Evolution And Ecology Of Cooperative Breeding In Galapagos Mockingbirds (nesomimus Spp.).

Abstract

Studies of avian cooperative breeding have emphasized the roles of environmental stability in territorial species and environ- mental uncertainty in colonial cooperative breeders. Galapagos mockingbirds (Nesomimus spp.) are territorial but they inhabit a climatically variable environment. To address evolutionary and ecological questions about cooperative breeding in this endemic genus, I combined intensive long-term research on N. parvulus with comparative study of its three allopatric congeners. Annual rainfall in the Galapagos varies, as does mockingbird breeding. Nevertheless, a population of N. parvulus on Isla Genovesa is characterized by demographic stability that promotes cooperative breeding. High adult survival favoring males and an apparent limit on breeding density produce an excess of non- breeding males. Social groups are maintained because dispersal by these birds is constrained by habitat saturation. Most helpers are non-breeding males in their natal groups. Climatic extremes cause temporary shifts in population structure, and relaxed constraints on dispersal and breeding cause changes in social organization. Group structure and interactions are unusually complex in N. parvulus. Groups contain 2-24 birds and up to three nesting pairs. Reproductive success is correlated with dominance because dominants interfere with nesting by subordinates. Conflict within groups also involves extra-pair copulations by dominant males. Groups with multiple pairs (plural breeding) may result from variable climatic conditions that favor opportunistic breeding by young mockingbirds. Helping by N. parvulus is flexible and kin-directed. The probability that a bird helps increases with its genetic relatedness to nestlings. Because most helpers improve the reproductive success of relatives, helping confers small indirect fitness benefits. Few receive large direct benefits, and helping is not a prerequisite for becoming a breeder. Comparative results further suggest that cooperative breeding is promoted by habitat limitation. N. trifasciatus and N. macdonaldi on two islands live in large cooperatively breeding territorial groups filling all habitat, but on San Cristobal, predation and habitat diversity are correlated with small group size in N. melanotis. Complexity of Galapagos mockingbird society in terms of group structure, conflict, and flexible helping behavior is attributable to the combined influences of habitat constraints and climatic variability.