ABSTRACT

Construction of nests containing multiple layers or “stories” has been observed in a number of passerine species. These atypical nest structures are frequently a response to brood parasitism, wherein the host female will bury the parasite’s egg under a new layer of nesting material. They may also occur as a result of either intra- or interspecific competition for limited nest sites, with one individual usurping another’s nest and adding new material on top. However, such structures have not previously been documented in the Cerulean Warbler (Setophaga cerulea), a Neotropical migrant species that breeds in the eastern U.S. and Canada. Here, we report our observations of an unusual “double-decker” nest that was constructed in 2014 in Brown County, southern Indiana. A female Cerulean Warbler first built a typical nest, and several days later we discovered a second visibly distinct nest superimposed on top of the original one. Subsequent retrieval and inspection of the two-story nest revealed no buried egg or other definitive evidence of Brown-headed Cowbird (Molothrus ater) parasitism, which is known to occur in this species. The second story may have been built by a second female who usurped the original nest, but too few birds in our study population were banded for us to distinguish between females. Consequently, the reason for the second story remains unclear because of the unique breeding biology of this canopy-dwelling species, which makes observing nesting activity a challenge.

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