We’ve been spending a lot of time in the company of Swainson’s hawks lately, at an undisclosed location in San Joaquin County. They seem to be everywhere, even in semi-suburban areas. It’s been an education. I didn’t know Swainson’s hawks hovered; they’re quite good at it, even in a stiff wind. I’d never heard their scream, much like a redtail’s but somewhat thinner and more wavering. And I’d never seen a pair mating or building a nest. All this has been happening just around the corner, within an easy walk of where we’re visiting.
Swainson’s hawks, like many other buteos (including red-tailed, rough-legged, ferruginous, broad-winged, and short-tailed hawks), come in multiple plumage morphs. “Morph” is the preferred term, not “phase,” because it’s permanent.The extremes are the light morph (brown bib, white belly) and the dark morph (all dark below), with rufous and intermediate variants. Proportions of different morphs in a population vary geographically, with dark birds predominant in the Central Valley and light birds on the Great Plains. Mixed pairs are not unusual.
We were out for a constitutional last month and watched three Swainson’s hawks—two dark, one light—interacting in what you could probably call a walnut savanna. The two dark birds were acting like a pair, calling back and forth, eventually touching down on a gnarly old walnut and mating. So I thought I had this relationship figured out: the dark hawks are on territory, the light bird is an interloper. But then we saw the larger dark hawk, the presumed female (female buteos being larger than males in most species) flying in tandem with the smaller light hawk and briefly locking talons with him. Sure looked like courtship.
I consulted the trusty Cornell Birds of North America site, which is really worth the cost of the annual subscription, and found a reference to Swainson’s hawks in northeastern California forming stable polyandrous trios. A female and two (related? unclear) males defend a territory, build a nest, and jointly care for the young, sometimes for several years in succession. Was this what I was watching? The following morning, though, I caught the two dark hawks carrying sticks into an isolated live oak, where they had already constructed a sizable nest, deep and bulky, near the crown. No sign of the light male; might have just been passing through, looking for an extra-pair copulation. As it turns out, that was the last time we saw him.
Polyandry, it seems, is not that unusual in buteos and related hawks. It’s more or less standard for the Galapagos hawk, which genetic studies indicate is the Swainson’s closest relative. (The i’o or Hawai’ian hawk is also near kin. Swainson’s is typically a long-distance migrant, with most of the population traveling from the North American plains to the Argentine pampas every year. You can see how accidental colonization of remote islands might happen.) Polyandrous mating groups also occur in the more distantly related Harris’s hawk. The advantage? Male raptors often provide prey for their incubating mates and nestlings. A female with two male providers would have a better chance of successfully fledging her brood.
Central Valley Swainson’s hawks migrate, but not to Argentina. Migrants travel only as far as western Mexico and Central America; a few spend the winter in the Valley or Delta. This, along with the different morph proportions, led to speculation that they might be genetically distinct from the Great Plains population. And they are, kind of, according to recently published research by Joshua Hull and colleagues at UC Davis–but not genetically different enough to meet the criteria for federal protection as a Distinct Population Segment. There’s apparently enough gene flow between Valley and Great Basin hawks to keep the populations from diverging too far. That’s too bad, given the Valley hawks’ widespread loss of habitat and precipitous (90 percent in the last century) decline in numbers.