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Lethal dehydration – a ticking clock for desert birds

While political leaders deny climate change, mass deaths of desert birds resulting from escalating temperatures are becoming a frequent occurrence. Scientists say the risk of lethal dehydration will only become worse as we move towards the end of the 21st century. Lethal dehydration occurs when the deficit of total body water becomes too great and causes all bodily functions to shut down, leading to death. 

The Head of the Department of Zoology and Entomology at the University of Pretoria, Professor Andrew McKechnie, is part of an international study looking at the effects of climate change on birds, particularly those living in very hot deserts. Their findings reveal that these birds are going to be under great strain in the future. Prof McKechnie explains: 'These birds are very well adapted to desert conditions, but the 4°C increase in temperature over the course of this century will likely overwhelm their physiological capacity to survive in extremely hot areas.'

The study comprises several components that were conducted in the deserts of Arizona in the USA, deserts in Southern Australia and the Kalahari in Southern Africa. Mid-summer temperatures in these environments can reach a scorching 50°C. In one component of the study, the team assessed five different species of birds living in the North American deserts in order to cover a range of body sizes. 'This is because the rate of evaporative water loss (EWL) depends largely on body size,' Prof McKechnie explains. Arid-zone specialists such as the cactus wren and the curve-billed thrasher were among the larger birds studied, while smaller, but more widely distributed subjects included the lesser goldfinch and the house finch, which occur over most of the western half of USA. The study reveals that very hot days in these deserts, with a 4°C warming scenario, exceed the threshold for lethal dehydration. Under future climate scenarios, the chronic effects of heat and water limitations will become even worse.

While goldfinches are very vulnerable to extreme heat, these deserts fortunately represent only a fraction of their range. Desert specialists have a lower rate of EWL because of their larger body mass; however, their geographical range is more restricted than smaller species, putting them at greater risk of extinction.

Like humans, birds' bodies are made up of about three quarters water. On hot desert days, by mid-morning, temperatures often reach a sweltering 40°C. By this point, birds become inactive and are no longer able to forage, because the more active the bird is, the more heat it produces. Perched in deep shade, which is difficult to find in any desert, they pant in a desperate effort to cool themselves down, but temperatures continue to soar well into the afternoon. On such days, birds lose water through EWL, but do not gain any by drinking for feeding. Lethal dehydration occurs when a bird loses 15% of its water.

Prof McKechnie explains that with average temperatures increasing over time, birds would be able to disperse and move out of increasingly hot areas into cooler ones. This study, however, undertook to understand what happens to birds living in extremely hot temperatures. Under these conditions, would birds also be able to migrate away from the heat? The findings suggest they cannot.

We cannot afford to lose birds – literally. The economic impacts that countries would incur would be devastating. The ecosystem services that birds provide make them incredibly important in the overall functioning of thriving ecosystems. Some of these services include predation, pollination, seed dispersal and ecosystem engineering. If one just considers their contribution in pest control, they – like bats – lessen the need for pesticides and save the agricultural industry millions by ensuring healthy crops.

While the study highlights the worrying fact that birds in very hot deserts are going to be in serious trouble sooner than most realise, McKechnie and his students are looking at ways to mitigate the impacts of higher temperatures on birds. Artificial water sources shaded by netting and types of vegetation that offer very dense canopies are supportive examples. Such measures are going to become very important in helping birds survive harsh conditions in their fight against extinction.


*This is a collaborative study with Dr Blair Wolf of the University of New Mexico, USA, and other researchers working in the USA and Australia. The paper, 'Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration', has received a lot of attention and appeared in the acclaimed Proceedings of the National Academy of Sciences of the United States of America journal.


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