Haydock, Joseph. Ph. D., Purdue University, (December 1993). Cooperative breeding in bicolored wrens, Campylorhynchus griseus. Major Professor: Kerry Rabenold.
Cooperative groups in many social species are built mainly because offspring remain with their parents past reproductive maturity (natal philopatry), delaying independent breeding and instead helping to rear young that are normally siblings. Despite many studies on social species, considerable controversy remains about the factors that are most important in the maintenance and evolution of cooperation and apparent altruism. The question for many species concerns the relative benefits and costs of philopatry compared to attempted dispersal to an independent breeding position.
I approached the study of bicolored wrens Campylorhynchus griseus with observational inquiry of environmental constraints, demography and life history strategies as well experimental appraisal of predation pressures and dispersal strategies. Bicolored wrens occur commonly throughout open woodlands of central Venezuela and northeastern Colombia. I studied a population in the central savanna of Venezuela. Young wrens in my study population remain on their natal territory for up to several years as nonreproductive helpers, though about half the groups are unaided pairs.
To examine costs, benefits, and constraints on dispersal for female bicolored wrens, I experimentally created breeding opportunities by removing breeding females. I found that females discriminate among reproductive opportunities of varying potential and that older, neighboring females are more likely to win contests for breeding status. To investigate parentage, I used mutilocus DNA fingerprinting for 222 juveniles produced during 99 group-years, demonstrating that 92% of the juveniles found on territories were produced by the behaviorally dominant pair and that only 2% were produced by subordinant helpers. Other juveniles were fathered outside the group (2%) and some were completely unrelated to the group in which they were found (4%), to investigate the effectiveness of aid by helpers, I measured territory quality and I experimentally enhanced group size. I found that helpers increased the reproductive success of their kin, but that variation in territory quality also affects reproductive success. The interaction of these factors can render helping ineffective. Overall, my results indicated that philopatry is probably more enforced by competition for breeding vacancies that favored by immediate benefits of helping. Potentially dispersive young encounter a wide range of conditions that can strongly favor philopatry through cooperation or competition.