Infrequent Cooperative Breeding in a Short-Lived Migratory Songbird, the Wilson’s Warbler

Cooperative breeding, or helping behavior, has long been recognized in birds. An ignored dichotomy, however, separates the helping found in many individuals of some long-lived, sedentary species from the helping occasionally found in the territories of isolated breeding pairs of some short-lived, long-distance migrant species. Both types of helping are called “cooperative breeding” in the literature. However, recognizing a dichotomy of “frequent” versus “infrequent” cooperative breeding would help justify the study of infrequent helping as a distinct discipline. Cooperative breeding in Wilson’s Warblers is infrequent, and among the unique behaviors found during this study were (1) solicitations by helper males, which aborted host male attacks and apparently initiated territorial acceptance, (2) an absence of sexual aggression between helper males and fertile host females, (3) attacks by helper males on intruding males during host female nest building, (4) helper males singing with impunity when host males were absent from territories, but being attacked when host males were present, and (5) a single male simultaneously serving as a helper in four adjacent host territories. Infrequent helping has essentially been ignored in studies and summaries of cooperative breeding. However, recognizing and studying infrequent helping as a distinct behavioral process could reveal interactions between helping and population ecology. Thus, infrequent cooperative breeding detected in a breeding population could reveal territorial saturation and could indicate that the population is likely ecologically healthy.