Handling the heat: managing microclimates for nesting desert hornbills

Pule, Obakeng

Handling the heat: managing microclimates for nesting desert hornbills

Thesis / Dissertation

2025

Publisher

Universiy of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Anthropogenic climate change poses a major conservation threat to organisms globally, including via negative impacts on breeding success. For example, over the last decade, rising temperatures in the Kalahari Desert, South Africa, have caused serious declines in the breeding success of Southern Yellow-Billed Hornbills Tockus leucomelas. Previous research suggests the impact of high air temperatures on hornbill nest success is mediated mostly (70%) through direct negative effects of high nest temperatures on chick growth. To address this, I tested whether an insulated nest box design could buffer negative effects of high air temperatures and improve hornbill chick growth and nest success in the Kalahari. I worked with a study population of hornbills breeding in nest-boxes at Kuruman River Reserve in the southern Kalahari, South Africa. I analyzed a long-term dataset (2008 - 2021) on chick growth and breeding success for this population, during which time nest box design changed from uninsulated (2008 - 2018) to insulated (boxes shaded on all sides with plywood paneling and mounted on the southeast [coolest] side of trees) (2019 - 2021). In this long-term dataset, mean maximum daily air temperature during the nesting period (Tmax), breeding season, and nest box type (insulated/uninsulated) all correlated with chick mass and tarsus length, and nest success; chicks were smaller during hotter weather, and less likely to fledge from uninsulated boxes compared with insulated boxes. However, nest box type, breeding season, and air temperature were also correlated with one another in the long-term dataset, making it difficult to assess the cause of these differences. I therefore also conducted a nest box insulation experiment (2022 - 2024) in which nest boxes were modified to either the insulated or uninsulated design at the time of egg lay (i.e. when hornbills had committed to a nest attempt in the boxes). This ensured both nest box types were used in the same breeding seasons. I collected data on Tmax, and maximum daily nest box temperature (Tbox), chick body mass (g) and tarsus length (mm), and nest success in both box types. I found significant interactions between Tmax and nest box type on chick body mass and tarsus length, such that negative effects of high Tmax were buffered in insulated boxes compared to uninsulated boxes (box type*Tmax interactions: chick body mass: est: 29.72±8.91 g, 95% CI: 12.87 g - 47.90 g; tarsus length: est: 2.83±1.20 mm, 95% CI: 0.55 mm - 5.18 mm). Nest success was also significantly higher (est: 4.19 ±1.50, 95% CI: 1.67 - 7.88); and mean maximum nest box temperatures significantly lower (e.g., by 3±1.09°C in unoccupied insulated nest boxes, 95% CI: 0.89 - 1.10°C) than uninsulated boxes. My findings confirm the importance of nest temperatures in shaping breeding outcomes in this species. My data also suggest nest box temperatures can successfully be manipulated with simple interventions such as using plywood to shade nest boxes and mounting these on the coolest side of trees, and this has significant benefits for breeding birds. These findings are important in a warming climate, especially for conservation projects that provide nest boxes with the intention of improving survival and breeding success of target species. page numbers in pdf

Keywords

African ornithology

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