Abstract
Dispersal is a fundamental process influencing evolution, social behaviour, and the long‐term persistence of populations. We use both observational and genetic data to investigate dispersal, kin‐clustering and intergroup relatedness in the white‐breasted thrasher, Ramphocinclus brachyurus , a cooperatively breeding bird that is globally endangered. Mark‐resighting data suggested sex‐biased dispersal, with females dispersing over greater distances while males remained philopatric. Accordingly, spatial autocorrelation analysis showed highly significant fine‐scale genetic structure among males, but not among females. This fine‐scale genetic structuring of the male population resulted in very high levels of relatedness between dominant males at neighbouring nests, similar to that seen within cooperative groups in many species where kin selection is cited as a cause of cooperation. By implication, between‐group as well as within‐group cooperation may be important, potentially creating a feedback loop in which short‐distance dispersal by males leads to the formation of male kin clusters that in turn facilitate nepotistic interactions and favour further local recruitment. The strength of spatial autocorrelation, as measured by the autocorrelation coefficient, r , was approximately two to three times greater than that reported in previous studies of animals. Relatively short dispersal distances by both males and females may have a negative impact on the white‐breasted thrasher's ability to colonize new areas, and may influence the long‐term persistence of isolated populations. This should be taken into account when designating protected areas or selecting sites for habitat restoration.