May Nature Almanac: Snake, Awake!
By Ruth Carol Cushman with Steve Jones
May 2025
Snakes that have snoozed away the winter in underground hibernacula or, possibly in your crawl space, awake and emerge in April or May. For many of us, finding a bullsnake (Pituophis catenifer sayi) in the garage is a welcome sign of spring, though a five-footer curled up in the valve box does make us jump back. As Emily Dickinson wrote, we feel “a tighter breathing, and zero at the bone.”
That’s a normal reaction after tripping over a snake, but too often snakes get a bad rap. Rattlesnakes are the only venomous snakes in Boulder County, though non-venomous snakes will bite or release foul-smelling cloacal secretions when harassed. But we’d be over-run with mice and voles without them.
A Bullsnake peeks shyly out of its hibernaculum, ready to retreat quickly. Photo by Dan Fosco
Bullsnakes are probably the most frequently encountered snakes in Boulder County. At the end of April, our friend Dan Fosco took several of us to a hibernaculum where he has been observing these snakes for ten years. In 2014, a year after the massive flood rearranged the landscape, he saw a kingfisher fly into a hole in a newly formed embankment. When he returned in 2015, he found bullsnakes in residence, and they have retained ownership ever since.
His first bullsnake of 2025 was sunbathing above the hibernaculum on March 28. A month later we watched two snakes poke their heads out from the burrows, but they were a bit wary. Only after we left and Dan returned alone, did they fully emerge.
One Bullsnake returns to the hibernaculum as another leaves by a different hole. Photo by Dan Fosco
These cold-blooded reptiles sometimes sunbathe in the middle of a road where they are crushed by cars. They are also killed by humans who mistake them for rattlesnakes because of their blotchy-patterned, yellow-brown scales. Or, by misguided humans who just don’t like snakes. Natural enemies include hawks, mink, weasels, coyotes, and foxes.
Averaging six feet in length and ranging up to eight feet, bullsnakes appear formidable. Individuals can flatten their heads into a triangle resembling a viper as they coil, hiss, and vibrate their tail, sounding like a rattlesnake in dry brush. Others, usually females, remain passive, even when picked up. Incidentally, snakes feel dry to the touch, not slimy.
A subspecies of gopher snakes, bullsnakes often share a hibernaculum with rattlers and other snakes, using the same site year after year. Hundreds of snakes sometimes inhabit the same den. After waking up and warming up, males slither out in search of females, flicking forked tongues in and out to follow scent trails. It’s hard to tell whether two snakes coiled together are jousting males or a copulating pair!
Are these bullsnakes mating or fighting? Photo by Steve Jones
Eggs, laid in June or July in underground burrows, hatch in about eight weeks. Many other snake species, including rattlesnakes, bear live young.
Most common below 7,000 feet, bullsnakes thrive in a variety of habitats: agricultural fields, grasslands, wetlands, woodlands, canyons, and residential areas. They remain active all summer, returning to winter quarters by late October.
Bullsnakes, adept at climbing trees, take bird eggs and nestlings as well as rodents and lizards. Monitors of the bluebird boxes at Walker Ranch reported that bullsnakes seemed to be the most common predator of nests, and osprey nest observers have seen them climbing 40-foot high poles that supported the nests. In the Nebraska sandhills, the majority of mallard nests within one study area were depredated by bullsnakes.
A jaw hinge enables the snakes to swallow large eggs and some live prey whole. Usually, however, these constrictors coil themselves around their victim, squeezing it to death.
Sometimes the tables are turned. We occasionally see Swainson’s Hawks carrying large bullsnakes to nestlings. Herpetologist Geoffrey Hammerson writes of a prairie dog burying a bullsnake in a burrow and of a group of prairie dogs attacking another.
Ten snake species occur in Boulder County: racers, smooth green snakes, milk snakes, northern water snakes, plains back-headed snakes, western terrestrial garter snakes, plains garter snakes, common garter snakes, western rattlesnakes, and bullsnakes. For more information, see Geoffrey Hammerson’s Amphibians and Reptiles in Colorado (book) or the Colorado Herpetofaunal Atlas (PDF).
Other May Events
Wild plums bloom in the foothills attracting Julia orangetips, red admirals, and other butterflies.
Sugarbowls, chiming bells, larkspur, and pink pediocacti also bloom. A new flower seems to open every day.
American White Pelicans, avocets, and White-faced Ibis fish or forage at local reservoirs.
Yellow-headed Blackbirds call raucously from cattail marshes while warblers sing sweetly from the treetops. Western Tanagers return from Central America.
Abert’s squirrels engage in mating bouts with up to seven males pursuing a single female through the ponderosa pines.
Nature Almanac is a monthly series by Stephen R. Jones and R. Carol Cushman, along with other guest contributors. Ruth Carol Cushman and Stephen Jones are authors of A Field Guide to The North American Prairie (Peterson Field Guides) and Wild Boulder County: A Seasonal Guide to the Natural World.