In the evolution of interspecific social parasites, the shift from facultative to obligate brood parasitism is thought to be driven by the cost of parental investment. Accordingly, avian brood parasites with precocial young are almost exclusively facultative parasites, whereas those with altricial young are almost exclusively obligate parasites. Surprisingly, then, North American cuckoos (Coccyzus spp.) have altricial young but are described as facultative brood parasites. Because little is known about parasitism by Coccyzus cuckoos, we explored the potential importance of heterogeneric brood parasitism to their reproductive strategy. In contrast to the existing set of anecdotal reports of cuckoos parasitizing songbirds, we found no evidence of cuckoo parasitism in 10 197 songbird nests, despite spatial and temporal overlap between cuckoos and potential hosts and despite varied food availability. Experiments revealed a lack of egg-rejection behavior in some of the most common potential hosts, suggesting that parasitic eggs would be accepted if laid and that we would detect cuckoo parasitism if it occurred regularly. We propose that reports of Coccyzus cuckoos parasitizing songbirds stem from errant attempts to parasitize other cuckoos. This resolves a theoretical paradox about interspecific parasitism and mode of offspring development, as we suggest that Coccyzus cuckoos have not evolved to parasitize other species. Copyright 2009, Oxford University Press.
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