The Case of the Missing Rough-legged Hawks

Most of the time, I am prompted to write about birds and other wildlife that I have encountered recently while out and about exploring.  Today, I am writing about a bird precisely because I haven't seen it recently, or at all this entire 2023-24 fall-winter season.

Rough-legged Hawks are special birds in California's Great Central Valley, and certainly one of my favorite raptors.  For one, they are simply gorgeous hawks.  But they're more than a pretty face: They are incredible migrants and amazingly hardy, nesting on cliffs and rocky outcroppings in remote tundra, boreal forest and alpine regions of the Arctic, where they spend the short summer breeding season feeding on lemmings and voles.    

But every winter, the world's entire breeding population of Rough-legged Hawks leaves the Arctic behind to migrate south, where they spend the colder months feeding on the rodents of open habitats across much of the U.S., including prairies, fields, shrublands and semi-desert regions.  

Rough-legged Hawk range in North America
allaboutbirds.org

And every winter, these birds bring a bit of Arctic mystique with them to their wintering grounds on the open grasslands along the eastern edge of the San Joaquin Valley.  

Or at least I thought it was every winter.

For the past four years, I've been able to find these incredible birds reliably in foothill grasslands not too far from my home, and have seen as many as three individuals wintering at one time in one specific valley.  The little valley they prefer is dotted with three or four widely spaced blue oaks and fringed with a smattering of the little rocky outcroppings Rough-leggeds love.  The hawks could be found almost without fail perched on outcroppings of rock or atop the oaks, surveying the meandering creek and valley below for rodent prey.  

This past November, I went out looking for "my" Rough-legged Hawks in their usual spot with no luck.  November is the month when they routinely show up here, but maybe, I thought, this year they were late.  I tried again in December: Still no hawks.  By the time I got out to the spot again in late January and found it sadly devoid of my favorite hawks, I was truly puzzled.  

Where are the Rough-legged Hawks this winter?  

Clearly, they had abandoned their favorite little patch of wintering habitat, and I began to muse over some possible explanations.

1. They could have found another more favorable patch of habitat, still in the area, perhaps, but behind the hills and out of sight of my binoculars.  The literature indicates that some overwintering Rough-legged Hawks continue to move around throughout the winter, so that could be the case.  (But it was strange that I hadn't seen any soaring or flying over either, and in previous years these particular birds have seemed incredibly sedentary during the winter.  They just didn't seem to be in the area at all.) 

2. Birds can show amazing site fidelity, the same individual bird returning year after year to precisely the same patch of habitat.  Maybe the birds of this little valley had died over the breeding season (or during their long migration) and new individuals had yet to find this patch of habitat.  (But that also didn't seem right, since it seemed unlikely that two or three birds would all go missing in the same year.  I would have expected at least one to turn up somewhere nearby.)

3.  Maybe it's just a "bad year" for Rough-legged Hawks.  (Is that even a thing?)  

This theory prompted another question: Have Rough-legged Hawks been seen anywhere in Central California during the winter of 2023-24?

Thanks to the incredible resource that is eBird, that question at least could be answered easily!  A quick search of Rough-legged Hawk reports in the area showed that there have been very few sightings this winter in Central California, and none at all as far south as the San Joaquin Valley.  (Okay, good: It's not just me!)

eBird map showing Rough-legged Hawk reports from this year, the winter of 2023-2024 (above) and last year, the winter  of 2022-2023 (below). 
Each pin represents a "hotspot" where at least one Rough-legged Hawk has been reported.  (Red pins are reports from within the last month.)  Note the lack of pins in and around the San Joaquin Valley (south of Sacramento) in the first map, compared with the number of pins in the same region on the second map.  These show spots where Rough-legged Hawks were seen last winter, and where they are absent this winter.

For comparison: This map shows all eBird records of Rough-legged Hawks in the region over the last century.  Note the density of sightings in a northwest-southeast trending strip along the eastern edge of the San Joaquin Valley; this strip corresponds with the foothill grasslands region, where I most often see Rough-legged Hawks.

So that answered that question.  There are essentially no Rough-legged Hawks in the San Joaquin Valley and surrounding foothills this winter.  But why not?

Time to do a little more research.

Audubon notes that "numbers [of Rough-legged Hawks] appearing south of Canada are quite variable from one winter to the next."  

HawkWatch International states that "during years of prey abundance, high numbers [of Rough-legged Hawks] are noted at subsequent fall and spring hawk watches," indicating that a larger number of individuals make the journey south to winter in the States after summers with greater prey availability and, presumably, greater breeding success.  Conversely, summers of low prey abundance result in lower breeding success and fewer migrating and overwintering hawks.

Cornell's Birds of the World indicates that "The number of breeding pairs and their reproductive performance fluctuate considerably with changes in prey abundance...  Numbers [of Rough-legged Hawks] seen on migration varies considerably among years...  Annual variation in numbers [is] attributed to species' reliance on cyclic lemming and vole populations for prey."

Simply put, it seems certain that the number of Rough-legged Hawks varies widely from year to year, and this fluctuation is based on prey availability.

So it could be that the population of Rough-legged Hawks that winters in Central California had a poor summer in terms of prey availability: a bad lemming year.  

Or could it be that they found a food source farther north and didn't need to come so far south?  How far south in California have Rough-legged Hawks come this winter anyway?

Back to eBird to find out.

These maps of Rough-legged Hawk reports in northern California show fewer sighting in the winter of 2023-24 (above) than the previous year (below), but the difference is not as striking as it is farther south.

It doesn't seem to be the case that the Rough-legged Hawks all decided to stop farther north this year.  There just seems to be fewer individual birds in California overall this winter.  And it sounds like that might have something to do with populations of Arctic lemmings and voles.

Googling "How was last summer's Arctic lemming population?," "Current Arctic lemming population," and "Lemming population summer 2023" didn't turn up too many promising results, however.

Maybe, from my limited perspective, its best to let the birds tell me something about the lemmings.  Fewer birds this winter must mean fewer lemmings last summer.  Snowy Owls, another even more iconic Arctic breeding bird, are also well known for experiencing population fluctuations that are linked to the abundance of their prey, with also consists largely of lemmings.

Cornell's Birds of the World continues to note that "peak flights [of Rough-legged Hawks] in Ontario coincided with peaks in Snowy Owl numbers, another arctic-nesting raptor that preys on lemmings and voles, but peaks in populations elsewhere have not correlated with Snowy Owl incursions.  This is not surprising given that fluctuations in lemming and vole numbers are often asynchronous among species and local populations of the same species.  Therefore, source populations sampled on migration may include birds from several areas with varying prey abundance." 

In other words, different populations of Rough-legged Hawks may experience very different rates of breeding success across the wide range of this species, based on the fluctuations of local prey populations.

I would dearly love to know exactly where in the Arctic "my" Rough-legged Hawks breed, and if it is true that their local lemming population was lower last summer.  

And another question: Do all of the Rough-legged Hawks that overwinter in California migrate from the same general region in the Arctic?  If so, was the lemming population in that entire region poor last summer?  

Lemming populations, which are well-known to fluctuate widely in cycles of boom and bust years, are probably far more complex than we know, and have causes and effects that are still beyond our understanding.  But it seems clear that there is a correlation between prey availability during the breeding season and numbers of Rough-legged Hawks that migrate south to overwinter in the Lower 48.  

And  the other thing that is clear is... more research is still needed on this topic!

For now, I still have more questions than answers.  But I would love to learn more!  

All I can say for certain is that it really does appear to be a "bad year" for Rough-legged Hawks in central California.  They just aren't around this winter, and the grasslands are certainly missing something without them.  

If you live near overwintering populations of Rough-legged Hawks, I would love to know: How are they this year?  Do the numbers seem high, low or average?  

Please share in the comments below!