Researchers and biologists begin study of bald eagles along Colorado’s Front Range

DENVER – Colorado Parks and Wildlife (CPW) along with numerous project partners have embarked on a four-year study to help better understand current population trends, habitat use and impacts of human disturbance on bald eagles along the state’s most densely populated corridor. 

The Front Range corridor of northern Colorado is an area that is experiencing rapid human population growth — up 18 percent since 2000. Between 2019 and 2029, the state is forecasted to grow by 832,000 people with 87 percent of that taking place in the Front Range. 

Amazingly, this densely developed area also contains a high concentration of bald eagles. In CPW’s raptor-nest database, as of 2020, there were more than 90 breeding pairs of bald eagles in this corridor from the Denver metro area to the Wyoming state line. 

“The reason we are focused on this area is the concentration of bald eagles along the Front Range, juxtaposed with the concentration of humans and human infrastructure along the Front Range,” said CPW Avian Researcher Reesa Conrey. “That intersection is a huge part of this project, in addition to monitoring what the eagles are doing in terms of their nest numbers and nest success.”

Populations of the American bald eagle — the bold national symbol of the United States — have quadrupled since 2009, according to a new report by the U.S. Fish and Wildlife Service and its partners.

However, eagles did not always thrive in this urban corridor.  

By the end of the 1970s there were only three known bald eagle nests in Colorado with none on the Front Range. Bald eagle populations declined in the early- to mid-20th century due to pesticides (primarily DDT), human disturbance, land conversion and loss of trees for nesting habitat.

Thanks to protections implemented for the species and DDT being banned for general use back in 1972, the slow recovery process started. In Colorado, that rebound has accelerated over the past few decades, concurrent with human population growth along the Front Range. The bald eagle was deliste​d from Endangered Species Act prote​​ction in 2007, although they still are protected by the U.S. Fish and Wildlife Service under both the Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act. CPW classifies the bald eagle as a Tier 2 “Species of Greatest Conservation Need.” 

Off-setting some of the urban development over the years was the construction of numerous reservoirs, which provide potentially usable habitat. Reservoirs, rivers and large streams surrounded by large cottonwood trees and riparian areas provide most of the conventional nest sites in the South Platte River Basin where the Front Range lies. 

However, discovery of new nest locations has revealed the presence of bald eagles nesting in upland locations far removed from conventional nesting locations. CPW’s study could help explain why bald eagles chose such unconventional settings. And though reduced in area and number, prairie dog colonies in this region still serve as an important year-round food source for raptors. Now, Colorado is home to eagles that live here year-round as well as a wintering population that breeds elsewhere.

Researchers, biologists and volunteers from CPW, the U.S. Fish and Wildlife Service, the Bird Conservancy of the Rockies (BCR), along with numerous city and county agencies have been working together for years to monitor Colorado’s bald eagles to better understand their environmental needs and disturbance tolerances. These partners are coming together to help with this new research project that launched in the second half of 2020 and is expected to have management implications to help conserve bald eagles in a region that will continue to see rapid human development and land use conversions. 

The study is expected to last four years and will be the most comprehensive bald eagle monitoring project ever done by CPW.

“We are looking at nest sites along a gradient of human activities and disturbances from urban to rural areas,” Conrey said. “We are especially interested in comparing areas expected to remain stable with those expected to see new development within the next few years. We can use spatial data over the past several decades to get at land use change as this area has been developed for residential and commercial uses, agricultural conversions, sand and gravel mining and energy, including oil and gas wells, solar, and wind energy facilities. We’re getting more transmission lines, cell towers, road traffic, use of trails and boating areas and all the other things that go along with human activity and an increasing human population.”

Researchers will mark a sample of bald eagles nesting along the northern Front Range with GPS-GSM transmitters. The transmitter data will allow them to intensively monitor habitat use and eagle movements year-round, during both the breeding and nonbreeding seasons. At the same time, staff and a large number of dedicated volunteers will continue to monitor reproductive effort and success by conducting observations at nest sites.

“The transmitters that we are using for this project are different from what typically have been used,” Conrey said. “Previous generations of wildlife transmitters required biologists to use antennae to pick up the signals or they connected to satellite networks. But these transmitters connect to the cellular communications network. It allows our transmitters to be lighter in weight. That reduces potential stress on the eagles and it was a good choice for us in the Front Range because we have a lot of cell towers in this area.” 

The transmitters can provide frequent location data – with time intervals as little as four seconds between locations. They’ll be placed on 25-30 bald eagles. The original transmitters plus harness weighed under 70 grams (0.15 pounds), but with continued innovation, the new units deployed in 2021 weigh less than 50 grams (0.11 pounds) – which is about one percent of the body mass of an average adult male. Each captured eagle is weighed, carefully inspected to assess its health and fitted with a numbered leg band as an identifier.

The first transmitter went out in July 2020 and the second in October. Further deployments continue, but most of the 14 marked eagles to date were captured from May to early July, the time of year when many adults are feeding large, hungry “teenage” eaglets preparing to fledge from the nest.

An army of volunteers, many from the Bird Conservancy of the Rockies and staff from city, county, state and federal agencies will all play a crucial role in data collection for this study. 

“We’re thrilled to see data collected by our volunteers contribute to this research effort,” said Matt Smith, outreach biologist with BCR. “Bird Conservancy has been monitoring the expansion of the bald eagle nesting population across the Front Range for 30 years and while eagle numbers have steadily increased during that time, so has the human population of the region. This gives us an opportunity to put those data to work and learn more about how eagles are adapting to the changes we’re making to the landscape. Hopefully, this will tell us more about what the future looks like for bald eagles in the years to come and what management actions can be undertaken to ensure a healthy population of this iconic bird in our state in perpetuity.”

BCR’s Bald Eagle Watch (BEW) program has a volunteer citizen science network that monitors nesting bald eagles along the Front Range and elsewhere. The program has been steadily increasing its number of citizen volunteers as well as its scope and intensity of monitoring.

“Observations from the BEW program complement the data gathering and population evaluations performed by CPW,” said Bruce Snyder, who began as a BEW volunteer in 2013 monitoring the bald eagle nest that at the time was located at the Applewood Golf Course. “BEW volunteer information fills gaps that CPW is unable to acquire because of resource and manpower limitations, but is important for making wise protection and mitigation decisions. CPW is able to more efficiently plan and implement its GPS tracking study because of the detailed observation information BEW observers provide.

“The things that continually have impressed me are the adaptability, hardiness and dedication of the bald eagle during the nesting phase,” Snyder added. “As one example, almost all nesting eagles continued incubating their eggs during the March 2021 snowstorm that dropped two feet of snow across much of the Front Range.”

Results from the monitoring effort will be used to model the bald eagle population trajectory and expected impacts of predicted future land use change. Biologists will then make data-driven recommendations on minimizing and mitigating disturbances of the bald eagle’s environment essential to its survival. It is anticipated that study results will identify opportunities for conserving and possibly developing key habitat conditions during the planning, permitting and implementation of various human developments along the Front Range. 

“The study will give us a better understanding of this species’ tolerance of and adaptability to human activities and land use changes,” Conrey said. “The results will greatly improve long-term bald eagle monitoring, conservation and management efforts in Colorado.”

Photos below
Row 1: Reesa Conrey releases a bald ealge along the St. Vrain River, July 3, 2021
Row 2: picture showing the GPS-GSM transmitters being used in the study
Row 3: Wildlife biologist Lance Carpenter holding the bald eagle pre-release on June 5, 2021
Row 4: Wildlife biologist Mike Sherman releasing a bald eagle

Podcast episode discussing the bald eagle research project Video showcasing the research project