Interspecific feeding is an uncommon yet widespread phenomenon where an individual of one species feeds the young of another. There are numerous hypotheses to explain why this behavior might occur, but in most cases the observer can only speculate on the cause because they lack information on the nesting histories of the species involved. We observed a color-banded male Eastern Bluebird (Sialia sialis) feed 10-day-old Tree Swallow (Tachycineta bicolor) nestlings in a paired nest box 29 times during a 3 h nest observation. The male bluebird had previously fledged young from the box where the Tree Swallows were currently nesting and currently was nesting in the paired box, suggesting that the probable cause for this reproductive error was both the close proximity of the box and an attachment to the box where he had previously raised young. This observation suggests that instead of identifying the young by call, the male was using a place-based decision rule when he responded to the calls from the box where he had previously nested. Pairing nest boxes, a common practice used to reduce competition for nest boxes between bluebirds and Tree Swallows, may inadvertently increase the probability of interspecific feeding as a result of location-based decision rules for feeding and care of young.
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