Extreme Heat Threatens Desert Songbirds With Death By Dehydration | @GrrlScientist
A recent study of songbird survival during heat waves in America’s desert Southwest finds that birds are at greater risk of lethal dehydration and mass die-offs when water is scarce, and this risk is predicted to worsen as climate change progresses
It’s no secret that the planet is growing warmer, and it has been doing so since 1880. According to NASA, most of the warming occurred in the past 35 years, with 15 of the 16 warmest years recorded since 2001 (Figure 1).
The year 2015 was particularly shocking because it was the first year ever when global average temperatures were at least 1 degree Celsius above the 1880–1899 average.
Thanks to climate change, extremely hot days are increasing in frequency, duration and severity every year, resulting in serious consequences for vulnerable populations. Already, extreme heat events have caused more human deaths annually than hurricanes, lightning, tornadoes, floods, and earthquakes combined.
But what about wildlife: how is climate change affecting animals and plants — especially those that live in already extreme environments, like hot deserts?
Desert-dwelling birds are vulnerable to extreme heat
To understand how climate change is affecting desert-dwelling birds, a team of scientists, led by Thomas Albright, associate professor of geography at the University of Nevada, Reno, studied heat stress in five bird species that live in the American Southwest.
“Birds are susceptible to heat stress in two ways,” said study co-author, Blair Wolf, a Professor of Biology at the University of New Mexico.
“When it’s really hot, they simply can’t evaporate enough water to stay cool, [so they] overheat and die of heat stroke. In other cases, the high rates of evaporative water loss needed to stay cool deplete their body water pools to lethal levels and birds die of dehydration; this is the stressor we focused on in this study.”
“By focusing on heat waves and dehydration in birds, it allows us to focus more carefully on one piece of the puzzle,” explained Dr Albright.
“It allowed us to use mechanistic understanding supported by actual physical measurements of evaporation from birds’ bodies.”
The research team collected physiological data, hourly temperature maps and used computer modeling to investigate how rates of evaporative water loss varied in five bird species with different body masses. They mapped effects of current heat waves on lethal dehydration risk for songbirds in the Southwest and pinpointed how rapidly dehydration can occur in each of their study species.
Measuring evaporative water loss in birds
To do this work, the research team went into the field in Arizona and captured 193 individual birds from five regionally common species (listed from smallest to largest: lesser goldfinch, Carduelis psaltria, house finch, Carpodacus mexicanus; cactus wren Campylorhynchus brunneicapillus; Abert’s towhee, Pipilo aberti; and curve-billed thrasher, Toxostoma curvirostre). These birds’ body masses ranged from 9.7 grams (slightly less than two US nickels) to 71.0 grams (roughly the same as a hen’s egg) and included both insectivores and granivores (Table 1).
Each bird was placed into a respirometry chamber — a device that estimates the rate of metabolism for whatever creature is placed inside. Its function is based upon the idea that whatever goes into an organism — food, air, or water — will come back out again sooner or later in the form of carbon dioxide and water. Measuring the CO2 and H2O gases as they build up inside the chamber provides an estimate of metabolism (or, as in this study, evaporative water loss).
To start, each bird was subjected to a temperature within its normal tolerance range (30 °C or 35 °C) to generate a resting baseline value for evaporative water loss. Next, the chamber was warmed in 2 °C increments over a time span of one to three hours to a final temperature somewhere between 40–54 °C, and respiratory gases measured once again.
The threshold of a bird’s thermal tolerance is around 40 °C.
“At about 40 C, they start panting, which increases the rate of water loss very rapidly,” said study co-author Alexander Gerson, an Assistant Professor of Biology at University of Massachusetts, Amherst, in a press release.
“Most animals can only tolerate water losses that result in 15 or 20 percent loss of body mass before they die,” continued Professor Gerson.
“So an animal experiencing peak temperatures during a hot summer day, with no access to water, isn’t going to make it more than a few hours.”
These data were used to create evaporative water loss “profiles” for each study species.
“Once we have these types of profiles for a number of different species, we can determine differential survival rates which will then drive differences in the overall avian community structure,” said Professor Gerson.
Smallest birds lose water the fastest
As expected, lesser goldfinches, the smallest birds in this study, lost water twice as fast as the much larger curve-billed thrasher, the largest birds in this study. These findings indicate that during a heat wave, goldfinches can reach their limits of dehydration tolerance in less than 2 hours — although even thrashers approached these same limits in as little as 3 hours.
Yet the goldfinches’ large distributions and relative abundances protected their populations against the effects of extreme temperatures because the majority of their populations live in places that face a low mortality from acute heat stress. Thus, although the goldfinches will likely experience mass mortality in desert areas in the future, birds living in less climactically challenging areas and in places where plenty of water is available should still survive.
That scenario differs sharply from expectations for desert specialists — Abert’s towhees, cactus wrens, and curve-billed thrashers. These specialists have relatively small distributional ranges and, because their populations are confined mainly to hot deserts, they are more vulnerable to, say, the 4 °C warming scenario that is predicted to occur by the end of this century.
Under this climate change scenario, desert specialists will be faced with extreme heat that will push them beyond their threshold for lethal dehydration on at least a few days every year. Since they only live in deserts, they’ve got nowhere to go to avoid extreme heat effects. Even more worrying is that a heat wave in an already hot desert can cause frequent and lengthy periods of drought — and the combined effects of extreme heat and water limitation can quickly prove lethal to even the toughest desert birds.
Larger desert specialists will also suffer mass die-offs
The research team constructed a computer model based on their data. This model was designed to estimate what might happen to the five study species in the future. The researchers found that desert-dwelling populations of goldfinches, which were the smallest study birds, and house finches will be confronted by an increasing risk of lethal dehydration under predicted climate change scenarios.
By 2100, goldfinches will be exposed to 60–120 days of potentially lethal conditions over large portions in their range annually, for example. Since goldfinches and house sparrows are granivores, and because grains are poor sources of water, both species have limited opportunities to obtain sufficient water from their food, which means they will be dependent upon open water sources.
The larger three study species — cactus wrens, Abert’s towhees, and curve-billed thrashers — may experience prolonged periods of heat stress only in Sonoran Desert portion of their ranges. Since wrens and thrashers are insectivores, their risk of water stress is mitigated somewhat because these species have constant access to preformed water in their diet. In contrast, granivorous Abert’s towhees could be in real trouble, unless they have access to open water sources.
Considering that extreme weather events will occur more frequently, for longer periods of time, and with greater intensity in the future, this makes me doubt whether small granivorous songbirds weighing between, say, 10–20 grams, can possibly persist in deserts over the long-term.
But, if anything, the future is dire for the larger desert specialists: because they have such small ranges and their populations are limited to deserts, the continued existence of these species is threatened by climate change.
Dr Albright agreed, pointing out that this research “shows that in these hot desert systems for these species, we have a potentially devastating mechanism that can lead to die-offs for some species.”
Further, as these species succumb to extreme heat, their loss will negatively reverberate throughout the entire desert community.
What we can do to help wild birds
The obvious solution to this problem — take all necessary steps to limit climate change — is unattainable, we must do other things to help desert-dwelling birds. For example, we can identify and conserve areas that are home to diverse plant and animal communities that provide essential shelter and water to desert birds; areas such as ravines where shade is available during the heat of the day, and riparian habitats that have open water.
“Using this type of data, managers identifying the best refugia can have a better idea of the temperature profile that will be suitable for these birds,” said Professor Gerson.
This work is part of a larger project that aims to better understand how the physiology of birds that live in the hottest places on earth is tied to massive die-offs that are already starting to happen elsewhere, such as Australia (ref). The researchers are planning to use their modeling data to identify which plant types and conservation strategies will best protect desert-dwelling birds in other parts of the world in the face of worsening climate change.
Thomas P. Albright, Denis Mutiibwa, Alexander. R. Gerson, Eric Krabbe Smith, William A. Talbot, Jacqueline J. O’Neill, Andrew E. McKechnie, and Blair O. Wolf (2016). Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration, Proceedings of the National Academy of Sciences, published online ahead of print on 13 February 2017. doi:10.1073/pnas.1613625114
Andrew E. McKechnie, Philip A. R. Hockey and Blair O. Wolf (2012). Feeling the heat: Australian landbirds and climate change, Emu — Austral Ornithology, 112(2):i–vii. doi:10.1071/MUv112n2_ED
ERRATUM: This piece included a picture that was misidentified as an Abert’s towhee. That bird was actually a canyon towhee, Melozone fusca. On 10 March 2017, that image was replaced with a correctly-identified Abert’s towhee.
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Originally published at Forbes on 28 February 2017.