Clutch Size, Egg Size, and the Consequences of Single Vs. Multiple Parasitism in Parasitic Finches

Clutch size and number of eggs in a season were determined for 11 species of parasitic African finches by examination of the ovaries. Clutch size averaged 3.14 for 10 species of Vidua and 2.91 for the Cuckoo—finch Anomalospiza imberbis. No significant species differences or latitudinal variation were found. Some Village Indigobirds V. chalybeata ♀ ♀ mate repeatedly for as long as 5 mo; 90 days is an average season. A ♀ lays 22—26 eggs in a breeding season. The eggs of the parasitic viduines and Anomalospiza are relatively smaller than those of other nesting finches (Ploceinae and Passerinae), and mean clutch size is larger than in the Ploceinae, the presumed ancestral group. Clutch size in V. chalybeata corresponds with the largest number of parasite young that a pair of foster parents Lagonosticta senegala rear in nature, and survival data (Morel 1973) indicate no net density—dependent effect on success of the parasites from egg to fledging in multiply parasitized nests. The frequency distribution of parasite eggs in all host nests in two wild populations is contagious, with more instances of multiple parasitism than if eggs had been laid randomly. An aviary ♀ laid one to three eggs in sequence in individual host nests. Nesting V. chalybeata are tolerant of other nestlings as expected if parasite young were often siblings. It is suggested that clutch size has likely been selected to correspond to the number of parasite young that the foster parents can rear. Nevertheless, half of all parasitized nests have a single parasite egg, and the most common laying behavior is scatter—laying. The results of scatter—laying parallel a probabillistic strategy that minimizes total failures, rather than one that maximizes a maximal number of successes. The observed mixed or conditional combined strategy of (1) scatter parasitism adapted to escaping high losses to predators and (2) multiple parasitism adapted to exploiting parental care of the foster parents may be peculiar to the finches among the parasitic birds.