Although animal societies often evolve due to limited natal dispersal that results in kin clustering and facilitates cooperation among relatives, many species form cooperative groups with low kin structure. Such groups often comprise residents and immigrants of the same sex that compete for breeding opportunities. To understand how such mixed-kin societies form, we investigated the causes and fitness consequences of dispersal decisions in male cooperatively breeding superb starlings (Lamprotornis superbus) inhabiting a climatically unpredictable environment. We show that the two alternative reproductive tactics—natal dispersal or philopatry—exhibit reproductive tradeoffs resulting in equal lifetime inclusive fitness. Surprisingly, an individual’s tactic is determined by the prenatal environment its parents experience prior to laying rather than the environment it experiences as a juvenile. Individuals that adopt the tactic not predicted by prenatal environmental conditions have lower fitness. Ultimately, climate-driven oscillating selection appears to stabilize mixed-kin societies despite the potential for social conflict.