Hot-dry weather is associated with worse reproductive outcomes regardless of group composition in a long-lived cooperatively breeding bird

Anthropogenic climate change is driving increases in temperature and droughts. Cooperative breeding, common in regions with greater environmental variation, has been proposed to buffer against such conditions, but findings across taxa are mixed. Life-history strategies may partly explain this, as long-lived, slow-developing species may invest less in reproduction under adversity. Here, we examined how climatic, social, and life-history factors affect reproduction in the long-lived, cooperative southern ground-hornbill (Bucorvus leadbeateri). Using 17 years of breeding data from 23 groups within the Greater Kruger National Park, South Africa, we tested for associations between temperature, rainfall and group composition, and breeding probability, laying date, fledging success, and nestling growth. Low winter rainfall decreased breeding probability, while higher temperatures delayed laying and reduced nestling body mass, regardless of group composition. Group composition influenced some outcomes: more adult helpers correlated with longer nestling tarsi, while juvenile members were associated with earlier laying and higher breeding probability, likely reflecting territory quality rather than juvenile contributions. In conclusion, hot and dry conditions negatively impact ground-hornbill breeding, and, as expected given their life history, group composition does not mitigate these effects. We suggest that life-history strategies should be considered when assessing cooperative breeders' responses to climate change.