Brian Millsap studied wildlife biology at Colorado State University in the 1970s, but during his four years there, he said bald eagles were a rare sighting.
鈥淲e saw one bald eagle, and it was a big deal,鈥� he said. 鈥淚t showed up on one of the little reservoirs east of Fort Collins and we were all out there looking at it, thinking, 鈥榃ow, this is cool. A bald eagle.'鈥�
Now, the former raptor coordinator for the U.S. Fish and Wildlife Service presumes there鈥檚 probably two to three nesting pairs near that same reservoir. He even saw some nests while driving to the Denver International Airport a few weeks ago.
鈥淲e saw three occupied bald eagle nests on the side of I-25,鈥� he said. 鈥淚t's been amazing for me to kind of watch the Front Range explode the way it has with bald eagles.鈥�
Eagles are crucial to regulating prey in the environment, and are also considered apex predators. That means if something happens to eagle populations, it could be a sign of a deeper problem.
In the 1960s, eagles had been nearly decimated nationwide by the pesticide , which was later banned in 1972. At the lowest point, there were only a little over 400 nests across the country, according to the .
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Fifty years later, Colorado's bald eagles and other raptors were hit hard by avian influenza in 2022. The disease caused birds to become puffed up and not responsive. In severe cases, infected birds would flap their wings excessively and act highly neurotic.
鈥淲e saw large numbers of dead birds,鈥� said Reesa Conrey, the avian researcher for Colorado Parks and Wildlife (CPW). 鈥淲aterfowl were very affected. There were some reservoirs where there were hundreds or even thousands of dead waterfowl. Bald eagles were likely impacted because they were feeding on infected waterfowl and becoming infected themselves.鈥�
The reduced the number of bald eagle nest success by about 20%, according to Conrey. It was difficult to confirm an exact number of deaths because eagle nests are high above the ground and hard to reach by researchers.
But now, eagles on the Front Range have made a remarkable recovery. CPW reported the highest numbers of bald eagles last year the state has ever seen 鈥� more than 300 nesting pairs.
There have been some cases of avian influenza in more than 67,000 chickens this year, according to . It's even affected and, according to , some people. But these recent cases haven't affected as many birds 鈥� let alone bald eagles.
Conrey expects eagle nesting pairs to be even higher this year, even though this year's eagle mortality rate is also higher than she had expected.
鈥淚t looks like an exponential curve that's perhaps starting to slow down a tiny bit,鈥� she said. 鈥淏ut we're still seeing more and more nests.鈥�
Millsap said it鈥檚 not surprising eagles on the Front Range are thriving.
鈥淭here's a lot of water there, a lot of fish, a lot of food, a lot of birds,鈥� he said. 鈥淏ut it's satisfying to see bald eagles adapting to that developed, residential, urban environment.鈥�
Michael Tincher, the rehabilitation and research coordinator with the Rocky Mountain Raptor Program, said the changes in climate over time could also be a factor behind the surplus.
鈥淏eing as we have milder and milder winters, we're finding more of these birds are not moving on come springtime over this past winter,鈥� he said. 鈥�(We鈥檝e) seen a lot more non-breeding bald eagles in the area, so things are good down here for them.鈥�
Conrey thinks the plummeting eagle population due to avian influenza in 2022 was 鈥渦nusual鈥� and 鈥渁 blip." She believes the lack of cases in 2023 was due to fewer eagles feeding on infected waterfowl as food sources.
鈥淚 see this as just a general long term recovery from the extremely low numbers of bald eagles that we saw, not just in Colorado but nationwide, back in the 1970s,鈥� she said.
But Tincher and other raptor experts are questioning if acquired immunity to avian influenza has also played a role.
鈥淚 don't think this virus just all of a sudden went away,鈥� he said.
The Rocky Mountain Raptor Program plans to collaborate with other organizations on a research project in the coming months that will take blood and tissue samples of birds to see if there are birds that have been exposed to avian influenza but don't show symptoms.
