Birding the High Line Canal Trail (2/17/2023)

This morning, a walk along the High Line Canal Trail in Greenwood Village proved eventful. For most of the walk, a drone — pilot unseen — beat me around every corner. It flew higher than nearly any of the birds, save a large flock of Red-winged Blackbirds.

Wednesday’s snow meant a fairly fresh canvas covered by mammal tracks. Indecipherable ones lined the canal — coyote perhaps, but I’ve also seen a bobcat in the area. Rabbit, raccoon, and rodent tracks crisscrossed the trail.

Down along the Little Dry Creek, signs of beaver activity. Should the beaver have any success at building, their work will certainly be undone. Homes, the trail, and nearby roads all come to close to the creek for Greenwood Village to abide beavers.

Beaver activity along the Little Dry Creek.
Beaver activity along the Little Dry Creek.

A Baffle of Bushtits

During my short walk, I had the good fortune to encounter a flock of 15-20 Bushtits. They were in a productive patch of the canal, along with Chickadees, White-breasted Nuthatches, Juncos, Songs Sparrows, and Spotted Towhees.

Bushtit along High Line Canal Trail (2/17/2023)

Every encounter with Bushtits is magic. They’re incredibly active birds, moving from spot to spot in ways that mean you can never be sure you’ll indeed meet them while birding.

This flock might have been satisfied feeding along the canal, undisturbed by me and my camera. But a Sharp-shinned Hawk had other plans. Entering the scene to briefly perch on a power pole, the hawk inspired the Bushtits to give an emphatic, noisy warning call. They went quite for a time, as the hawk stayed perched. But as soon as the sharpie took off again — flying directly away from the Bushtits — the Bushtits called again.

I’d not heard this vocalization from Bushtit before. Apparently, they reserve it for small avian predators, namely Sharp-shinned and Cooper’s Hawks. Joseph Grinnell documented this in a 1903 essay. Referring to the call as a “confusion chorus,” Grinnell speculates that its radiating intensity causes accipiters to lose individual Bushtits to the din.

A flock of bush-tits will be foraging as usual, with the ordinary uncertain medley of location-notes, when suddenly one or two birds utter several of the sharp alarm notes and then begin a shrill quavering piping. This is taken up by the whole flock, until there is a continuous monotonous chorus. At the same time every member of the scattered company strikes a stationary attitude in just the position it was when the alarm was first sounded, and this attitude is maintained until the danger is past. In nearly every case the
danger is in the shape of a hawk, more especially of the smaller species such as the sharp-shinned or sparrow hawks. No matter how close the hawk approaches, the shrill chorus continues and even intensifies until the enemy has passed. The remarkable thing about this united cry, is that it is absolutely impossible to locate any single one of the birds by it. The chorus forms an indefinably confusing, all pervading sound, which I know from personal experience to be most elusive. This confusion-chorus, as I think it might be appropriately called, is a sure sign of the appearance of a small hawk even a long way off.

It seems reasonable to infer that this monotonous chorus of uncertain direction, at the same time as it sounds a general alarm, serves to conceal the individual birds, all of which at the same time maintain a statuesque, motionless attitude. Their colors also harmonize closely with the shadows of the foliage. The whole evidently forms a composite protective device, which must be, as a rule, effectual.

Elsewhere, spring?

Beyond the Bushtits, I encountered a female Northern Flicker in a cavity in a cottonwood. She sat or perched deep in it at times, her head barely visible in the cavity.

Northern Flicker in a cottonwood cavity along the High Line Canal Trail (2/17/2023)

She stayed in the cavity for some time, perhaps watching me watch her.

Northern Flicker along the High Line Canal Trail

What was she doing? We’re months away from when flicker nesting usually begins. I didn’t notice any obvious nest building activities — no excavation of the cavity, no interactions with a mate.

Perhaps the sun, after snow, sparked something — as, for all of us, it tends to.

Fri 17 Feb 2023
9:58 AM
High Line Canal Trail–Greenwood Village / Centennial (Arapahoe Co.)

16 species observed

Sharp-shinned Hawk, 1
Northern Flicker, 4
Blue Jay, 2
Black-billed Magpie, 7
American Crow, 2
Black-capped Chickadee, 5
Bushtit, 15
Red-breasted Nuthatch, 1
White-breasted Nuthatch, 2
European Starling, 3
American Robin, 10
House Finch, 9
Dark-eyed Junco, 2
Dark-eyed Junco (Slate-colored), 1
Dark-eyed Junco (Pink-sided), 1
Song Sparrow, 2
Spotted Towhee, 2
Red-winged Blackbird, 35

A Briefness of Bohemian Waxwings

Bohemian Waxwings are having a moment. These far north birds are venturing into Colorado. But what’s brought them here for the first time in a decade?

Bohemian Waxwings are having a moment. This winter, these far north breeders have ventured as far south as Santa Fe, New Mexico. Here in Colorado, Bohemian Waxwings have been relatively common in the Front Range this winter.

It’s been been a decade since Bohemians have been so numerous and conspicuous in Colorado. What’s brought them here? We’ve had deep snow and extreme cold in the Rocky Mountain west. While I don’t know if this has been the case farther north, where this species usually overwinters, one suspects that it is.

But perhaps it’s a fool’s errand explaining the Bohemian Waxwings southward movements. Cornell’s Birds of the World says this of the bird.

Notoriously irruptive, irregularly appearing in large numbers in regions south of normal wintering range. For example, only rarely reported from New Mexico, but in Apr 1959, huge numbers (about 10,000 individuals) arrived in Santa Fe and remained until early May (Ligon 1961). Late arrival at this southern locality suggests individuals moved into the area after depleting fruit supplies in more northern areas. 

At least what we can say what the birds are doing here: devouring the berry-like drupes of Common Buckthorn, an invasive, non-native plant.

Just south of Denver, along the High Line Canal, a single Bohemian Waxwing amid 50-100 Cedar Waxwings quickly became a flock of 30-50 Bohemians among 100-150 Cedar Waxwings.

Today, January 15, that flock tripled in number. For the first time, I had the good fortune of standing amid a large, active, and noisy flock of Bohemians. They filled a small Buckthorn on the Canal’s edge. Dozens occasionally jammed in to my camera’s field of view.

While the photos of these gatherings aren’t particularly impressive, I like how much movement and activity they convey. Waxwings and a robin or too point in every direction. Some are in flight. Others are swallowing Buckthorn. Others are about to collect it. The birds only seem to do one thing in unison: throw themselves into flight, as a single, impressive flock.

Waxwings Gathering in a Buckthorn

When not eating, the Bohemian Waxwings trilled from high perches in the Canal cottonwoods. By comparison to the Cedar Waxwing’s call, the Bohemian’s call is slow and low. Between the robins, Bohemian Waxwings, and a few magpies, the Canal filled with a complex mix of sounds.

Waxwings and the Suburban Landscape

This year, many of the best places to see Bohemian Waxwings near Denver have been south and slightly east of the city, along the High Line Canal Trail. eBird shows that birders have reported Bohemian Waxwings throughout a 4 mile, as a car drives, and 7.5 mile, as the Canal winds, stretch of the trail from southeast Denver through Cherry Hills Village.

eBird Species Map of Bohemian Waxwings sighted along the High Line Canal
eBird Map of Bohemian Waxwing Sightings

The Canal itself is a ditch dug by hand over a century ago. It used to carry water from S. Platte River, southwest of Denver, to the prairies north east of Aurora. Along it, a riparian corridor of cottonwoods, willows, chokecherry, Golden Currants and American plums developed.

These native plants provide important resources to the trail’s wildlife. Cottonwoods, willows, chokecherries, and plums are important host plants for moths and butterflies. These are themselves important food sources for birds during nesting season. Currants, chokecherries, and plums bloom early, providing insects and birds with nectar. The cottonwoods provide nesting habitat for native and introduced species, particularly Northern Flickers, Eastern Screech Owls, and European Starlings.

The chokecherries feed songbirds in autumn; they also serve as host plants for moths and some swallowtail butterflies. Meanwhile, the plums are both a critical host plant for moths and butterflies and an important late autumn food source for mammals, particularly coyotes.

The corridors has also proven a suitable home for weedy and invasive non-native trees and shrubs, particularly honeysuckles, Russian olives, Siberian Elms, and Buckthorn. These can spread voraciously. Tree of Heaven, European Spindle, Privets, the occasional lilac, and Golden Rain Trees, which are overused in landscape plantings, have also begun to move in.

So here in the southern suburbs of Denver and Aurora, our encounters with waxwings happen in a landscape altered by human activities. Meanwhile, waxwings and other birds continue to alter it, spreading seeds of Buckthorn across the canal and adjacent open spaces.

ISO Buckthorn

Birds ignore Common Buckthorn drupes until mid-to-late winter. It’s often the only fruit left standing at this time of year. So to find uncommon or rare birds, it often means being in search of the plant. In addition to the waxwings and robins, I’ve also seen White-throated Sparrows, Townsend’s Solitaires, Hermit Thrushes, White-crowned Sparrows, Song Sparrows, and Spotted Towhees feeding on or associating with Buckthorn.

White-throated Sparrow in Common Buckthorn
White-throated Sparrow in Common Buckthorn along High Line Canal Trail (2017)

All this leaves me with an ambivalence. Rarely do I seek out patches of invasive plants with any kind of joy or hope. But if I hope to find waxwings, I also implicitly hope to find these. And this stretch of the Canal is a wall of Buckthorn. Many of the adjacent yards also have patches of the plant, perhaps owing to the activities of robins and waxwings.

Not to be Ignored, Cedar Waxwings

While Bohemian Waxwing steal the show, Cedar Waxwings are also worth watching. They, too, acrobatically feed on Buckthorn, in Cirque du Soleil feats of balance and flexibility.

Cedar Waxwing on the High Line Canal, Cherry Hills Village, CO

Cedar Waxwing and Buckthorn

Meanwhile, I’ve watched waxwings mellow out the laxative properties of Buckthorn by voraciously drinking. They’ll do this at puddles on pavement, as well as high in cottonwoods, where they gobble up melting snow.

Cedar Waxwings Eating Snow in a Cottonwood
A Cedar Waxwing Munching on Snow

A Cedar Waxwing with Snow on Beak
Cedar Waxwings on the High Line Canal, Cherry Hills Village, CO

An apparent pair of Cedar Waxwing used the snow to maintain their bond, exchanging it as a sort of gift. (Another Colorado birder reported similar behavior between Cedar Waxwings on January 7, and I saw it again on 1/15.)

This “passing behavior” is common during nesting season, when birds affirm their bonds through exchanges of food and water. According to Birds of the World, this behavior is not especially well documented in the winter among Cedar Waxwings: “The extent to which mates associate in winter and migratory flocks is not known.”

However, Bohemian Waxwings display this behavior, according to this 1978 article in Blue Jay, the long-running journal of Nature Saskatchewan. And according to the Birds of the World account of Cedar Waxwings, some pairs do indeed remain bonded beyond the nesting season.

For now, the birds remain committed to the Canal. Eventually, they’ll exhaust it as a food source and look elsewhere. They’re remarkably nomadic birds, after all.

Until then…

waxwing ripen
the winter
Bohemian Waxwing

For more on the remarkable behavior of winter waxwings, see Mary Holland’s incredible post and photographs of a Bohemian Waxwing feeding on Bald-faced Hornet larvae.

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