Persistence of egg recognition in the absence of cuckoo brood parasitism: pattern and mechanism

Broad ecological shifts can render previously adaptive traits nonfunctional. It is an open question as to how and how quickly nonfunctional traits decay once the selective pressures that favored them are removed. The village weaverbird (Ploceus cucullatus) avoids brood parasitism by rejecting foreign eggs. African populations have evolved high levels of within-clutch uniformity as well as individual distinctiveness in egg color and spotting, a combination that facilitates identification of foreign eggs. In a companion study, I showed that these adaptations in egg appearance declined following introductions of weavers into habitats devoid of egg-mimicking brood parasites. Here, I use experimental parasitism in two ancestral and two introduced populations to test for changes in egg rejection behavior while controlling for changes in egg appearance. Introduced populations reject foreign eggs less frequently, but the ability of source and introduced populations to reject foreign eggs does not differ after controlling for the evolution of egg color and spotting. Therefore, egg rejection behavior in introduced populations of the village weaver has been compromised by changes in egg appearance, but there has been no significant decline in the birds' ability to recognize foreign eggs. This result reconciles earlier studies on this system and provides insights into the ways behavior can change over generations, especially in the context of recognition systems and the avoidance of brood parasitism.