Home on the Range
Across North America, birds breed on grasslands grazed by domestic cattle. However, much of this region was once occupied by vast herds of American Bison (Bos bison). Grasslands evolved in the presence of grazers, with many species adapted to the type of disturbance large ungulates create. Although the majority of grassland habitat is maintained as rangeland for cattle, in some places people have begun reintroducing bison back onto the landscape. Most of the reintroduction efforts are relatively recent, occurring in the two decades. But interest in bison reintroduction is growing. So, what is the birding like where the “buffalo roam”? Turns out, it can be pretty great!
Smithsonian’s National Zoo and Conservation Biology Institute (NZCBI) scientist Dr. Andy Boyce, (who is a University of Montana alum and affiliate faculty member) and his colleagues at NZCBI’s Migratory Bird Center wanted to know how bird diversity on rangeland is affected by what animals graze there. The scientists are particularly interested in the grasslands of eastern Montana, which have the greatest diversity of breeding grassland- and sagebrush-obligate songbirds in North America. With the cooperation of public and private landholders, the American Prairie, and the Fort Belknap Indian Community, the scientists selected two types of grazing sites: areas that were grazed seasonally by domestic cattle, which were manually moved among fenced enclosures; and areas that were grazed year-round by bison, which were free-moving within a perimeter fence. The researchers found only slight differences in the associated bird communities. Two bird species, Vesper Sparrows (Pooecetes gramineus) and Grasshopper Sparrows (Ammodramus savannarum) were detected at higher densities in bison-grazed areas than in cattle-grazed areas. But other bird species appeared to be unaffected by the different grazers. This is important because one of the arguments against large scale reintroduction of bison on range areas is the fear that without rotational controls, such as are used in cattle ranching, bison may overgraze areas, leading to wildlife habitat degradation. This research suggests otherwise.
In a follow-up article, Dr. Boyce and his colleagues compared the effects of cattle and bison grazing on riparian habitat contained within these same grassland study areas. For this study, they looked at differences in the use of the habitat by deer and elk, as well as associated bird diversity. What they found was that, in general, bison reintroduction has strong positive effects on overall biodiversity in riparian habitats. Bison pastures had higher riparian plant diversity and shrub cover, were used more by white-tailed deer and elk, and had more breeding bird species.
The reason for this difference is due, in part, to the fact that cattle require constant sources of water, while bison do not. Unless manually moved by ranchers to new ground, cattle tend to center their activities around water sources, overgrazing the surrounding vegetation. In contrast, bison naturally move around a lot more, because they are able to wander further away from water. Ranging away from rivers and streams means bison are less likely to degrade riparian habitats by trampling plants and eroding banks. This could also be good news for fish and amphibian diversity. The authors suggest bison may function as a “passive riparian restoration strategy” enhancing the diversity of both mammals and birds.
In addition to moving around more, bison also graze differently than cattle. They are more effective at trimming “overstory” grasses, allowing sunlight to reach smaller flowering plants growing below. Having access to added sunlight stimulates the undergrowth, attracting more native insects and birds. Where bison wallow, they create depressions that fill with rainwater, creating micro-communities of diverse organisms. Because bison are generally managed to be free-ranging, they require less fencing than cattle which could spell good news for animals such as pronghorn (Antilocapra americana) and Greater Sage-Grouse (Centrocercus urophasianus), which are occasionally killed by fencing. Overall, reintroducing bison can improve water quality and habitat conditions, with likely benefits to the broader biodiversity of an area. In fact, bison may be the ideal grazers for restoring ecosystem health and bird diversity in grassland systems. However, Boyce and his colleagues note that, because the grasslands evolved with grazers, good range management practices can “create habitat for a highly diverse assemblage of grassland birds” on ground occupied by either bison or cattle.
As more national parks, Indigenous groups, and conservation organizations are reintroducing bison to land in the U.S., Canada, and Mexico, it is comforting to know that songbirds may continue to find a home on the range.
To learn more about this research, check out the papers listed below!