Estimating conspecific nest parasitism in the northern masked weaver based on within-female variability in egg appearance

-The Northern Masked Weaver (Ploceus taeniopterus), like many weavers, exhibits a dramatic degree of intraspecific variability in egg appearance. While the rule is that all eggs within a nest look alike, this rule frequently is violated. By observing marked females laying eggs, I determined that within-female variability in egg appearance is both significantly and substantially lower than the variability seen among eggs laid by different females. Because communal breeding does not occur in this species, and because I ruled out parasitism by heterospecifics, nests containing odd-looking eggs must have been victimized by conspecific nest parasites. Based on the maximum variability in egg appearance observed in nonparasitized clutches, and by noting clutches with peculiar laying sequences that are indicative of parasitism (e.g. two eggs appearing on the same day), I determined the rate of conspecific nest parasitism in the Northern Masked Weaver to be between 22.8% and 34.7%. This is one of the highest rates of conspecific nest parasitism reported for passerines. My results support the conclusion that conspecific nest parasitism is a widespread and common behavior among Ploceus weaverbirds. Received 6 December 1990, accepted 13 January 1992. THE Northern Masked Weaver (Ploceus taeniopterus), like many of its congeners, exhibits a dramatic degree of intraspecific egg-color variability (Mackworth-Praed and Grant 1955, Moreau 1960, Freeman 1988, pers. observ.). Eggs range from dark brown to tan to blue-green. They can be either speckled or immaculate and, if speckled, can have various patterns of speckling. Typically, as in other Ploceus species with variable eggs, all the eggs within a nest look alike, suggesting that eggs produced by individual females do not vary in appearance. Initial support for this conclusion comes from studies of a captive colony of Village Weavers (P. cucullatus; Victoria 1972, Collias 1984); eggs laid over a lifetime by individually marked females varied little in either color or speckling. However, the rule that all eggs within a nest look alike is frequently violated in the Northern Masked Weaver, as well as in other weavers. In a study of Ploceus egg collections, Freeman (1988) found that among 16 species with variable eggs, between 2% and 29% of the nests of a species contained an odd-looking egg. If females produce only one egg type, these oddlooking eggs must have been laid by a brood parasite. Communal nesting is unknown among I Present address: Burke Museum DB-1O, University of Washington, Seattle, Washington 98195, USA. Ploceus species and, in this study of the Northern Masked Weaver, I never observed two females tending to the same nest. Freeman (1988) concluded that the parasites were conspecifics, rather than Chrysococcyx cuckoos, common brood parasites of many tropical Old World passerines, because he found no correlation between the incidence of cuckoo parasitism reported in the literature and the incidence of odd-egg clutches. In addition, Jensen and Vernon (1970) incubated 12 oddly colored eggs they discovered in weaverbird nests, and all of them hatched into weavers of the species in whose nest they were found. Finally, Chrysococcyx cuckoos are relatively unimportant brood parasites of the Northern Masked Weaver (Jackson in press a). Thus, conspecific nest parasitism (CNP) appears to be a common occurrence among Ploceus weaverbirds. Conspecific nest parasitism is now known to be widespread and common in birds. In an extensive review of the phenomenon, Rohwer and Freeman (1989) listed more than 140 species that engage in this behavior. The number known to exhibit this behavior has increased dramatically over the years and is likely to continue growing (see Jackson 1990). This growth is the result of improved detection rather than changes in the rates of CNP. Because in most species there is little variation in egg appearance, either within a nest or between nests, researchers rare-