Apart from obligate and facultative brood parasitism in birds, mixed-species clutches and broods are uncommon. Competition among birds for limited nest sites can result in antagonistic interactions leading to nest usurpation. When one species evicts and takes over the nest of another it can, on rare occasions, result in a clutch containing eggs of both species. Here I report an instance of Carolina Chickadees (Poecile carolinensis) usurping a Prothonotary Warbler (Protonotaria citrea) nest that resulted in the first documented instance of chickadees fledging a brood containing both chickadee and warbler nestlings. During a nest box study in southern Illinois during 1994–2006, this was the only nesting attempt (n = 6,076) where there was interspecific nest usurpation. In this case, a female chickadee contributed to building the nest cup in the box and laid eggs on 3 of the same days as the female warbler. The chickadee pair usurped the nest from the warblers, and ultimately fledged a mixed brood containing 3 chickadees and 3 warblers. The displaced adult warblers nested in a nearby nest box where they fledged 3 warbler nestlings, subsequently reused the original usurped nest box for an attempt at a second brood after the chickadees' brood fledged, and fledged 5 more warbler nestlings from it. This was not a case of facultative brood parasitism because the warblers were first to start building a nest in the box, only to have the chickadees contribute to nest building before evicting the warblers after there were 3 eggs of each species in the nest. The potential for the Neotropical migrant warbler nestlings/fledglings to mis-imprint on the resident chickadees that raised them likely diminished the warblers' ability to successfully survive their first year of life and/or subsequently mate with other Prothonotary Warblers. The rarity of nest usurpation in this bottomland forest study system suggests that cavities that serve as nest sites are not limiting.

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