Southern Yellow-billed Hornbills are socially monogamous and widespread in southern Africa,
where they occupy savanna and woodland habitats [12]. The individuals used in this study
were captured at Leeupan Guest Farm, Northern Cape, South Africa (S 26.95652° E
021.86913°). The study site is in the southern Kalahari Desert where arid savanna dominates
the vegetation along a dry riverbed and dune landscape [13]. The site is characterised by cool,
dry winters and hot summers with an annual mean rainfall of 215.5 ± 13.0 mm, mean daily
summer maximum air temperatures of 34.7 ± 0.05°C. In the last decade, air temperatures
exceeded 40°C on 31.1 ± 6.2 days per year during hornbill breeding season (data from the Austral
summer months October to March, 1960–2015, Twee Rivieren, 150 km from the study
site, South African Weather Service). Temperatures and humidity within tree cavities occupied
by a female Southern Yellow-billed Hornbill with one or more chicks at our study site can
range between 20.8–43.1°C and 13.8–97.1% relative humidity (van de Ven unpub. data). During
incubation and early nestling-rearing female hornbills are confined to the nest cavity, therefore,
in contrast to males, they cannot make use of cool microsites within the wider landscape.