September 17, 2021
A male Bobolink on his nesting grounds, wearing an aluminum ring with a unique number that allows researchers to identify him as an individual.
Around the world, grassland bird populations are in steep decline, warranting urgent conservation action. In North America, grassland birds evolved alongside American bison, which European immigrants counted in the tens of millions during their settlement of the North American Great Plains in the 1800’s. Although overhunting subsequently drove bison to near extinction, bison numbers have rebounded from a few hundred survivors to hundreds of thousands of individuals today, due to modern conservation efforts. But how are bison reintroductions affecting grassland birds? Surprisingly little research has been done on this question, which IBCP addresses in collaboration with other conservation scientists in a new scientific paper. Our publication focuses on an iconic grassland songbird, the Bobolink.
Bobolinks are long-distance migratory birds that travel over 20,000 kilometers each year between their North American nesting areas and South American wintering grounds. Bobolink populations have declined by over 60% in the past half-century, due in part to the loss and degradation of their grassland nesting habitat, making the conservation of grasslands where they nest an especially urgent priority. We conducted research on Bobolinks nesting on private conservation grasslands in the North American Great Plains where reintroduced bison recently replaced cattle as the main grazers.
An American bison herd, which was recently reintroduced to conservation grasslands where Bobolink nest, including a calf and bull in the foreground.
We expected that reintroducing bison would improve the conservation value of Bobolinks’ grassland nesting habitat. Instead, we were surprised to find dramatic declines in both adult and young Bobolinks following the reintroduction of bison! At the same time, Bobolink numbers remained stable in neighboring grasslands grazed by cattle and grasslands where hay was harvested after the bird breeding season. Many of us are familiar with the famous American folk song, “Home on the Range,” which describes the free-roaming bison of the past. Most modern bison, however, are unable to roam freely, and are instead confined in limited areas throughout the year.
The sedentary state of modern bison can lead to overgrazing, trampling, and other negative impacts, which in turn contribute to declines of birds nesting in these grasslands, as we found in our study. Cattle, by contrast, are domesticated livestock that are safer and easier to handle than bison, and are likewise easier for land managers to manage with short-term rotational grazing, potentially better replicating the ephemeral grazing with which Bobolinks and other grassland birds on North American prairies evolved. A cautionary tale for grassland bird conservation, this study provides important information for managers of conservation land that serves as potential or actual bison reintroduction areas.
First author Rachel Kaplan worked collaboratively to conduct fieldwork and analyze data to understand the effects of bison grazing as compared with cattle grazing and hay harvesting in grasslands where Bobolinks nest.