Nature Almanac, March 2021: Pampas Hawks Arrive on Schedule but Find Fewer Vacancies

For millennia, pampas hawks (Buteo swainsoni)* have lived geographically symmetrical lives, wintering in mid-latitude grasslands of Argentina and nesting in mid-latitude grasslands and desert shrublands of western North America. Traveling in flocks of up to 200, they fly as far as 7000 miles during spring and fall migration.

With their long, tapered wings and relatively light bodies, pampas hawks can soar for miles on fall thermals, completing a more than 5000-mile migration to grasslands in southern South America. Photo by Stephen Jones.

We once saw a flock of more than 100 individuals riding a thermal over the southern Wyoming prairie in early October and were astonished to see a golden eagle dive down and snatch a hawk out of the sky. One autumn, Ruth Carol Cushman, Larry Crowley, and other local naturalists saw an equally large flock circling over a plowed field east of Lafayette, where a subdivision now spreads across the land. Other Boulder County observers have seen dozens following tractors across fields in late summer, presumably catching grasshoppers stirred up by the plowing.

Pampas hawks typically begin arriving in Boulder County around the first week of April, but in recent years sightings have been reported as early as March 20. We wondered if climate change might be affecting their long-distance migration.

Surprisingly, neither the Boulder County Audubon 1978-2014 wildlife inventory nor eBird 1900-2020 data show a trend toward earlier first arrivals. The vast majority of these long-distance migrants continue to arrive in Boulder County in April, with only a few trickling in toward the end of March.

This handsome youngster fledged from a nest in a large cottonwood in Ruth Carol and Glenn Cushman’s front yard. Photo by Glenn Cushman.

However, we’ve observed another trend that may implicate climate change. These open country specialists have begun nesting on the urban fringe. Seven pairs nested within the Broomfield city limits during a recent spring, and a pair has begun nesting in downtown Louisville across the street from the public library. 

Ruth Carol and Glenn Cushman enjoyed watching a nesting pair in a large cottonwood along their driveway near Teller Lakes Open Space from 2005-2009. This nest fledged 1-3 young per year, and the adults had no compunction about divebombing Ruth Carol and Glenn when they walked down the driveway to pick up their mail.

This handsome youngster fledged from a nest in a large cottonwood in Ruth Carol and Glenn Cushman’s front yard. Photo by Glenn Cushman.

Meanwhile, red-tailed hawks seem to be taking over historic pampas hawk nesting territories in remnant grasslands of Boulder County. Red-tails reside here year-round and have increased in number. They typically begin incubating eggs as soon as deciduous trees leaf out, which now begins as early as mid-March. So when most of the pampas hawks arrive in April, the aggressive red-tails have already claimed the best rural nesting territories.

Is this shift in nesting habitat limiting pampas hawk numbers? The 1987-1992 and 2007-2012 Colorado Breeding Bird Atlas data indicate no decline statewide, nor do North American Breeding Bird Survey data.

All birds gravitate toward easy food sources, and pampas hawks have long nested around agricultural fields, where they hunt grasshoppers and small rodents. During late spring and early summer, when small mammals tend to dominate their diets, semi-urban life may suit them just fine, since rodents typically thrive around human settlements. For what it’s worth, the nest in downtown Louisville fledged two young last July. 

However, urban nesting can have its perils. In the San Francisco Bay Area, burrowing owls have nested in rodent burrows near airports and landfills for decades, but productivity in these areas remains low due to predation on the young by urban-adapted carnivores.

Either way, we would hate to live in a world where most birds face the choice of adopting an urban lifestyle or vanishing, as have long-billed curlews, mountain plovers, and other grassland-nesting birds of Boulder County. That’s one powerful incentive for continuing to lobby for conservation of native grasslands here and throughout Colorado.

*For this article, we’ve used a descriptive name derived from the term for South American grasslands. One Arapaho name is heeyei biitei, denoting “hawk ghost,” and the American Ornithological Society (AOS) moniker remains Swainson’s Hawk. Ruth Carol deferred to Steve on this issue but prefers using the AOS name believing that a standardized name facilitates communication and lessens the possibility of misunderstanding. For more about the growing controversy over patronymic naming of other beings, see Ted Floyd’s recent article in Birding magazine. 

Additional March events:

• Listen for chorus frogs chirrup in wetlands throughout the plains.

• Look for early emerging hoary comma and mourning cloak butterflies in foothill canyons and on the plains. They are among a half-dozen Boulder County butterflies that overwinter as adults.

• Watch flocks of migrating greater sandhill cranes fly over on their way to the North Platte River near Lewellen, Nebraska, where they will spend the month fattening up on corn and invertebrates before continuing north.

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Nature Almanac is a series by Stephen R. Jones and R. Carol Cushman. For many years, Boulder-area readers have enjoyed the wonderful perspectives of our local natural world that Steve and Carol so beautifully portrayed.  We thank Steve and Carol for agreeing to continue inspiring us.  Stephen Jones and Ruth Carol Cushman are authors of Wild Boulder County and The North American Prairie.