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 these discrepancies, as long-lived species should invest less in reproduction. We examined how climatic, social, and life-history factors affect reproduction in the long-lived, cooperative southern ground-hornbill (<i>Bucorvus leadbeateri</i>). Using 17 years of data from 23 groups within the Greater Kruger National Park, South Africa, we tested for associations between temperature, rainfall and group composition, and several reproductive parameters. Low winter rainfall decreased breeding probability, while higher temperatures delayed laying and reduced nestling mass, regardless of group composition. Nestlings had longer tarsi in groups with more adults, and groups with more juveniles bred earlier and were more likely to breed, likely reflecting territory quality rather than group composition. In conclusion, hot and dry conditions negatively impacted ground-hornbill breeding and, as expected given their life history, group composition did not mitigate these effects. We suggest that life-history strategies and non- reproductive benefits of collective behaviour, such as resource defence and survival, should be considered when assessing cooperative breeders' responses to environmental fluctuations.