Thursday, March 26, 2015

Swainson's Hawks in Borrego Springs

Flock of Swainson's Hawks over Borrego Springs
Swainson's Hawks over Borrego Springs. This photo represents nearly the total
number of Swainson's Hawks I've previously seen in over 40 years of birding!
March 22, 2015. Borrego Springs, California. Greg Gillson.
Although a fairly common breeder early in the 20th Century, Swainson's Hawks no longer breed in southern California. Breeding widely in open country across western North America--even to Alaska, they form large flocks as they migrate to the pampas of Argentina for the winter.

In 2003 Hal Cohen moved to Borrego Springs from the Midwest, where he enjoyed many hawk watches along the shores of the Great Lakes. The West isn't famous for hawk watches like the East and Midwest, but Hal started counting migrating Turkey Vultures in his new home in late March. On April 11th he discovered about 1000 Swainson's Hawks coming into the Borrego Valley to roost at dusk. They fed on caterpillars in the morning, then departed as the thermals formed. It turns out this this is an annual event not previously discovered.

Borrego Valley Hawkwatch site tracks the comings and going of these birds. The official counts start February 15. Peak numbers are reached in mid to late March. The season ends on April 15. This website is great, because the blog tells you if, and how many, new birds entered the Valley that evening and where the roost is, and what strategy is best for viewing the next day based on wind forecasts. Over 9000 Swainson's Hawks were counted during the 2014 season.

Why are they here?

sphynx moth caterpillar

Yep. That's a caterpillar... and a big one, over 3 inches long. It is the caterpillar for the Sphynx Moth. There are literally millions of these juicy grubs crawling everywhere across the desert (see the tracks?) [not to mention the thousands squished on the roads!]. Why? Queue next photo.

desert flowers

In February there were rains on the desert and those seeds that laid dormant for months, or years, came to life. The moths laid eggs, and the caterpillars hatched and were hungry. The Swainson's Hawks took advantage. And the birders followed! (See this recent post on the Borrego Valley Hawkwatch site.)

desert flowers

I made a quick visit last weekend to Borrego Springs to locate some desert birds for my county year list. I was most interested in finding Crissal Thrasher in the mesquite there at dawn. I knew I could watch birds at the waste treatment plant and the Mesquite Bosque and be done mid-morning and still have time to possibly see Swainson's Hawks. They had been reported roosting in the evening near the landfill.

After birding the desert scrub I headed over to where the Swainson's Hawks were forecasted to appear. I noted cars parked on the side of Peg Leg Road and people in clumps looking through scopes and binoculars--birders!

Hawk watchers at Borrego Springs

There were still a few hawks in the field here, but most had lifted off--almost 150 were in the air, along with ravens and a few vultures. That's 3x the number of all the previous Swainson's Hawks I've ever seen--in over 40 years of birding! I snapped the photo that introduces this post.

The Swainson's Hawks will still be around for a couple more weeks. If you get the chance you should really check it out! I think I'll make plans for next year and take Marlene for a long weekend. Maybe I can get some closer photos of the hawks while they are still perched in their roosting trees or feeding out in the desert.

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