Ecology and evolution of breeding adaptations in the village weaver <italic>Ploceus cucullatus</italic>.

Abstract

Introduced species can provide rare opportunities to test hypotheses of adaptation and evolution by natural selection. The main part of this dissertation is a test of predictions from two hypotheses regarding the evolution of traits in a colony-nesting African passerine bird, the village weaver (Aves: Ploceidae): (1) Egg appearance characteristics and egg rejection behavior function to direct parental care to offspring; and (2) these traits are maintained by natural selection primarily as counteradaptations to brood parasitism by the egg-mimicking diederik cuckoo <italic>Chrysococcyx caprius</italic>. Predictions from these hypotheses were met by observations and egg-replacement experiments in four populations (The Gambia, South Africa, Mauritius, and Dominican Republic) between 1999 and 2001. A comparative study between two natural weaver populations in sympatry with the diederik cuckoo, and two introduced populations that have been in allopatry with the diederik cuckoo for 115 and >200 years respectively, demonstrates a decline in anti-parasite adaptations in the absence of the cuckoo. Within-clutch variability in egg color and spotting increased, and between-individual variability decreased. Decay of these traits correlated with duration of divergence from the respective source populations. The efficiency of egg rejection behavior also declined, but solely as a byproduct of the evolution of egg appearance characteristics; no decline in the perceptual or cognitive basis for egg recognition occurred. A general hypothesis based on solar damage to embryonic viability may explain observed directional evolution of egg color in introduced village weavers, as well as the blue to blue-green color of many arboreal birds' eggs. A graphical model explains the interaction between egg recognition and egg variability in brood parasite---host systems. The evolution of the latter but not the former in the present study is explained by the hypothesis that nonfunctional morphological traits may generally be more subject to direct selection than nonfunctional behavioral traits are. This work contributes to our understanding of (1) coevolution or reciprocal adaptation between brood parasites and their hosts; (2) natural selection and evolution in natural populations over brief time spans; and (3) the dynamics of trait decay.