marshbird nests had similar nest and egg survival. Experimental nests baited with natural passerine eggs such as those of Shiny Cowbirds seem to approximate the natural nest and egg predation risk for passerines in our study area.
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Vol. 74 • No. 3