Ecological mechanisms of a resource polymorphism in Zenaida Doves of Barbados

Despite the important place of resource polymorphisms in ecological and evolutionary theories, the reason why a group of individuals sharing a common environment should diverge in the use of the resources remains poorly known. Given the existence of distinct open niches and the relaxation of interspecific competition, niche theory suggests two possible mechanisms: resource competition between conspecifics and trade-offs in resource use efficiency. We investigated the importance of these explanations in a foraging polymorphism recently developed by Zenaida Doves (Zenaida aurita) on the West Indian island of Barbados. In this polymorphism, individuals either aggressively defend feeding territories from conspecifics or feed in large unaggressive groups with conspecifics. Combining field observations and a cage experiment, we found no evidence that the polymorphism is primarily driven by age- or sex-related differences, or that it results from morphological feeding specializations or dietary preferences. Instead, our results support the hypothesis that individual specializations arise from contest competition. In the study area, competition for territories was intense, with very little undefended space left between territories and owners frequently involved in territorial contests. Replacement of territory holders from year to year was low compared to the number of potential recruits, implying that many doves were incapable of securing a territory. Approximately half the doves at territorial sites did not hold a territory but wandered between them as floaters. Compared with territory holders, floaters were smaller and had shorter wings, traits that in this species mediate territory defense. Yet floaters did not differ from group feeding doves on these morphological traits. This suggests that group feeders are floaters that shift to an alternative resource. The new resource appears to be suboptimal, as indicated by the fact that group feeders were in worse body condition than doves from the territorial sites. Taken together, our results suggest that the resource polymorphism in Zenaida Doves is primarily driven by competition for territories, which forces less competitive individuals to use alternative, subobtimal resources.