Swainson’s Hawk

Every year birds migrate all across the globe. They log millions of miles collectively, perhaps billions. The longest migration of all belongs to the relatively diminutive Arctic tern, which travels almost 56,000 round-trip miles every year! Unfortunately you’d be hard pressed to witness an Arctic tern in Prescott. Luckily though, our corner of the world hosts no shortage of marathon aviators. One such species is the handsome Swainson’s hawk.

Every spring Swainson’s hawks begin arriving around late April. As they funnel in they’ve just completed a 12,500-mile journey that originates deep within Argentina, taking them just shy of three months to complete. Once they arrive in northern Mexico they fan out across the entire western half of the continent.

Swainson’s prefer open grassland with scattered perches to build nests on. When they breed adjacent to human activity they often find trees that border on agricultural fields. This generally provides ample resources during their most vulnerable period. In Arizona I always picture their ideal habitat as the Chihuahuan desert grasslands typical of the southeastern corner of our state. They like to perch on the tall yuccas and sotols that dot the vast sea of grass. Whenever I find myself in the Las Cienega Grasslands near Sonoita I have Swainson’s on my mind, and have viewed them fulfilling their romantic image in my head on several occasions.

During breeding season Swainson’s tend to stick to small mammals and reptiles. By focusing on that prey base they can deliver large portions at once to the hungry mouths waiting in the nest. Similar to the common black hawk I wrote about in our August edition, they have a varied diet that includes a wide variety of small creatures, generally up the size of a rabbit. Where they really distinguish themselves, though, is in their heavy reliance on insects, specifically large grasshoppers. While other raptors will dine on insects as well, Swainson’s sustain themselves almost entirely on insects when they are not actively breeding. They like to “hunt” grasshoppers in a very un-raptorlike fashion; they will simply walk along the ground and pluck up as many grasshoppers as they can get their talons around. This is a simple and efficient way to take in enormous numbers of calories while expending relatively few.

As breeding season comes to a close and Swainson’s prepare for the long journey back to the pampas of Argentina, they gorge themselves on abundant September grasshoppers. It’s not uncommon to see them in groups numbering in the hundreds, walking their way through farm fields in search of fat, protein-packed snacks. I have even heard reports of them following closely behind combines busily harvesting a crop. In the process the machines also kill and portion out the grasshoppers into smaller bites! The hawks have figured this out; why make it harder on yourself?

Unfortunately this heavy reliance on grasshoppers has at times put the species in peril. By the 1990s US Forest Service biologists had been tracking summering populations in northern California for decades. One year there was a sudden dramatic decline in the number of adult birds returning from the agriculturally rich La Pampas region. On investigation they discovered dead Swainson’s numbering in the thousands, and were able to conclude that a pesticide called monocrotophos was to blame. While it had been banned in the US, it remained in use in South America. Farmers were using it to control grasshopper populations in sunflower and alfalfa crops, and it was having a devastating effect on the Swainson’s population. Fortunately a coordinated effort by the farmers, multiple world governments, and even the company producing it led to the ban on the pesticide in Argentina. Coupled with a massive public outreach campaign that reduced reported mortality due to the pesticide from over 35,000 individuals in one year to just 24 a couple of years later. To this day Swainson’s maintain a large, thriving population.

During migration Swainson’s tend to fast, having stored up enough fat-energy to carry them home, but they have to travel in a highly efficient manner. While all raptors use thermal columns to some degree, Swainson’s rely more heavily on them. This reduces their energy output because they can be lifted on the rising columns of hot air and glide for hundred of miles before stopping to rest. They migrate in kettles of as many as 10,000 individuals, and it’s common to see groups in the hundreds right in our area.

Look for these slender, midsized raptors as you travel through Prescott or Chino Valley over the coming month. If you’re lucky you’ll see one atop a fencepost, plucking apart a grasshopper.

‍Be sure to check this column at 5ensesmag.com for past articles and updates on our 2023 Bird Challenge.