JWM study: Ferruginous hawks benefit from some human development

There’s no shortage of stories these days about wild animal populations decreasing due to habitat loss from human development. However, sometimes there’s a story in which human development actually benefits a species. A recently published study in the Journal of Wildlife Management provides such an example.

The authors examined the diets of ferruginous hawks (Buteo regalis) in rural and exurban areas of New Mexico. William H. Keeley, a wildlife ecologist at Boise State University and lead author of the study, defines exurban development as “an expansive human land use pattern characterized by one dwelling for every 20-40 acres. It usually occurs on the outskirts of urban areas where folks prefer a bit more land than urbanites. It also has a profound effect on habitat quality as roads and other human infrastructure effectively fragment habitat.”

From 2004-2005, Keeley and his co-authors studied the diets of two separate ferruginous hawk populations, one in Estancia Valley (exurban) and one in the Plains of San Agustin (rural). Both habitats are grasslands that share similar elevations, levels of precipitation, maximum temperatures and vegetation. However, Estancia Valley has significantly more human development (one house per 26 hectares) than the Plains of San Agustin (one house per 11,939 hectares).

They analyzed prey remains and regurgitated pellets to determine what and how much the hawks had been eating. The hawks living in the exurban areas were well-fed compared to their rural counterparts where houses are more spread out. In addition, the exurban hawks experienced greater nesting success and produced more offspring than the rural hawks.

Further, the exurban hawks delivered 2.5 times more biomass to their fledglings than the rural hawks. They were also more likely to feed on Gunnison’s prairie dogs (Cynomys gunnisoni) than the rural hawks, which were more likely to eat rabbits and hares.

“The results surprised us a bit,” said Keeley. “Our results indicate that the presence of prairie dogs may mediate the disturbance or other habitat modifications caused by humans in our exurban area and allow the hawks to maintain a relatively high level of nesting success and productivity.”

In other words, the exurban hawks were doing well because prairie dogs were abundant in Estancia Valley.

But why?

“My theory is that with more people owning land in the exurban area, categorical persecution of prairie dogs occurred less,” Keeley said. “In rural areas, one landowner, who owns a lot more land, could exterminate entire prairie dog colonies on his property. Whereas in exurban areas, the prairie dog colonies might cross the properties of multiple different landowners, some of whose values might align with prairie dog conservation, and therefore, the chance of extermination might be less.”

“To our knowledge, ours is the first study to document greater nesting success and productivity for ferruginous hawks nesting in an anthropogenically altered environment compared to a rural setting,” the co-authors wrote.

While Keeley and his colleagues urge caution when interpreting the results, due to the small sample size, they recommend that future resource management plans for ferruginous hawks include efforts to conserve prairie dogs and other prey populations, as well as their habitats.

Richard Levine is a freelance science writer.