Wintering raptor sightings have declined in Boulder over the past 30 years
Boulder County has seen massive changes over the past century when it comes to human populations and development. This has resulted in habitat loss and fragmentation and, consequently, the loss of local biodiversity. Thirty years of wintering raptor data indicate that sightings of several raptor species in Boulder have declined dramatically, with two notable exceptions.
Raptor with prey / Unsplash
Boulder County has seen massive changes over the past century when it comes to human populations and development. And recent population growth continues this upward trend: according to the U.S. Census Bureau, since 2010, Boulder County’s population has grown by roughly 11%, or 32,000 people, with the surrounding counties growing at equal or even higher rates. This growth results in conversion of open lands to development, and may threaten the species that depend on these open lands, including Boulder’s birds of prey.
Steve Jones, Environmental Consultant and Volunteer at the Boulder County Nature Association and Boulder County Audubon Society, has led surveys of Boulder’s raptors every winter for the last 30 years. As his data show, raptor sightings have declined rapidly over that time period with several species now mostly absent from surveys (Figure 1). For example, according to the Boulder County Audubon Society, Ferruginous Hawks (Buteo regalis, shown as the orange line in Figure 1), were observed regularly in winter in Boulder County during the 1980s and early 1990s, but the number of sightings has sharply declined for this species over the past 30 years.
But not all species have suffered.
Observations of two raptor species, the American kestrel (Falco sparverius) and Red-tailed hawk (Buteo jamaicensis, the red line in Figure 1, below), have actually increased over time. Experts believe the success of these two species is linked to their generalist lifestyles. Both have varied and opportunistic diets and are relatively tolerant of human structures, so they have adapted to the changing landscape in ways that other species have not.
Birds of prey are predators at the top of the food chain, and their presence serves as a barometer for ecological health. The decline in raptor sightings over the last thirty years underscores the need for Boulder County to continue acquiring large parcels of open land for biodiversity conservation (Figure 2).
explore the data
Wintering raptors: a 30-year study
species
Figure 1. Bird detection data from six Boulder County survey routes, conducted each winter (October – March), from 1990 to 2020. Data are presented as the number of individuals per square kilometer, which represents the average number of each species of wintering raptor sighted across the six survey routes each year. Generally, raptor observations in Boulder County have declined over the thirty-year period. These declines are likely the result of habitat loss, specifically the loss of large open lands that support their mammal prey. Two species are exceptions to this trend: the American Kestrel (Falco sparverius) and red-tailed hawk (Buteo jamaicensis), whose sightings have increased over time. Data provided by Steve Jones, Environmental Consultant and Volunteer at the Boulder County Nature Association and Boulder County Audubon Society.
Meet the exceptions:
American Kestrel (Falco sparverius)
The American kestrel (Falco sparverius), also called a sparrow hawk, is the smallest and most common falcon in North America. Their diet typically consists of grasshoppers and other insects, lizards, mice, and small birds (e.g. sparrows). This broad diet has contributed to their wide success as a species, and is likely responsible for why the kestrel has persisted in Boulder, while most other raptors are declining. Source: Audubon Society Guide to North American Birds.
Red-tailed hawk (Buteo jamaicensis)
The Red-tailed hawk (Buteo jamaicensis) is one of the most common hawks worldwide. Their diet includes large birds, reptiles, and an array of small mammals, as well as amphibians, fish, and insects. Their diet is highly variable and reflects their status as opportunistic generalists. These generalist tendencies are considered the primary reason the red-tail is expanding in Boulder, while most other raptors are declining. Source: Audubon Society Guide to North American Birds.
recommendations
Accelerate the acquisition and restoration of open lands throughout Boulder County.
Boulder County Parks and Open Space, as well as the city of Boulder Open Space and Mountain Parks, have acquired and continue to acquire large parcels of land for conservation, agriculture, and recreation (Figure 2).
The decline in sightings of nearly all raptor species indicates that these efforts may not be enough — that land is not being acquired fast enough to offset the habitat being lost. Moreover, these species may be in decline due to land use changes throughout the front range region of Colorado, not just in Boulder County.
Several important native grassland species have already disappeared from Boulder County, such as the mountain plover (Charadrius montanus, a ground-nesting bird that is threatened throughout much of its range) and pronghorn antelope (Antilocapra americana, a species that has been in North America for over a million years!).
To reverse the observed declines in raptor sightings, we recommend continuing local efforts to conserve native habitats, as well as continuing to form partnerships with organizations to enhance habitat quantity and quality for these key species. For example, the Colorado Open Space Alliance (COSA) is “a statewide organization of publicly funded local and regional open space programs, working cooperatively to share information, create public awareness and foster partnerships needed to protect and preserve the special places of Colorado” and is actively engaged in collaborations around shared values of protecting open lands and biological diversity.
Become an eco-steward! The raptor observations displayed here are the result of a robust, local volunteer program designed to monitor and protect natural areas. Join them for a future survey!
Figure 2. These maps showcase the efforts of Boulder County Parks and Open Space to acquire land for parks and open space from 1990 to 2020. The dark gray spaces represent developed (urban) areas of the county and the green spaces represent open lands owned or managed by the Boulder County Parks and Open Space (BCPOS) department. The urban area in the lower-middle part of the maps is Boulder, and in the upper right corner of the maps is Longmont. Louisville, Lafayette, and Superior are in the lower right portion of the maps. BCPOS is working to acquire more land, with the goal of protecting native plants and animals, including birds of prey. Note that this map does not include open space lands surrounding the city of Boulder that are owned or managed by the city of Boulder Open Space and Mountain Parks department. Source: Data provided by Boulder County Geospatial Open Data. Symbology was modified to clearly identify Boulder County Parks and Open Space properties.
