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鈥檚 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鈥檚 Hawks. They take to the skies with the last wisps of warm air鈥攂irds 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鈥檚 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鈥檚 800,000 Swainson鈥檚 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鈥檒l need to survive the last leg of their marathon migration and ensure they鈥檝e 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鈥攍ured by a local dish many predators can鈥檛 stomach.

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鈥檚 Hawks. The raptors don鈥檛 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 and director of 探花精选鈥檚 , says researchers have counted up to 30 insects in the hawks鈥 pellets. 鈥淚t鈥檚 a full-on binge,鈥 she says.

That is, when the caterpillars flourish. Scientists and birders drawn to the park鈥檚 springtime bounty have linked the hawk鈥檚 abundance to that of the caterpillar鈥攚hich in turn is influenced by wildflower booms and busts. But no one鈥檚 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. 

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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 risqu茅 bouquets.

In 2017, after five years of drought, seven inches of precipitation provoked the region鈥檚 biggest 鈥溾 since 1999. A record number of visitors descended on Anza-Borrego to witness the phenomenon. RVs, Porta-Potties, and  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鈥 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鈥攁ll the more reason for Swainson鈥檚 to make a detour to Anza-Borrego. The stats collected by the volunteer-run , which launched the first of its annual surveys in 2003, support the hypothesis: The top two years for Swainson鈥檚 Hawks, 2011 and 2017, line up with the last two super blooms.

鈥淲ith the caliber geography and food source, Anza-Borrego and its surrounding lands are an absolutely critical stopover location,鈥 says Peter Bloom, a zoologist who monitors the California Swainson鈥檚 Hawk population. In the spring of 1979 he tallied fewer than 300 hawks in the desert during a statewide survey to determine the species鈥 abundance and distribution. Last year the hawkwatch recorded 11,690. "I look at the stats and I'm just astonished,鈥 Bloom says. 鈥淚t's amazing how much they've gone up.鈥

Bloom thinks the migration bump in recent years echoes a rallying population. The Swainson鈥檚 Hawk was in California in 1983 after a 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. 鈥淲e're seeing more birds than we ever dreamed of four decades ago,鈥 Bloom says.

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

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nlike fickle flower chasers, super bloom or not, Hal Cohen returns to Anza-Borrego every spring. A retired marine biologist from Chicago鈥檚 South Side, 16 years ago Cohen helped found the Borrego Valley Hawkwatch, the only springtime hawkwatch on the West Coast. Now, he鈥檚 the local Swainson鈥檚 expert with a  and a t-shirt that shows off the species鈥 three different morphs. Birders know to look for his Prius鈥攍icense plate 鈥淗AWK GUY鈥濃攖earing down the backroads. Wherever Cohen is going, that鈥檚 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鈥檚 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鈥攂ut only if they cross a specific junction in the park, marking their departure for the season. Even that isn鈥檛 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 and 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鈥檚 tiring, she says; but even after 11 years, it never gets old. 鈥淭his site, the feeling of the birds all coming together, it鈥檚 just special,鈥 Leong says. The volunteers also keep tabs on other raptor species, including declining Golden Eagles, Peregrine Falcons, and Cohen鈥檚 favorite, Merlins. After pooling together each season鈥檚 stats, Cohen sends them to the .

Soon, there will be even more intel to share with birders, state officials, and anyone else who gives a caterpillar鈥檚 frass about Swainson鈥檚 Hawks. Backed by a private donation, Cohen is planning to buy radio transmitters to dispatch on four birds during the 2019 hawkwatch. He鈥檚 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鈥 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.

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uch like Swainson's Hawks, white-lined sphinx moths have been on the rise in Anza-Borrego, where records of caterpillar outbreaks . 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鈥檚 reproductive cycle has shifted to coincide with the eruption of flowers after winter rains. 鈥淭hey can be so successful [in breeding], they eat themselves out of home,鈥 says David Wagner, a lepidopterist at the University of Connecticut. 

While Francois, the University of Arizona entomologist, thinks the hawks are keeping Anza-Borrego鈥檚 caterpillars in check, Wagner is skeptical that the birds are making much more than a small dent in the population. 鈥淎t this point they鈥檙e far exceeding all their enemies,鈥 he notes. Still, he stresses, the insects aren鈥檛 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鈥檚, hundreds short of the 16-year average of 4,737, Cohen says. It鈥檚 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鈥檚 (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鈥檛 been shifting. That鈥檚 not a problem for the caterpillars: Females lay eggs when heat and moisture dictate鈥攖he 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. 鈥淭hey will almost certainly go elsewhere or be reduced,鈥 Bloom says.

It鈥檚 not just a matter of uncertain food sources for hawks. 鈥淥bservations show that it鈥檚 getting dryer,鈥 Anza-Borrego Desert State Park ranger Steve Bier, who died last year and to whom the 2018 hawkwatch was dedicated, told 探花精选 last spring. 鈥淲e have trees falling down from the lack of irrigation, and hawks sitting on the ground, open to coyotes.鈥 

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

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鈥攁 sure sign of a caterpillar massacre, Bier said. He鈥檇 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鈥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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