Fair-weather friends: Group size boosts reproduction in cool, wet years in a cooperative breeder

Abstract Climate change is altering global temperature and rainfall patterns, with measurable impacts on reproduction and population persistence. These effects are pronounced in dryland ecosystems that are characterized by high temperatures, and low, but highly variable rainfall. Cooperatively breeding species (where helpers provide parental care to other group members’ offspring) are common in drylands, but it remains unclear whether cooperative breeding buffers against reproductive failure under unfavourable conditions (social buffering hypothesis) or enhances reproductive success under favourable conditions (social capitalisation hypothesis). Distinguishing between these hypotheses requires comparing the effects of helpers on breeding success under varying conditions, but such interaction analyses are rarely performed. Using data collected from 2006 to 2010 on apostlebirds Struthidea cinerea, a cooperative breeder inhabiting the climatically variable Australian drylands, we show that wet years were favourable and all groups produced more fledglings. However, compared to smaller groups, larger groups with more helpers had more successful breeding attempts and produced more fledglings particularly during cooler breeding seasons, supporting the social capitalisation hypothesis. This benefit likely reflects increased nestling provisioning in larger groups. Still, larger groups could not offset negative impacts during the hottest, driest breeding season when no groups successfully reproduced. Our findings highlight the importance of interactions between helper number and environmental conditions in shaping offspring production and thus, potentially population persistence under adverse climatic conditions. Further research could determine whether increased breeding success in favourable years maintains population viability despite adverse years.