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 helping found in many individuals of some long-lived, sedentary species, from helping occasionally found in 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 study of infrequent helping as a distinct discipline. Cooperative breeding in Wilson’s Warblers is infrequent, and among unique behaviors found during this study were: 1) solicitations by helper males, which abort host male attacks, and apparently initiate 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 7) a single male simultaneously serving as a helper in four adjacent host territories. Infrequent helping essentially has 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 indicate that the population is healthy. SIMPLE SUMMARY Cooperative breeding, where more individuals than just a parental pair, contribute to raising young, happens in many bird species. Most studies have investigated long-lived, non-migratory species, many of which get brood help from genetically-related relatives. Additionally, however, cooperative breeding can happen infrequently in isolated pairs of short-lived, migratory species. Helpers in these cases have no indirect kinship relationships with the birds they help. Their benefits in helping can come from direct genetics, having sired some young in host nests, or from direct behavioral benefits, such as inheriting future territories. In this study, I documented several unique behaviors associated with infrequent helping. These include solicitation, a behavior through which male helpers may gain acceptance into host territories, defense of host territories by helper males, helper singing when host males are absent, but not when present, and a single male simultaneously being a helper in four adjacent territories. Infrequent cooperative breeding most often may be found in healthy breeding populations, where most or all prime breeding territories are occupied. However, infrequent helping has not been studied with regards to its possible relationship with population ecology.