Facultative dispersal by juvenile males in the cooperative stripe-backed wren

We present genetic and demographic data documenting juveniledispersal in the cooperatively breeding stripe-backed wren (Campylorhynchusnuchalis) of Venezuela. Parentage and DNA fragment-sharing analysesrevealed 12 cases in which juveniles were unrelated to othergroup members. Of these 12 foreign juveniles, (1) all were males,(2) eight of 12 had been found with breeding pairs lacking helpersrather than with groups containing helpers, and (3) four outof seven of those observed as adults courted or sired offspringwith the dominant females in their new groups despite the strongincest avoidance of this species. Furthermore, juvenile maleshad a significant tendency to disappear from natal groups intheir first year, and singleton juveniles observed with pairsafter the breeding season were mostly males. These data supportthe hypothesis that foreign juveniles were dispersers from intactgroups and not products of conspecific brood parasitism or adoptionfollowing group dissolution. We suggest that unassisted pairsmight accept juvenile males into their groups as helpers toincrease their future reproductive success and that dispersersthemselves might leave large natal groups in which their helpingis superfluous to join small groups of nonrelatives in whichthey might soon reproduce.