Thrush On the Move

By Terry Rich, Golden Eagle Guest Blogger

The thrush family (Turdidae) is one of the most successful on earth, with 192 species found all over the place. I’m always impressed when I visit a country on a birding trip to see some species of thrush behaving very much like the thrushes in Idaho. Once you see that distinctive size and behavior, or hear the melodic song, you know you have a thrush.

Robins and bluebirds get a lot of attention. But I’m writing today about two thrushes that spend the summers in shady Idaho forests where they keep low profiles. These are Swainson’s Thrush and Hermit Thrush.

Different Travel Plans

Although these two species look similar, their ecologies are somewhat different. Both are a subdued brownish, with subtle spotting on the breast, and eyerings that differ only in the most minor details. Hermit Thrushes have a more contrasty reddish tail, which birders will detect but which may escape more casual observers.

Their songs are also similar – lovely fluting melodies coming from deep within the woods. But there is a difference in their songs that is easy to detect. Swainson’s Thrushes always sing a song that goes up the scale from the first note. Hermit Thrushes, on the other hand, start with a long, single note. The following notes may then go up the scale, down the scale, or stay at about the same pitch.

Although their appearance, breeding habitat, and vocalizations are similar, the way they spend the winter is very different. The majority of Swainson’s Thrushes leave Idaho in the fall and don’t even slow down for winter until they hit the mountains of central Mexico, and many individuals continue on down the Andes all the way to northern Argentina.

Hermit Thrushes, on the other hand, winter all over the southern U.S., and along the Pacific Coast, from southern Baja to north of Vancouver, BC.

Above left, Swainson’s Thrush by Ceredig Roberts. Right, Hermit Thrush by Jeff Black

Migration Mystery

Southwestern Idaho is not shown as winter range for the Hermit Thrush on the eBird map. Tell that to the birds. You can now find this thrush on any winter day in Kathryn Albertson Park, Hulls Gulch, and Eagle Island State Park, to name a few locations. I think we have more hermit thrushes now during the winter than we used to. But this is just my impression. What do the numbers say? 

Community Data Reveals a Trend

My first stop is the Christmas Bird Count (CBC). Turns out, there are not enough data for Idaho or even for North America to give a report on CBC trends. This is really surprising, given the extensive winter range of the species in the U.S.

I then checked the Survival by Degrees website, also from Audubon, which has predictive models for various species under different climate warming scenarios. Hermit Thrush was not modeled. So, there aren’t even hypotheses on whether it might be increasing or decreasing in winter or summer. Ugh.

Hermit Thrush by Terry Rich

My next stop is the eBird website, to see where Hermit Thrushes have been reported in each winter over the years. The problem with that information is there are no built-in analytical tools. You have to just look at checklist reports and make an assessment. So, let’s do that.

From December 2023 through February 2024, 30 – 40 Hermit Thrushes were found in and around Boise. It’s not possible to totally rule out multiple reports of the same birds. But by looking at detections from separate locations, we can minimize double counting.

Ten years earlier, from December 2013 through February 2014, there were 7 Hermit Thrushes reported. Over those same dates in 2003-2004 and 1993-1994, there were no reports of this species in the same geographic area.

Why the Change?

The most logical explanation in my mind is climate change. We just had the hottest year in the recent history of the earth. Other species – Anna’s Hummingbird, Bewick’s Wren, and Lesser Goldfinch, for example – are taking advantage of milder winters to hang around. Migration is full of dangers – unknown food sources, unpredictable predators, and things to collide with. Why migrate farther than you have to? It makes a lot of sense. 

One other factor to consider is the population trend of the species. Maybe there are simply more Hermit Thrushes in Idaho now than in the old days, so more overwintering birds are just an inevitable consequence.  

Breeding Bird Survey data show a decline in Hermit Thrushes in Idaho over the period 1966 – 2022 of -2.43% per year. That’s pretty steep. But we also need to look at trends in British Columbia and Alberta, the provinces north of us that may be sending us thrushes for the winter. The trend in British Columbia is flat, and the Alberta trend is -1.82% per year. So, the notion that there are simply more thrushes around isn’t supported.

What about Swainson’s Thrush? We need to keep our eyes open. One bird was reported from Provo, Utah, on December 24, 2017, and one from Willard, Utah, on January 20, 2020. I already mentioned what a serious long-distance migrant this thrush is. But birds are flexible and adaptable. We shouldn’t assume all our birds are hermits. Get a good look. The treasure hunt continues.

You can reach Terry at terryrichbrd@gmail.com