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During a study of Red-necked Grebes Podiceps grisegena in Lake Slagsmyren, Sweden, a pair of Common Terns Sterna hirundo were observed to have placed their two eggs on a deserted, floating nest of a Red-necked Grebe. Prior to this, the grebes had laid one egg of their own in the nest and this egg was included by the terns in their clutch and incubated by them. The species assignment of the odd egg was confirmed with DNA sequencing. A lack of natural nest sites for the terns to use at the lake could have caused this somewhat unusual choice of a nest site. I discuss possible explanations for adopting a foreign egg, including the adaptive behavioural response to roll an egg into the nest bowl to salvage lost eggs, the incubation stimulus that foreign eggs or egg-like objects potentially provide, and the limited egg discrimination abilities of Common Terns. The incubation of a foreign egg may reasonably be assumed to cost energy but to be of little benefit, if any, to the incubator.

Common Terns Sterna hirundo incubating a Red-necked Grebe Podiceps grisegena egg