Abstract. Heterothermy plays an important role in lowering the costs of thermoregulation in endotherms by reducing water and energy requirements. We tested predictions that birds in arid habitats should express fine-scale variation in their thermoregulatory patterns as a function of prevailing climatic conditions. We assessed effects of air temperature (T air ) and water vapor pressure deficit (D) on body temperature (T b ) in free-living White-browed Sparrow-Weavers (Plocepasser mahali ) during summer in two arid habitats in the Kalahari Desert, South Africa, using data from a dry period at a hot, desert site (n ¼ 7 birds), and during a dry period (n ¼ 4 birds) and a wet period (n ¼ 5 birds) at a milder, semi-desert site. The desert birds maintained a significantly higher set-point T b (41.58 6 0.28C, mean 6 SD) than semi-desert birds (40.28 6 0.28C). During the warmest part of day (12:00-18:00 hours), T b increased significantly during periods of high T air and/or high humidity, and mean and maximum T b were up to 1.48 and 2.38C, respectively, above normal levels. However, as T air increased, birds at the desert site maintained T b at or below set-point levels for a greater proportion of the time than birds at the semi-desert site. Birds at the desert site also expressed a greater magnitude of daily heterothermy (heterothermy index, HI ¼ 2.48 6 0.38C, mean 6 SD) than birds at the semi-desert site: the latter population showed a greater magnitude of heterothermy during a dry period (HI ¼ 2.18 6 0.38C) than during a wet period (HI ¼ 1.68 6 0.28C). Birds continued foraging throughout the warmest part of the day, despite the fact that heat dissipation (percentage of time spent panting and wing-spreading) increased significantly with increasing T air . Our findings reveal that populations can vary in their thermoregulatory responses in both space and time and suggest that small changes in T air can have significant effects on thermoregulation in free-ranging desert birds, even when T air , T b . These data have important implications for assessing vulnerability of species to climate change, suggesting that sensitivity should be assessed at the population, rather than species, level.

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The population of the Endangered African penguin Spheniscus demersus has decreased by > 65% in the last 20 years. A major driver of this decrease has been the reduced availability of their principal prey, sardine Sardinops sagax and anchovy Engraulis encrasicolus. To date, conservation efforts to improve prey availability have focused on spatial management strategies to reduce resource competition with purse-seine fisheries during the breeding season. However, penguins also undergo an annual catastrophic moult when they are unable to feed for several weeks. Before moulting they must accumulate sufficient energy stores to survive this critical life-history stage. Using GPS tracking data collected between 2012 and 2019, we identify important foraging areas for pre- and post-moult African penguins at three of their major colonies in South Africa: Dassen Island and Stony Point (Western Cape) and Bird Island (Eastern Cape). The foraging ranges of pre- and post-moult adult African penguins (c. 600 km from colony) was far greater than that previously observed for breeding penguins (c. 50 km from colony) and varied considerably between sites, years and pre- and post-moult stages. Despite their more extensive range during the non-breeding season, waters within 20 and 50 km of their breeding colonies were used intensively and represent important foraging areas to pre- and post-moult penguins. Furthermore, penguins in the Western Cape travelled significantly further than those in the Eastern Cape which is likely a reflection of the poor prey availability along the west coast of South Africa. Our findings identify important marine areas for pre- and post-moult African penguins and support for the expansion of fisheries-related spatio-temporal management strategies to help conserve African penguins outside the breeding season.