In the early to mid-1800s, when Europeans and Euro-Americans began arriving in what is now Colorado, they encountered a landscape that was significantly different from what we see today. The changes that have occurred to the landscape since then have had significant impacts on the state’s natural history. In a well-known example, the fur trade, removal of Indigenous people, hunting, and mass conversion of prairie to farmland resulted in the near-extinction of the bison. The effects of changes in land use on bird populations are less familiar, but they are easy to observe and provide a great way to appreciate the avian fauna of our state as well as how humans are seamlessly connected to our living world.
This article is focused on land use and bird life in Boulder County on Colorado’s Front Range. However, many of the same kinds of human-avian relationships can be observed elsewhere in the state, even if they play out in slightly different ways.
More Trees, Different Birds
Perhaps the most noticeable difference in the natural history of Boulder County over the past 150 years is the vast increase in trees on the plains and in some foothill areas. A pair of photographs in Silvia Pettem’s book Boulder: Evolution of a City shows Mapleton Avenue in Boulder during the 1890s, when silver maple saplings were first planted, juxtaposed with a view of the same area in the 1990s, illustrating how the presence of trees has created an entirely new habitat for birds over the past 100 years. Meanwhile, in the foothills and mountains west of Boulder, many areas were heavily logged in the mid-1800s and have slowly grown back. Thomas Veblen and Diane Lorenz show this change in paired photographs from the nineteenth and late twentieth centuries in their book The Colorado Front Range: A Century of Ecological Change. Because the logged trees were native species that eventually grew back, this change did not affect bird populations as much as introducing many new trees to the plains.
Before the arrival of European Americans, the only trees on the plains were riparian—species associated with stream channels. Plains Cottonwoods, Peachleaf Willows, and a few other native species provided limited habitat for tree-dwelling and tree-nesting birds. Many of these birds, such as the yellow warbler and Bullock’s oriole, are still present today. But as the new immigrants planted trees in Boulder County and other towns and cities across Colorado, many Eastern woodland birds slowly extended their ranges into Colorado; examples include the blue jay, blue-gray gnatcatcher, northern flicker, bushtit, black-capped chickadee, and northern mockingbird. Even the northern cardinal and eastern phoebe have recently been sighted regularly in Boulder County and other parts of eastern Colorado. The spread of forest and urban habitats has also brought more common nonnative species, such as the European starling, the house sparrow, and, more recently, the Eurasian collared dove.
The creation of urban forest habitats has also resulted in increased populations of red-tailed hawks and great horned owls owing to increased nesting and perching sites, and of Cooper’s hawks and sharp-shinned Hawks, which nest in trees and prey on woodland birds.
As trees—along with urban development and agriculture—have displaced native prairie habitats, there has been a predictable decrease in bird species adapted to prairie life. Some, such as the plains sharp-tailed grouse, are long gone from Boulder County, while others—such as grasshopper, savannah, and fox sparrows—have become less common as their habitat or food sources declined. Lark buntings, horned larks, and species of longspurs are much less numerous now because of reduced prairie habitat. According to the bird-conservation group Partners in Flight, there has been an 86 percent decline in the population of Colorado’s state bird, the lark bunting, since 1970. The North American Breeding Bird Survey shows that populations of mountain plover also declined by 80 percent between 1966 and 2014.
More Ponds and Lakes, Different Birds
Perhaps even more striking than the effect of trees on bird populations on the Colorado Front Range is that of the large number of ponds and reservoirs built over the past 100 years. Before the arrival and settlement of Europeans and Euro-Americans in the mid- to late 1800s, there were no naturally occurring lakes or ponds on the plains of Colorado, other than the occasional widenings of streams and rivers. The landscape from a bird’s-eye view was one of arid grasslands and long, narrow, streamside habitats. Just as these riparian habitats provided trees for limited populations of woodland birds, the quiet, pondlike sections of rivers and streams supported limited populations of water birds. That all changed in the late nineteenth century.
Boulder’s first reservoir was built in 1876. Starting at about the same time, ponds and larger reservoirs began to be built in earnest all over the Colorado plains. A satellite view today reveals hundreds of reservoirs and ponds all over the Front Range and eastern plains of Colorado, especially near towns and cities. These features have provided huge new habitats for many birds that previously were either not present or present in very small numbers. The vast proliferation of human-made ponds and reservoirs has resulted in the long-term and common presence of double-crested cormorants, common and red-breasted Mergansers, ospreys, bald eagles, various species of grebes and shorebirds, and virtually all species of diving ducks on the plains of Colorado.
The American white pelican is a visible and fitting example of a bird whose presence in Colorado has been vastly changed by the presence of human-made reservoirs. Although white pelicans had always migrated through Colorado, which lies between their southern wintering areas and natural prairie lakes in Canada and the northern United States, their long-term presence in Colorado is almost entirely due to reservoirs. On the Colorado Birding Trail website, for example, all locations for white pelicans (including new, breeding populations) are reservoirs.
Sometimes these pelicans have helped solve other human-made problems in Colorado’s reservoirs. For example, in the early 2010s, one Boulder County reservoir, the twelve-acre Teller Lake No. 5, became infested with thousands of nonnative goldfish. State wildlife biologists considered various solutions, including draining the lake or shocking it and removing the stunned goldfish. But in spring 2015, white pelicans descended on Teller Lake No. 5 and ate virtually all the goldfish, presenting state biologists with a hassle-free solution.
Other Land-Use Changes
Smaller-scale, more subtle human land-use patterns have also had an effect on Colorado bird populations. According to Steve Jones, Boulder County naturalist and author of A Field Guide to the North American Prairie, when ranching on semiurban grasslands is replaced by industrial use or land speculation for development, there is a short-term burst in prairie dog numbers because there are fewer incentives for landowners to poison them. Jones believes that in the 1980s this rise in prairie dog population fueled an influx of wintering ferruginous hawks into Boulder County. Jones also notes that the removal of cattle from mountain meadows, foothill shrublands, and plains riparian areas during the past fifty years has contributed to the proliferation of birds such as yellow-breasted chats, gray catbirds, ovenbirds, and more.
Finally, although climate change is a result of worldwide land-use changes, its impact in Colorado should not be neglected. Warming average temperatures may already be affecting Colorado bird populations. Many migratory species are arriving earlier in the spring and nesting at higher elevations. Habitat for species that nest above the tree line is shrinking, contributing to a population decrease among brown-capped rosy finches and white-tailed ptarmigan. Over the next several decades, further drying of Colorado’s grasslands, shrublands, and foothills forests may cause the composition of our nesting bird populations to resemble that of present-day New Mexico.
Beyond Boulder County
Importantly, the human-bird relationships observed in Boulder County can also be observed in different parts of the state in different ways. For instance, populations of greater sage grouse in western Colorado have been negatively affected by the replacement of sagebrush grasslands by agriculture and oil and gas activity, as well as by more frequent fires resulting from the introduction of nonnative, weedy plants such as cheatgrass. In addition, the creation and fish stocking of large reservoirs on the Western Slope, such as Blue Mesa Reservoir in Gunnison County, have attracted many of the same species that are now more common in Boulder County, such as herons and pelicans. Essentially, wherever humans have made drastic changes to the land and water in Colorado, drastic changes in the bird populations have followed, a consistent reminder of our inescapable place within the natural world.