As global temperatures rise due to climate change, it is critical to understand how heat affects wildlife, particularly during critical breeding seasons. High temperatures can disrupt breeding success in many species by affecting parental care, increasing physiological stress, and reducing food availability. Researchers predicted that higher temperatures would negatively affect female and nestling body conditions, reduce male provisioning rates, and lead to lower fledging success.

A research study funded by the NRF investigated the success of the Southern Yellow-Billed Hornbill mating, provisioning rates, and fledgling quality in South Africa’s Kalahari Desert. The study was conducted over three breeding seasons from 2012 to 2015 at the Kuruman River Reserve. The researchers monitored 50 hornbill nesting attempts and collected data on nest temperatures, adult and nestling body mass, provisioning rates (the number of feeding trips males made), and fledgling success. Nest and air temperatures were recorded, with maximum daily air temperatures (Tmax) ranging between 33.2°C and 39.1°C.

To assess the impact of these high temperatures on reproductive outcomes, temperatures were measured both inside and outside the nests. The study’s results showed a clear decline in breeding success with rising temperatures. When mean Tmax exceeded 35.7°C, the likelihood of a nest producing at least one fledgling fell below 50%. This temperature threshold, now regularly surpassed in the Kalahari due to climate change, signals a serious risk to the species’ reproductive success now and in the future. High temperatures impacted the male hornbills’ ability to forage and provide food for the nest. As Tmax approached 40°C, males spent more time panting and seeking shade to avoid overheating, reducing their efficiency in gathering food for females and young. Lower provisioning rates led to less food for nestlings, affecting their growth and survival.

Previous studies referenced in the article reported over 90% reduction in foraging efficiency when males were forced to pant while foraging. Nesting females experienced significant weight loss due to prolonged exposure to high nest temperatures. When temperatures rose above 31.6°C, females’ diurnal mass gain ceased entirely, marking a threshold beyond which they could not maintain body condition. This weight loss likely compromises females’ health and ability to care for their young. Nestlings exposed to high temperatures showed stunted growth, losing an average of 7.3% of overnight mass at Tmax above 40°C, with insufficient daytime weight gain to compensate. Fledglings from hotter conditions were about 51% lighter than those reared in cooler temperatures, impacting their survival potential.

Higher nest temperatures also impaired structural development. For every 1°C increase in Tmax, fledgling body mass dropped by roughly 20 grams, and tarsus length declined by approximately 0.9 mm. Heat stress was further linked to developmental delays, with fledging age increasing by an average of 2.6 days for every 1°C rise in mean Tmax. The study concludes that high temperatures hinder Southern Yellow-Billed Hornbills’ breeding success by decreasing male provisioning rates, resulting in insufficient food for females and nestlings and negatively affecting body condition in both.

Climateinduced temperature increases are expected to exacerbate these effects, potentially reducing fledging success and offspring quality, with long-term implications for population stability. If temperatures continue to rise as predicted, hornbill numbers in the Kalahari Desert could decline. To mitigate these risks, the study recommends conservation strategies that account for climate change, such as managing habitats to expand shaded foraging areas. Further research on long-term impacts on population dynamics and potential adaptive responses to heat stress is also recommended. Protecting cavity-bearing trees that provide thermally buffered nesting sites may be essential for supporting hornbill populations in warming environments.