The term “resource polymorphism” refers to the existence of alternative phenotypes in relation to resource use, as a result of disruptive selection. Evidence for resource polymorphism is widespread in fish but remains scarce in birds. Although Zenaida Doves (Zenaida aurita) usually defend year-round territories, doves on Barbados can also be observed foraging at seed-storage sites in large flocks with little, if any, inter-individual aggression. On the basis of morphological variation, it has been suggested (Sol et al. 2005) that this represents a case of resource polymorphism, primarily driven by competition for territories. Using new data, we revisited the evidence for resource polymorphism in Zenaida Doves on Barbados. In particular, we added replicates in time and space for territorial and flock-foraging birds and used molecular markers to assign sex to adults and juveniles. In addition, we used microsatellite markers to assess potential genetic differentiation between flock-feeding and territorial doves. Our results confirm previous observations that territorial adults were larger than flock-feeding ones, whereas the reverse was observed in juveniles. Contrary to previous observations, we found a significant excess of females among flock-feeding adults, whereas the sex ratio was balanced in territorial adults and in juveniles. In addition, we observed no significant difference in body condition and no genetic differentiation between territorial and flock-feeding individuals. Overall, our data question the existence of resource polymorphism in Zenaida Doves in Barbados. We suggest alternative, more parsimonious explanations, based on age- and sex-related differences in the relative benefits of holding a territory.
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Vol. 128 • No. 1