Gerhard Assenmacher / Courtesy photo Western tanagers are passing through Boulder on their way to conifer forests to nest.
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If you mixed together the colors of a cardinal, a canary, and a crow, you might come up with a western tanager, a songbird that graces the cover of many bird guides, including the second Colorado Breeding Bird Atlas.

In May, on their flight north from Central America to breeding areas in western conifer forests, these robin-sized birds pass briefly through Boulder. If you see pedestrians gawking into the sky at an intersection, they may be watching a tanager catching insects in the style of a flycatcher. That was our introduction to these bewitching birds, and our initial thought was they were exotic escapees.

Instead, western tanagers are fairly common but sometimes difficult to see because they prefer to stay in the tree canopy. To find them, listen for their up-and-down song that sounds like a sore-throated robin rapidly asking and answering questions.In Colorado, they usually nest above 5,000 feet in Douglas-fir, ponderosa pines and lodgepole pines. They’ve also nested in spruce-fir forest, pinyon-juniper woodlands and even oak shrublands. Pairs normally begin courtship and nest building in mid-May. Cornell’s website “All About Birds” includes a description of a display in which the male tumbles past a female, flashing yellow and black plumage. Males sing almost constantly to establish territories and, once mated, stay close to their mate, sometimes feeding her at the nest.

Fledglings appear from June through mid-August and are fed a smorgasbord of bugs including wasps, ants, termites, beetles, grasshoppers and dragonflies whose wings, head and legs are clipped off before being swallowed. It’s thought that a rare pigment, rhodoxanthin, contained in many of the insects the birds consume causes the red head feathers in adult males.In addition to insects, western tanagers eat fruit, and winter stragglers sometimes come to feeders for seed. During this year’s mid-April blizzard, Dave and Peg Fletcher reported an early-arriving male western tanager battling starlings for a perch on their suet feeder in Niwot. Dave says they seem to like suet that contains fruit.

Formerly classified in the tanager family, western tanagers (Piranga ludoviciana) are now considered part of the cardinal family. Five tanagers (summer, scarlet, stripe-headed, hepatic and western) migrate to North America from the tropics. Of these, the western tanager ranges farthest north, even making it into frigid northwestern Canada.

However, climate change may adversely affect these colorful migrants. The National Audubon Society’s climate model projects a 70 percent loss of current summer range by 2080. The model predicts that by that date the range of western and scarlet tanagers may overlap, and they may be interbreeding.

Other May Events

• Painted ladies (black, orange and white butterflies) may put on a spectacular show this year, thanks to the heavy rains in southern California.• Miller moths pass through in large numbers on their way to breed in the mountains; tent caterpillars emerge from cobwebby tents; and little blue butterflies puddle in muddy streams.• Sugarbowls, larkspurs, shooting stars, pink ball cacti and wild iris bloom in the foothills.• Abert’s squirrels engage in mating bouts with up to seven males pursuing a female through the ponderosa pines.• Colorado chipmunks, striped skunks, and white-tailed deer are born.• World Migratory Bird Day, May 18, celebrates returning migrants at St. Vrain State Park. This year, migratory birds seem especially vulnerable because the Interior Department is rolling back the prohibition on “incidental take” from the Migratory Bird Treaty. Guided bird walks will be held on the half hour, starting at 8:30 a.m. For details, check www.boulderaudubon.org.


Ruth Carol Cushman and Stephen Jones are authors of “Wild Boulder County” and “The North American Prairie.”

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