USGS study finds ravens more likely to occur in areas where cattle grazeWritten by Saige Albert
On March 2, the U.S. Geological Survey (USGS) released a study noting that the presence of cattle increases the occurrence of ravens, which have been shown to prey on Greater sage grouse nests within sagebrush ecosystems.
“We’ve been investigating factors that influence raven populations and interactions between ravens and nesting sage grouse for quite some time,” says USGS Research Wildlife Biologist Peter Coates. “In this study, within southeastern Idaho’s Curlew Valley, we set out to understand what factors are contributing to raven populations at this site and their relationship with cattle.”
He continues, “We specifically investigated how cattle might influence the predator community, rather than how they might influence vegetation related to nest concealment, which is generally the focus of most grazing studies.”
The main conclusion from the study is that ravens are nearly 50 percent more likely to inhabit areas in sagebrush landscapes if cattle are prevalent. Ravens also preferentially select sites near sage grouse breeding grounds.
“Evidence suggests that livestock operations may provide ravens with a number of direct and indirect resource subsidies,” Coates adds.
Raven populations across the American West have increased three-fold during the last 40 years, according to Coates, who adds that the birds have also been identified as an important predator of sage grouse nests through previous USGS research.
“We have learned, from previous video-monitoring work, that ravens are often the most frequent predator observed depredating sage grouse nests,” he explains.
While depredation of sage grouse nests has occurred throughout the history of the bird, Coates notes that the dramatic increase in raven populations in recent decades is concerning to sage grouse and other species.
The primary goal of this research was to help inform management actions aimed at reducing raven access to subsidies and interactions between ravens and nesting sage grouse.
In this study, Coates and his team worked to detect ravens utilizing intensive surveys that were randomly assigned across the sagebrush landscape consisting of approximately 400 square miles.
“We conducted nearly 350 10-minute point counts, and within those, we estimated the distance of ravens from the observer,” he says. “We then measured landscape and site level characteristics at survey areas to investigate which environmental factors were associated with the probability of raven occurrence.”
The major finding of the study showed that the incidence of ravens increased by 45.8 percent in areas where cattle were present.
“Ravens were also more likely to occur around lek sites,” Coates says. “We found that for every one kilometer of increased distance from a lek, the probability of ravens decreased by 8.9 percent.”
Leks are important breeding areas for sage grouse, he says, noting that they generally serve as hubs for nesting grouse.
“Anecdotal evidence suggests ravens are cued into the lek areas where they can more easily find nesting females,” he explained. “They also appear to be relying on resource subsidies that come with cattle – like food and water sources.”
In other words, ravens are able to utilize range improvements that usually accompany cattle grazing, including water tanks and troughs.
Anthropogenic resource subsidies
More broadly, Coates notes that ravens utilize a variety of anthropogenic resource subsidies – or resources that benefit ravens made by humans – to thrive in the arid, desert landscapes across the West.
“We also investigated other potential resource subsidies on the landscape, like fence lines, power poles, structures, water troughs and so forth,” he says. “At this site, the presence of cattle was the most influential of all the subsidies and landscape characteristics that we investigated. Other subsidies added to the probability that ravens would be present, however.”
Coates continues, “It seems highly likely that water troughs designed for cattle use contribute to raven occurrence because these structures provide an available water source in an arid environment throughout the spring and summer months.”
Territorial ravens may even set up their nests around water troughs.
“It is likely that cattle are not specifically what ravens are after, though they have been shown to provide food subsidies in multiple ways,” Coates adds. “The resources provided to cattle are used by ravens and likely contribute to raven reproduction and survival.”
Ravens are highly intelligent birds that rapidly adjust to human-induced changes for their benefit, and the ecological consequences of unintended resource subsidies for ravens should continue to be evaluated and recognized as important, he continued.
“With more human development in remote areas, we see an increase in reproduction and survival of ravens,” he says. “Features like transmission lines, landfills, roads or cropland that wouldn’t otherwise exist on the natural landscape are typically beneficial to raven populations.”
Coates qualifies the research by saying that limitations are present in the study.
“Although we had a fairly large number of surveys, the study represented a single site,” he says. “We can always continue to make improvements in the number of samples and replicate the design in other areas to really understand the system.”
He also notes that they are currently replicating the study across multiple sites in Nevada to investigate if the effects of cattle vary across sagebrush steppe ecosystems. For example, they are studying remote areas where cattle are grazing further from ranch headquarters and homesteads to see if discoveries there are consistent with these findings. More remote locations also reduce other human-created features on the landscape, such as fences and roads.
Coates notes that although ravens seem to be more prevalent in areas with cattle, several management actions may help to reduce raven occurrence.
“Limiting access to water troughs might help to reduce the number of ravens in the area,” he says. “Managers might also consider timing of cattle use in areas where sage grouse nest, which could help reduce interactions between ravens and sage grouse.”
In addition, Coates says that management to reduce adverse effects from human-induced modifications to the landscape might be most beneficial. He notes that direct removal of ravens will likely be less effective in the long-term.
“Some studies have reported short-term success with direct removal of ravens through lethal action,” he explains. “Ultimately, however, studies have also indicated that reducing access to anthropogenic resource subsidies and improving nesting habitat conditions for sage grouse will likely be the most effective at reducing raven densities for longer periods of time and avoiding elevated nest predation by ravens.”
As a result, he believes that research would support reduction in the number of anthropogenic subsidies on the landscape to reduce raven populations over the long-term.
Coates comments, “Further awareness and research regarding factors influencing raven populations and their adverse impacts to sensitive wildlife species will likely lead to effective management of our natural resources for generations to come.”