Our Evergreen Audubon Birders are a flexible bunch, as at the last minute our bird walk location was moved to Rocky Mountain Arsenal National Wildlife Refuge. Despite the late change, seven stalwart birders arrived at the Arsenal, binoculars in hand and spotting scopes at the ready, excited to enjoy a cool but sunny early Spring morning.

Rocky Mountain Arsenal National Wildlife Refuge is a prime example of how a former shortgrass prairie ecosystem has rebounded after human use, first as farmland, then for wartime and cold war chemical weapons production. The soil, water, plants, and wildlife were heavily contaminated with a toxic mix of chemical weapons components, organic chemical pesticides, and other contaminants. In a prior highlights post, Chuck Aid, Director of Bird Monitoring, provides a more detailed description of this sorry history.

Fortunately, the Department of Defense’s remediation efforts to decontaminate the Arsenal property and to restore it to the original shortgrass prairie ecosystem have been largely successful, even as ongoing monitoring of soil and water is necessary. In the meantime, U.S. Fish and Wildlife Service management has opened the landscape to hiking so that visitors may experience a shortgrass prairie ecosystem in close proximity to Denver’s urban and suburban areas.

This is exactly what our group of Evergreen Audubon birders did! The south edge of the Arsenal has several reservoirs and riparian areas that are magnets for migrating birds. We began our trip at the southwest edge of the Arsenal, where Lake Mary and the northwest corner of Lake Ladora are readily accessible. The weather started out sunny and cool (~40F), so a quick walk to Lake Mary warmed us up and got us started on the day’s list with sightings of Canada Geese, a singing Song Sparrow, and several drumming and calling Northern Flickers.

We then spent a significant period scoping out waterfowl and shorebirds at the northern end of Lake Ladora. This was our opportunity to spend significant time studying and comparing species. Spotting scopes were necessary to identify waterfowl at this stop, as most of the birds were at some distance to the south. Here we were pleasantly surprised to see a large flock of Ruddy Ducks and were able to easily identify a flock of American Avocets wearing their rust-red breeding plumage. Other birds were more of a challenge to identify, such as the Semipalmated Sandpiper that we observed and discussed for several minutes. Horned Grebes provided a different challenge: Eight of them were close to the shore and readily observed, but the variations in head plumage had us comparing back and forth between Horned and Eared Grebes before settling on our identification. Spotting a pair of Red-Breasted Mergansers before we continued on to other points was a great way to finish birding this hotspot.

Next, we drove along the south edge of the Arsenal, and birding the south end of Lake Ladora, adding more birds to our list, including Great Blue Herons, Green-winged Teal, a Least Sandpiper, and more ducks. Lower Derby Lake, which can range om bone-dry to full, proved to be a challenge. There were many waterfowl and shorebirds present but low water put them at a great distance and the rising wind and shimmer off the water and lake bed cofounded our efforts to identify them. We stopped at Upper Derby Lake, which was, unsurprisingly, dry, and decided to take the auto tour.

Bison were in abundance, and the chance to see Horned Larks and Say’s Phoebes made the trip worthwhile. We were disappointed not to see the Burrowing Owl that had been reported at the North East corner of the Arsenal, despite our diligent scanning. A lone Coyote, with a bare tail, lets us know that mange continues to affect Front Range canids.

The Arsenal is a great place for experiencing the prairie at all times of the year, but right now is an especially good time to get out there as migration continues to drive many species through this shortgrass oasis in the midst of Front Range sprawl.

Hope to see you out there!

Ed Furlong
Evergreen Audubon

Rocky Mountain Arsenal NWR, Adams, Colorado, US
Apr 15, 2023 7:38 AM – 1:31 PM
46 species (+2 other taxa)

Canada Goose  30
Blue-winged Teal  10
Northern Shoveler  85
Gadwall  25
Mallard  8
Green-winged Teal  14
Canvasback  1
Lesser Scaup  8
Bufflehead  5
Hooded Merganser  4
Red-breasted Merganser  2
Ruddy Duck  35
Horned Grebe  11
Western Grebe  12
Mourning Dove  4
American Coot  3
American Avocet  23
Killdeer  7
Least Sandpiper  1
Semipalmated Sandpiper  1
Greater Yellowlegs  4
Lesser Yellowlegs  1
large shorebird sp.  7     Observed at the limit of scope magnification at lower Derby Lake. Distance and
light shimmer precluded more specific ID.
Ring-billed Gull  3
Great Blue Heron  2
Turkey Vulture  3
Northern Harrier  4
Bald Eagle  1
Red-tailed Hawk  8
Northern Flicker  6
American Kestrel  3
Merlin  1
large falcon sp.  1     Good probability that this was a Prairie Falcon, but distance and obscuring branches
precluded a more definitive ID.
Say’s Phoebe  1   
Black-billed Magpie  4
American Crow  1
Black-capped Chickadee  1
Horned Lark  5     
Tree Swallow  1
Barn Swallow  1
European Starling  50     conservative estimate
American Robin  30
American Goldfinch  1
Vesper Sparrow  1
Song Sparrow  1
Western Meadowlark  25
Red-winged Blackbird  34
Common Grackle  1

Rocky Mountain Arsenal,
Apr 15, 2023
46 species