Natural populations might exhibit resilience to changing climatic conditions if they already show adaptive flexibility in their reproductive strategies. In cooperative breeders, theory predicts that mothers with helpers should provide less care when environmental conditions are favourable, but maintain high investment when conditions are challenging. Here, we test for evidence of climate-mediated flexibility in maternal investment in the cooperatively breeding superb fairy-wren Malurus cyaneus. We focus on egg size because in this species egg size influences offspring size, and females reduce egg investment when there are helpers at the nest. We report that females lay larger eggs during dry, hot conditions. However, the effect of temperature is modulated by the presence of helpers: the average egg size of females with helpers is reduced during cooler conditions but increased during hot conditions relative to females without helpers. This appears to reflect plasticity in egg investment rather than among female differences. Analysis of maternal survival suggests that helped females are better able to withstand the costs of breeding in hot conditions than females without helpers. Our study suggests that females can use multiple, independent cues to modulate egg investment flexibly in a variable environment.