Assessment, age and proximity in dispersal contests among cooperative wrens: field experiments

Abstract

Experimental removals of breeding females were conducted in a completely marked population of 35–85 territories in the cooperatively breeding stripe-backed wren, Campylorhynchus nuchalis, over 3 years in central Venezuela. The ability of non-breeders to discriminate among 19 vacancies with varying reproductive potential was tested and competitive asymmetries among contestants were examined. Significantly more females responded to openings (both experimental and natural) in large groups (four or more adults) than to openings in smaller ones. Data from 12 years show that breeders in large groups have much greater reproductive success than those in smaller groups. As many as 10 contestants fought with intense aggression for several days over vacancies in large groups. In contrast, vacancies in small groups attracted few contestants with little aggression and frequent abandonment of the opportunity. Nearly all older females (greater than 1 year of age) in the population competed for at least one vacancy, while only half of the younger females did so. When competitors responded from both age classes, an older female normally gained the position. Females from adjacent territories won valuable vacancies in large groups more often than did competitors from farther away. The escalation of these contests to intense aggression can be attributed mainly to the scarcity and necessity of the resource (multiple helpers) and the potential for monopolizing it. Advantages of a e and proximity probably promote delayed short-distance dispersal that is characteristic of this and other cooperative species.