Bird baths in unlikely places: How cattle stock tanks could contribute to the survival of raptors in the southwest

A newly published study in the Journal of Raptor Research suggests that cattle stock tanks may be a crucial source of water for raptors in the Southwestern United States. The paper illuminates how raptors cope with heat stress, and how human-made structures influence the presence of these important top predators as they move through a rapidly changing landscape. The study, led by Clint Boal from Texas Tech University, represents the first examination of the topic, which may become increasingly relevant to wildlife managers as they endeavor to understand how rising temperatures and falling levels of precipitation will affect the survival of wildlife species across the globe. In the Southwest, cattle stock tanks, referred to as “free water,” may offer a helping hand for raptors in their quest to stay cool, hydrated, and flight ready.  

Raptors obtain water from three sources: free water, preformed water, and metabolic water. Free water means surface water. Preformed and metabolic water are absorbed through consumption of prey. For decades there has been a consensus that raptors don’t require free water, but the results of this study throw that assumption into new light.

It turns out that raptors utilize free water for a variety of health-related behaviors including drinking, bathing, and preening. If drought and other dangerous environmental conditions intensify, raptors may be less likely to find and consume prey, which could further jeopardize their ability to remain adequately hydrated. In places like the Southwest, cattle stock tanks are the main source of free water on the landscape. The main goal of this study was to figure out how often raptors use free water, and which, if any, environmental factors were correlated with that use.

The researchers set up camera traps in west Texas to investigate the use of human-made water sources by 14 species of raptors during March of 2009 through January 2013. For nine months of this study an extreme drought swept through the region, heightening the reliance of several species on stock tanks. Of the identified raptors, barn owls (Tyto alba), northern harriers (Circus hudsonius), and Swainson’s hawks (Buteo swainsoni) were the three most common visitors to the tanks, likely because they are locally the most abundant species depending on the season.

All three species increased their visitation to stock tanks during the drought. Most individuals drank from the tanks and many bathed as well. Bathing is about more than just cooling off — it allows birds to refresh the condition of feathers, aiding them in many ways, not least of which is ensuring efficient flight. Notably, vultures were never observed bathing, and diurnal raptors, meaning those active during the day, were more likely to bathe than owls. Temperature and precipitation levels were important predictors in the frequency of tank use by northern harriers and Swainson’s hawks. Surprisingly, although drowning at stock tanks has been cited as a danger to raptors, no such events were observed over the course of this four-year study. Raptors that landed in deep water were able to escape. 

Boal, lead author on the paper, notes that although adults utilize free water, nestlings are unable to do so since they cannot fly, sometimes spending up to 70 days in the nest. “If they are not able to get enough water via their food, they can ultimately die from thermal exposure and dehydration.” Boal suggests future studies on the proximity of nests to water in arid landscapes, as well as investigation into whether bathing adults may influence nest microclimate with sodden feathers.

Drought frequency is on the rise. Temperatures are predicted to increase and levels of precipitation to decrease. This paper’s findings confirm that raptors ramp up their use of free water during times of heat stress in arid environments. And, says Boal, they “raise awareness of the potential negative implications for those species nesting in already arid environments.” Wildlife managers may benefit from considering the mosaic of human-made water sources on the landscape in conservation planning, especially given the importance of raptors as indicators of environmental health wherever they reside.

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Paper

Clint W. Boal, Brent D. Bibles, and Trevor S. Gicklhorn "Patterns of Water Use by Raptors in the Southern Great Plains," Journal of Raptor Research 57(3), 444-455, (25 May 2023). https://doi.org/10.3356/JRR-21-70

Notes to Editor:

1. The Journal of Raptor Research (JRR) is an international scientific journal dedicated entirely to the dissemination of information about birds of prey. Established in 1967, JRR has published peer-reviewed research on raptor ecology, behavior, life history, conservation, and techniques. JRR is available quarterly to members in electronic and paper format.

2. The Raptor Research Foundation (RRF) is the world’s largest professional society for raptor researchers and conservationists. Founded in 1966 as a non-profit organization, our primary goal is the accumulation and dissemination of scientific information about raptors. The Foundation organizes annual scientific conferences and provides competitive grants & awards for student researchers & conservationists. The Foundation also provides support & networking opportunities for students & early career raptor researchers.