Differences in the use of surface water resources by desert birds are revealed using isotopic tracers

Abstract

The scarcity of free-standing water sources is a key determinant of animal and plant community structure in arid environments, and an understanding of the extent to which particular species use surface water is vital for modeling the effects of climate change on desert avifauna. We investigated interspecific variation in the use of artificial water sources among birds in the Kalahari Desert, South Africa, by (i) observations at waterholes and (ii) tracing spatial water-use patterns during summer by isotopically labeled water sources and blood sampling. More than 50% of the avian community (primarily insectivores and omnivores) were not observed to drink. The majority (53%) of species drinking at waterholes were granivorous, and their use of surface water was best predicted by their relative abundance in the community. Species representing the remaining dietary guilds drank significantly more on hot days. Blood samples revealed that only 11 of 42 species (mostly granivores and a few omnivores) showed evidence of drinking at a waterhole with enriched deuterium values; on average, in the latter birds, water from the enriched waterhole accounted for ~38% of their body water pool. These findings illustrate that 2 methods employed in this study provide different, but complementary data on the relative importance of a water source for an avian community. Although our results suggest that most avian species are independent of surface water, drinking patterns on the hottest days during our study period suggest that free-standing water might become more important for some of the non-drinking species under hotter climatic conditions.

Chronic, sublethal effects of high temperatures will cause severe declines in southern African arid-zone birds during the 21st century

“…These reductions are driven by mechanical constraints on panting and foraging simultaneously and by lower foraging returns in shady locations (7,10,24). Conflicts between foraging and thermoregulation are particularly acute in arid zones, where many birds [∼50% of species in the Kalahari (25,26)] do not drink and gain water only through food.…”

Regularly drinking desert birds have greater evaporative cooling capacity and higher heat tolerance limits than non‐drinking species

“…All the above species have M b <70g and occupy the sandy arid regions of western Southern Africa where the availability of surface water is limited. We used observations from the literature to categorize each species as either a "regular drinker" (significant numbers of the species have been observed drinking from surface water sources), or a non-drinker (not observed drinking from surface water even when available in their habitat) following Abdu, McKechnie, Lee, & Cunningham, 2018;Smit, Woodborne, Wolf, & McKechnie, 2019;van der Merwe & Smit, 2019. Some species were considered opportunistic drinkers (e.g.…”

“…Some species were considered opportunistic drinkers (e.g. sociable weaver and white-browed sparrow-weaver) as they drink regularly when water is available (Smit et al, 2019); because most work was…”

“…The relationships between drinking, evaporative cooling capacity and heat tolerance documented here are relevant to understanding several aspects of the ecology of birds in arid regions and the coevolution of their diets, behavioural ecology and thermal physiology. Interspecific variation among desert birds in terms of dependence on surface water is strongly related to dietary water content, with regular drinking far more prevalent among granivorous species compared to insectivores or omnivores (Fisher et al, 1972;Serventy, 1971;Smit et al, 2019). Water dependence, in turn, has a strong influence on movement ecology, with water-dependent species often flying considerable distances daily to water sources in order to drink (e.g., Willoughby & Cade, 1967).…”

“…Surface water is a critical resource for many animals inhabiting arid environments, particularly those taxa whose diets contain little preformed water (e.g., dry seeds) (Bartholomew & Cade, 1963;Fisher, Lindgren, & Dawson, 1972;MacMillen & Hinds, 1998). Within desert bird communities there is considerable variation in dependence on surface water, with some species drinking regularly, others drinking occasionally, and some species apparently never drinking (Fisher et al, 1972;Lynn, Chambers, & Rosenstock, 2006;Lynn, Rosenstock, & Chambers, 2008;Smit, Woodborne, Wolf, & McKechnie, 2019; Author Manuscript This article is protected by copyright. All rights reserved Smyth & Coulombe, 1971).…”