Why conspecific nest parasitism is more frequent in waterfowl than in other birds: a reply to M. D. Sorenson

Abstract

We hypothesized that conspecific nest parasitism is rare in altricial birds because hosts face a high cost of being parasitized, and consequently have developed strong defenses against parasitism. Sorenson (1992) suggests that, on the contrary, parasitism is rare in altricial birds because parasitic females gain little reward for parasitism. We reject Sorenson's hypothesis for two main reasons. First, the abundant literature on egg addition experiments shows that altricial birds typically can raise additional young, but do so at a cost to their future reproduction and survival. Second, the high rates of parasitism observed in colonial and cavity-nesting altricial birds are simply inconsistent with his hypothesis. In this note we briefly review our hypothesis, introduce and assess an alternative hypothesis for the distribution of conspecific nest parasitism (CNP), and furnish an extended criticism of Sorenson's hypothesis. We previously demonstrated that CNP is considerably more common in precocial birds than in altricial birds (Rohwer and Freeman 1989). Our explanation of that pattern focused on hosts. We presumed that hosts obtain no benefits from CNP, but rather experience some reduction in reproductive success. We suggested that this cost of parasitism was great for altricial birds but minor for precocial birds. These differing costs of being parasitized relate to limits on reproductive output in the two groups of birds. Moreover, in response to the differential in costs of parasitism, we suggested that most altricial birds had developed defenses against CNP, whereas precocial birds had not developed antiparasitic strategies. A second explanation for the disparity in rates of CNP between the two groups of birds, which neither we nor Sorenson discussed, is that the parasitic strategy has rarely evolved in altricial birds. This argument presumes that the behavior would be a successful way to increase reproductive output when coupled with, or substituted for, normal parental care. We know of no means to test this evolutionary lag hypothesis. It appears to be seriously flawed, however, because parasitism occurs in a diverse array of altricial birds (Yom-Tov 1980; Rohwer and Freeman 1989), which implies repeated independent evolution. Likewise, parasitism is common in waterfowl and game birds, although we do not know how many times CNP has independently arisen in these latter two groups. Sorenson's explanation for the scarcity of CNP in altricial birds emphasizes a disparity between altricial and precocial parasites in the benefits gained from laying eggs in the nests of conspecifics. We believe that Sorenson's conclusions are seriously flawed, for several reasons. Although Sorenson attempts to take the parasite's perspec-