Denver Direct

By Dave Felice

The cormorants at Duck Lake in Denver’s City Park are carrying on with their nesting, despite the disturbance of lake renovation and development of the Asian Tropics exhibit at the Denver Zoo.

The migratory double-crested cormorants nest in the trees on the one-acre island in Duck Lake and feed on fresh-water fish. Observers say more cormorants are flying to find food this year, apparently because of the lower water level of the lake.

“The double-crested cormorant is the most widespread North American cormorant and the only one that occurs in large numbers near inland freshwater,” says Wildlife Ecologist Ashley DeLaup of Denver Parks and Recreation (DPR). She tells Denver Direct the island in Duck Lake has long been a rookery for the cormorants, along with black-crowned night herons and snowy egrets.
“Duck Lake is a resource in peril. The man-made lake has had marginal water quality conditions due to a buildup of nutrients and organic debris on the lake bottom, while the island is losing trees annually due to erosion,” observes George Pond, Vice President of Planning and Capital Projects for the Zoo. He describes Duck Lake as an important urban wildlife habitat.

According to a Zoo statement, construction crews are draining the 5-and-a-half acre lake to re-contour the lakebed, control erosion, and stabilize the banks. Indigenous fish are being moved to nearby Ferril Lake while the sediment at Duck Lake is drying.

“The Zoo has some of the best bird people around and I know they have always had a special interest in the rookery,” DeLaup tells Denver Direct. She says the renovation has to be done carefully because “as a migratory bird, the cormorants are federally protected, (and) mitigating impact on nesting would be federally mandated.”

Park officials have not said if the lake would get a liner to control seepage. Purple Pipes carry minimally processed water from the Lowry Landfill to Ferril Lake which flows to Duck Lake. The water is “recycled” to provide irrigation for park properties.

As part of the renovation, DPR crews will plant wetland vegetation along the banks of the lake, and build small observation areas. New trees will be planted on the island later in summer 2010.


Writing on the COBirds web site, George Kemena of Denver says: “The double-crested cormorant nesting colonies at City Park's Duck Lake (about 30 pairs) and Ferrill Lake (about five pairs) are a stunning sight. Nest refurbishment is well underway. It's great to see the double-crested cormorants in full double-crested mating plumage. As a group, they are a loud colony of birds. There are no leaves to obstruct views, and you can easily view from your car. Duck Pond is on the northwest side City Park whereas Ferrill Lake is just west of the Denver Museum of Nature and Science (the bigger lake). Anyway, this is a great location for a drive after work or a lunch hour.”


The double-crested cormorants live near rivers, lakes, and coastlines from Alaska to Florida. The all-black birds, about 32-inches long, get a double crest of black and white feathers in breeding season. After being in water, the birds have to dry their feathers, which are not waterproof. Use of DDT once threatened cormorant populations, but the birds have increased noticeably in recent years.

In some locations, people consider the cormorants a nuisance because they are better at fishing than humans. The birds are known to occasionally raid fish farms in open water.

According to the national Audubon Society’s Colorado office, birds in City Park face “minor” threats from introduced animals (Canada geese), pollution from the heron colony, and disturbance to birds and habitat from heavy recreational use of the park.

On Sunday, May 16, Kathy Bollhoefer of the Audubon Society of Greater Denver will conduct a tour and present a lecture on the bird colonies at City Park. The group meets at 5:30 p.m. in the parking lot of the museum of Nature and Science, and the tour lasts about two hours. Bollhoefer says she will have more information about how the renovation of Duck Lake may affect the nesting.

Brad Parks, Director of Public Programs for Denver Zoo, says Ferrill and Duck Lakes are significant breeding sites for the birds. “This improvement will help ensure that migratory birds have a safe rookery for years to come. Without improvement, Duck Lake will continue to decline, becoming a less viable option for these amazing animals.”

Brad Parks conducts annual counts of nests and birds for Denver Zoo in partnership with the Rocky Mountain Bird Observatory. Ashley DeLaup can be contacted at 303-455-0785 or ashley.delaup@denvergov.org.