Reanalysis gives further support to the ‘shotgun’ model of shiny cowbird parasitism of house wren nests

The ‘shotgun’ model of brood parasitism suggests that parasites randomly spread a large number of eggs in host nests, instead of carefully selecting a host nest for each egg they lay. Kattan (1997) reported data on the brood parasitic use made by shiny cowbirds, Molothrus bonariensis, of the nests of house wrens, Troglodytes aedon, in the Cauca Valley, southwestern Colombia, where the wrens are the cowbirds’ only effective host. Because shiny cowbirds do not precisely synchronize egg laying with their host’s laying cycle, and because they do not avoid multiply parasitized nests, Kattan concluded that cowbirds follow a shotgun strategy, relying more on high fecundity than on ‘targeting’, that is, placing each egg precisely in a nest that would guarantee a high success. Kattan tested for randomness in cowbird egg laying by comparing the distribution of cowbird eggs in wren nests with a Poisson distribution. This analysis, however, included the zero-egg class, and Lowther (1984) previously noted that inclusion of unparasitized nests biases the data, because some nests may remain undetected by cowbirds. The present note reports a reanalysis of Kattan’s data using a truncated Poisson distribution, and provides further support for his conclusions. The analysis is then extended to show that, contrary to Lowther’s conclusions, the shotgun model can also account for all data so far reported on brown-headed cowbird, M. ater, egg distribution. Kattan (1997, Figure 3) investigated the distributions of the numbers of cowbird eggs placed in wren nests in natural nests and nestboxes. In its simplest form, the shotgun strategy would predict that these distributions should conform to the Poisson distribution. If the cowbirds were selecting unparasitized nests, on the other hand, we should find an excessive number of nests with one or perhaps two eggs, relative to the Poisson distribution (i.e. cowbirds should avoid already parasitized nests). In fact, both in nestboxes and in natural nests, the distribution departed significantly from the Poisson distribution, so it is clear that egg laying was not random. But the data did not suggest that the cowbirds were targeting unparasitized nests. In the case of nestboxes, the deviation took the form of an excess number of nests with the largest numbers of eggs (11 or 12), the opposite of what would be expected if unparasitized nests were preferred. In the case of natural nest sites, there was an excessive number of nests containing zero eggs. Kattan accounted for this in terms of the cowbirds’ inability to enter cavities with small openings (see also Kattan, in press). This physical limitation means that some nests were unavailable to the cowbirds. Thus, the analysis does not address the question of whether the cowbirds’ use of those natural nests they can enter is consistent with random or selective laying. Similar situations have been noted in the distribution of eggs of the brown-headed cowbird with a variety of host species; data reported by a number of authors have been summarized by Preston (1948), Mayfield (1965), Lowther (1984) and Orians et al. (1989). Preston found systematic departures from a Poisson distribution, but he argued that the data could be accounted for if, within the sample of parasitized nests, the first egg was placed systematically, but subsequent eggs were randomly distributed. Lowther recognized that nests with no eggs constituted a special case, because some of them might not be available for parasitization, the same suggestion as Kattan (1997) made for the shiny cowbird. Lowther argued that if the Poisson model could be fitted without using the number of unparasitized nests, a stronger test of the shotgun model would be achieved. He therefore Correspondence: S. E. G. Lea, University of Exeter, Department of Psychology, Washington Singer Laboratories, Exeter EX4 4QG, U.K. (email: s.e.g.lea@exeter.ac.uk). G. Kattan is now at Apartado 25527, Cali, Colombia.