Super swarm: Here's why hawks are amassing at Calif. state park

Flocks of Swainson hawks are blanketing the skies of Anza-Borrego Desert State Park in Southern California, a contrast to the colorful wildflower fields on the ground below.

Thousands of wildflower seekers have inundated the park for a glimpse of the spectacular super bloom display, but the hawks aren't in Anza-Borrego for the sights. They're seeking a snack, and juicy sphinx moth caterpillars do just the trick.

Named for their defensive posture, the white-lined sphinx moth caterpillars play a complicated role in the ecosystem. The caterpillars eat the wildflowers – sometimes hundreds of acres of 'em – but the moths pollinate them, destroying and sustaining their food source simultaneously.

The hawks then eat the caterpillars. And like the flowers, they've come to Anza-Borrego in droves this year.

It's eat or be eaten in Anza-Borrego State Park. This year, a record number of Swainson's hawks have descended upon the park as a stopover on their yearly winter migration. The hawks eat the white-lined sphinx moth caterpillars, which in turn munch the blooming wildflowers. less It's eat or be eaten in Anza-Borrego State Park. This year, a record number of Swainson's hawks have descended upon the park as a stopover on their yearly winter migration. The hawks eat the white-lined sphinx ... more Photo: Iris Kirkpatrick Photo: Iris Kirkpatrick Image 1 of / 40 Caption Close Super swarm: Here's why hawks are amassing at Calif. state park 1 / 40 Back to Gallery

Hal Cohen, retired biology professor and self-proclaimed father of the Borrego Valley Hawkwatch, wants to set the record straight about the raptors. He says the Swainson hawks are hungry with good reason, as they've stopped in Anza-Borrego on their annual 6,000-mile migration from South America to the plain states of the U.S.

Cohen's blog shows staggering numbers of the birds this year. March 15 saw 328 hawks. March 16, 788. March 18, 3,713 – the most hawks spotted in Anza-Borrego in one day ever. Hawkwatch, says Cohen, is just 73 hawk-sightings away from the highest recorded number of hawks in the region ever.

Each day, a fresh group of hawks touch down, only to take off by night for the next stop on their journey. The hawks come to Anza-Borrego with the promise of caterpillars, although Cohen reports the insects' numbers are lower than in past years. He chocks up the high volume of birds to rising Swainson hawk populations globally.

"Especially in South American countries, they're finding it costs less to not use a lot of chemicals in agricultural production," Cohen said, explaining that pesticides decimated the hawk population in the 1990s. "Across the board, Swainson hawks are starting to do pretty darn well."

If you're looking to see Anza-Borrego's feathered visitors, Cohen says you better plan a trip soon. The hawks came earlier this year, and they typically disappear by mid-April.

"We've been having monumental days," said Cohen, ensuring the sights will be worth the trip. "This is the most Swainson hawks you'll see anywhere north of Mexico right now."

Read Michelle Robertson’s latest stories and send her news tips at mrobertson@sfchronicle.com.