The effects of environmental warming on the reproductive performance of birds are most easily studied in desert habitats where birds already experience air temperatures (Tas) close to their upper thermal tolerance. Many desert birds coincide breeding with periods of food availability triggered by rainfall during the summer season. Daily maximum air temperatures (Tmax) during the Kalahari summer season frequently reach the lower forties (°C) and recent years havernbeen characterised by reduced rainfall and increased Ta. Breeding Southern Yellow-billed Hornbills (Tockus leucomelas) could be particularly vulnerable to high Ta due to their breeding strategy whereby the females are confined to the nest cavity for most of the nesting period. During this time their male partners are solely responsible for food provisioning, which imposes a considerable energetic demand. In this thesis, I investigated the extent to which Ta affects the ability and willingness of breeding males to provision their female partners and offspring. And consequently, the extent to which male investment and the thermal environment affect female body mass (Mb) and chick development rates in Southern Yellow-billed Hornbills in the Kalahari.