High temperatures are associated with substantial reductions in breeding success and offspring quality in an arid-zone bird.

During hot weather, terrestrial animals often seek shaded thermal refugia. However, this can result in missed foraging opportunities, loss of body condition and impaired parental care. We investigated whether such costs could compromise breeding success in a widespread southern African bird: the Southern Yellow-Billed Hornbill Tockus leucomelas. We predicted that hornbills might be especially vulnerable to temperature-dependant reductions in parents' foraging capacity due to extreme asymmetry in sex-specific roles during breeding: females are confined within the nest cavity for most of the nesting period and the burden of provisioning falls solely on the male during this time. We followed 50 hornbill nesting attempts in the Kalahari Desert between 2012 and 2015, collecting data on provisioning rates, adult and nestling body mass, fledging success and size of fledglings. Mean daily maximum air temperatures (T) during nesting attempts ranged from 33.2 to 39.1 °C. The likelihood of successful fledging fell below 50% at mean T > 35.1 °C; a threshold now regularly exceeded at our study site due to recent climate warming. Additionally, offspring fledging following the hottest nesting attempts were > 50% lighter than those fledging following the coolest. Sublethal costs of keeping cool including loss of body condition, production of poor-quality offspring and breeding failure are likely to become issues of serious conservation concern as climate change progresses; even for currently widespread species. Missed-opportunity costs associated with behavioral thermoregulation and direct sublethal costs of temperature exposure should not be overlooked as a potential threat to populations, especially in environments that are already hot.