Interspecific territoriality in red-breasted meadowlarks and a method for estimating the mutuality of their participation

Abstract

1. Three species of red-breasted meadowlarks (Aves, Icteridae, Sturnella) coexist on the pampas of southern Buenos Aires Province, Argentina. The species differ somewhat in size and shape but have very similar color patterns. 2. In the nearly homogeneous habitat provided by wheat fields, S. defilippii and S. militaris superciliaris have nonoverlapping territories that are maintained by interspecific aggression and that are not related to vegetation discontinuity. Maintenance of interspecific territoriality (IST) was not mutual, for the larger species (defilippii) had a much stronger tendency to attack and exclude the smaller species than vice versa. However, once a superciliaris male did establish a territory, it was capable of defending it against defilippii males. A third species, S. loyca, had territories that overlapped those of the other two species, and although the largest of the three, it seldom engaged in interspecific aggression. 3. The observed interspecific aggression seems due to close similarities in plumage and behavior of defilippii and superciliaris, reflecting their close phylogenetic relationship. No evidence has been adduced indicating how or whether the IST is adaptive, and no reason for predicting character convergence emerges from this study. 4. This paper describes a method for quantifying the relative contribution of two or more species to interspecific aggression, including methods for correcting for the actual availability of heterospecific and conspecific males.