Acid-Covered Caterpillars Are a Delicacy for Migrating Swainson’s Hawks

Each spring, thousands of the ravenous raptors descend on California's Anza-Borrego desert to gorge themselves on a buffet of moth larvae.

Sunset rolls through the California badlands like a rogue wave, dragging shadows across the canyons and flower-choked fields, setting whites, purples, and yellows aglow against a dusty terrain. It’s mid-March, but the thermometers have already broken the 100-degree mark. Dusk, thankfully, brings relief to the Anza-Borrego desert.

As the atmosphere cools and heat rises off the dunes, so do thousands of Swainson’s Hawks. They take to the skies with the last wisps of warm air—birds with eight-foot wingspans becoming specks of umber, clay, and white as they circle higher and higher. The flocks twist like gyres over the flats, eventually settling into the safety of eucalyptus groves for the night.

In the past few decades, more and more Swainson’s Hawks have been stopping here, making a detour on the 6,000-mile trek the birds have made for millennia from their winter rendezvous in the South American pampas to their northern breeding tracts. The journey begins in late February, as most of the world’s 800,000 Swainson’s move up the eastern seaboard of Mexico to spread out onto the Great Plains. The rest head up the Pacific coastline to grassland nesting sites in Central California, Oregon, and British Columbia, moving as slow as Coachella traffic. They stop often in prairies and ranchlands to recharge on bats, grasshoppers, dragonflies, and other insects, replenishing the fat they’ll need to survive the last leg of their marathon migration and ensure they’ve got stamina for sex. Since the early 2000s, each spring up to 12,000 birds have peeled away from the coast and winged it 60 miles inland to Anza-Borrego Desert State Park—lured by a local dish many predators can’t stomach.

Swainson's Hawks hang in the warm air lifting off the desert floor at dusk. Photo and video: Mac Stone

In Anza-Borrego, the raptors waddle from dusk to dawn among the creosote and invasive mustard bushes, stalking the neon-colored larvae of the white-lined sphinx moth, also known as the hawk moth. The fat-bottomed caterpillars wriggle across the desert during spring blooms, decapitating flowers and dressing car tires with their sticky green innards. While they might seem enticing to hungry hunters, between their neon-orange butt spike and the unappetizing acid they douse themselves in when threatened, few predators other than coyotes and javelinas dare eat them. 

And Swainson’s Hawks. The raptors don’t dine on the caterpillars; they devour them, sometimes overfilling their gullets to the point that they vomit up mushy cakes after liftoff. Cristina Francois, a white-lined sphinx moth researcher at the University of Arizona and director of Audubon’s Appleton-Whittell Research Ranch, says researchers have counted up to 30 insects in the hawks’ pellets. “It’s a full-on binge,” she says.

That is, when the caterpillars flourish. Scientists and birders drawn to the park’s springtime bounty have linked the hawk’s abundance to that of the caterpillar—which in turn is influenced by wildflower booms and busts. But no one’s quite sure how climate-related drought will affect this cycle, and whether migratory Swainson's will be able to continue to count on the ephemeral food source that they flock to in the desert. 


At 600,000 acres, Anza-Borrego is the vastest state park in California, with an area that rivals Rhode Island. It sits at the intersection of the Pacific and North American tectonic plates, between San Diego and the Salton Sea. Geologic upheaval over the past 100 million years carved out a rugged realm for survivalists. Bighorn sheep careen down ravines to sip at isolated palm oases; sidewinders leave quaking tracks across sandy hummocks; and ocotillos sprout from rocky slopes.

After heavy winter rains, the normally bleak flats are nuked by wildflowers. The canvas of tiny lilies, evening primrose, and sand verbena melts into the scorched horizon. Squat barrel cacti and finger-like cholla erupt in risqué bouquets.

In 2017, after five years of drought, seven inches of precipitation provoked the region’s biggest “super bloom” since 1999. A record number of visitors descended on Anza-Borrego to witness the phenomenon. RVs, Porta-Potties, and fashion models with their entourages lined the main roads of the park. By the third weekend of March, the restaurants in Borrego Springs had run out of beer and ice, and the visitors’ center was charging admission for bathrooms.

These violent blooms seem to lure in a greater number of hawks, too. With more flowers come more caterpillars—all the more reason for Swainson’s to make a detour to Anza-Borrego. The stats collected by the volunteer-run Borrego Valley Hawkwatch, which launched the first of its annual surveys in 2003, support the hypothesis: The top two years for Swainson’s Hawks, 2011 and 2017, line up with the last two super blooms.

“With the caliber geography and food source, Anza-Borrego and its surrounding lands are an absolutely critical stopover location,” says Peter Bloom, a zoologist who monitors the California Swainson’s Hawk population. In the spring of 1979 he tallied fewer than 300 hawks in the desert during a statewide survey to determine the species’ abundance and distribution. Last year the hawkwatch recorded 11,690. "I look at the stats and I'm just astonished,” Bloom says. “It's amazing how much they've gone up.”

Bloom thinks the migration bump in recent years echoes a rallying population. The Swainson’s Hawk was listed as threatened in California in 1983 after a report authored by Bloom revealed that only 375 breeding pairs survived in the state. And while the species is still well below historical levels of more than 17,000 breeding pairs, Bloom says he sees encouraging signs, such as hawks returning to abandoned nesting grounds in the Central Valley. “We're seeing more birds than we ever dreamed of four decades ago,” Bloom says.

Of course, to know how many hawks are out there, someone has to count them. 


nlike fickle flower chasers, super bloom or not, Hal Cohen returns to Anza-Borrego every spring. A retired marine biologist from Chicago’s South Side, 16 years ago Cohen helped found the Borrego Valley Hawkwatch, the only springtime hawkwatch on the West Coast. Now, he’s the local Swainson’s expert with a blog and a t-shirt that shows off the species’ three different morphs. Birders know to look for his Prius—license plate “HAWK GUY”—tearing down the backroads. Wherever Cohen is going, that’s where the flocks are.

At 8:00 a.m. and 5:30 p.m., from February through April, Cohen and his crew take their perch on a dune, eyes trained on the distant treeline and skies. The first half hour in the morning and evening can be slow, Cohen explains, as the birds skim the mesquite snags while mulling their plans. A decade and a half of study has taught him one of two things: The hawks will either land in the groves to catch some winks, or hightail it out of the valley using the winds wafting off of Coyote Canyon.

Getting a firm grasp on the overnighting flocks is no easy task. There’s far more guesswork and adrenaline involved at Borrego Valley than at your typical hawkwatch, due to the geography. Most surveys are set on a ridge, where raptors neatly stream by on their way out of the area. In Anza-Borrego, however, the birds could pop up from a number of sleeping or roosting spots. So, scouts armed with walkie-talkies spread out over miles of sand, ready to report back on the size and direction of flights. The birds are then tallied in the air—but only if they cross a specific junction in the park, marking their departure for the season. Even that isn’t simple; counting individuals against clear skies and granite backdrops has induced a few migraines. 

When the bloom is at its mid-March peak, plenty of birders are on hand to help with the count. Members of the Sea and Sage and San Diego Audubon chapters, tourists from Canada and Europe, and photographers in guerilla gear all mill about the hawkwatch sites, gasping at the low-hanging kettles. But only four or five stalwarts will stick around for the entire migration. British Columbia resident Rosemary Leong has been a disciple ever since she read a newspaper article about the annual count during a birding vacation at the Salton Sea. Yes, it’s tiring, she says; but even after 11 years, it never gets old. “This site, the feeling of the birds all coming together, it’s just special,” Leong says. The volunteers also keep tabs on other raptor species, including declining Golden Eagles, Peregrine Falcons, and Cohen’s favorite, Merlins. After pooling together each season’s stats, Cohen sends them to the Hawk Migration Association of North America.

Soon, there will be even more intel to share with birders, state officials, and anyone else who gives a caterpillar’s frass about Swainson’s Hawks. Backed by a private donation, Cohen is planning to buy radio transmitters to dispatch on four birds during the 2019 hawkwatch. He’s now working with the park and tracking experts at the Golden Gate Raptor Observatory to figure out how to temporarily trap the skittish birds to fasten the gadgets. The transmitters, which resemble small battery packs and have a lifespan of five years, will trace the hawks’ migration after they hit the desert. This will help Cohen and others find out not only where the birds ultimately breed, but also identify a string of refueling stations that connect to Anza-Borrego, and where the flocks head when caterpillar pickings there are slim.


uch like Swainson's Hawks, white-lined sphinx moths have been on the rise in Anza-Borrego, where records of caterpillar outbreaks date back to 1966. In their native range in Arizona, the caterpillars emerge to chomp on the burst of vegetation that follows the summer monsoons; in the Anza-Borrego desert, the insect’s reproductive cycle has shifted to coincide with the eruption of flowers after winter rains. “They can be so successful [in breeding], they eat themselves out of home,” says David Wagner, a lepidopterist at the University of Connecticut. 

While Francois, the University of Arizona entomologist, thinks the hawks are keeping Anza-Borrego’s caterpillars in check, Wagner is skeptical that the birds are making much more than a small dent in the population. “At this point they’re far exceeding all their enemies,” he notes. Still, he stresses, the insects aren’t immune to hardships such as drought; they will move north in search of food if conditions become too bleak.

This spring offered a glimpse of what the desert looks like in drought. Compared to the deluge in 2017, Borrego Springs only got an inch or two of rains last winter. There was barely a wildflower bloom, barely a moth emergence, barely a massive, swirling kettle to stir the hawkwatch crowd. By the time the count ended on April 1, volunteers had totaled 4,172 Swainson’s, hundreds short of the 16-year average of 4,737, Cohen says. It’s unclear how the birds knew not to stop by. Bloom posits that they might size up the poor flower cover from the air, or notice few other Swainson’s (when a bird sees a kettle, it knows that food might be nearby and is more likely to stick around). Those that did stay this year had to scrounge by on flying ants.

Feast and famine are the harsh reality of the desert. And conditions may get even harsher yet. Climate change, Cohen says, is already dragging the hawk migration earlier into the season. Hawkwatch volunteers used to see the birds through early April; now most move through in March. Meanwhile, Wagner says, wildflower blooms haven’t been shifting. That’s not a problem for the caterpillars: Females lay eggs when heat and moisture dictate—the same cues that trigger flower blooms. But for the wayfaring hawks, their shifting migration window could end in a narrow miss,  given that it tracks pampas temperatures, not Anza-Borrego rains. “They will almost certainly go elsewhere or be reduced,” Bloom says.

It’s not just a matter of uncertain food sources for hawks. “Observations show that it’s getting dryer,” Anza-Borrego Desert State Park ranger Steve Bier, who died last year and to whom the 2018 hawkwatch was dedicated, told Audubon last spring. “We have trees falling down from the lack of irrigation, and hawks sitting on the ground, open to coyotes.” 

These added pressures of extended drought could tip the balances for the hawk and hawk moth’s relationship. “The [evolutionary] stage is set, but the players are trying to figure this out,” Wagner says. “How many more caterpillars? How many more hawks? It feels like we’re watching something unstable.” Even if the moths moved north, though, Cohen thinks the hawks would find a way to carry on; they’ve already carved out a niche in Borrego Springs. He’s counted dozens of the birds napping at the country club where he plays pickleball; he’s spotted them picking up barbecued snakes around controlled burns on farms; and he’s found them hovering in the hot air above the town’s solar plant. If the caterpillars are no longer on the menu, the Swainson’s worldly palate may allow it to pick up a different diet during migration. “They’re probably one of the most adaptive hawks,” Cohen says. “They’re surviving quite well.”

In the desert, synergy is both a weakness and a strength. But after 12 years in Anza-Borrego, ranger Bier understood that the hawks had to take that risk to survive. On a March evening, he padded along the dunes, following the roller coaster tracks left behind by young sphinx. The ruts stopped abruptly at the edge of the trees—a sure sign of a caterpillar massacre, Bier said. He’d come closer to the hawks than anyone else during the super bloom.

But it was too close for the birds. Just as he crept up on the roosting trees, about 12 feet away from the birds, they fled—500 silhouettes against the slipping sun. Then, across the potato fields from an opposing grove, a new kettle rose. It churned, once, twice, three times, before the desert absorbed it again.


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