Cassin’s Finch – A Typical Migrant?

by GEAS Guest Blogger Terry Rich

We think of the Cassin’s Finch as the altitudinal replacement for the House Finch in this part of the world. When we’re in the valleys, we see the House Finch. When we travel up into the mountains for camping and other summertime recreation, we see the Cassin’s Finch.

I’m always a bit surprised when I see a Cassin’s Finch among our spring migrants in Boise, but I shouldn’t be. According to the Bar Chart option in eBird for Ada County, the species makes a pronounced, but brief, appearance from April 22 to May 15, with a peak on May 8 (see graph). Occurrence in Ada County plummets for the breeding season as Cassin’s Finches spend the summer in higher elevation mixed conifer forests. Of course, without further research, we can’t be sure the Cassin’s Finches we see in Boise in May are the same ones we see in Stanley in July.

Cassin’s Finch by Terry rich

Graph – eBird Bar Chart frequency data for Cassin’s Finch for Ada Co., Idaho, over all years.

Altitudinal Migration or More?

We might be tempted to say the Cassin’s Finch is simply an altitudinal migrant, but it’s pretty obvious from our experience and the eBird data that they’re not in Ada County in the winter in any numbers. They are quite unlike our most common altitudinal migrants – the Dark-Eyed Junco and White-crowned Sparrow – who are found around the valley and in our yards all winter.

Looking at the range map for the Cassin’s Finch, we see that all of Idaho is classified as “Year-round” habitat. But we also see a substantial winter range from the mid Colorado Plateau deep into central Mexico. This suggests that our springtime Cassin’s Finches might be latitudinal migrants like the Western Tanager. Maybe their stopover in Ada County is one of many fueling stops in a long migration from Mexico. They blow through here pretty quickly heading not only to higher elevations but to more northerly locations as well. The fact that we see a second peak in observations in Ada County between September 15 and October 1 reinforces the latitudinal and altitudinal migration idea. The Cassin’s Finch appears to have a migration pattern as good as any tanager or bunting.

Cassin’s Finch Range Map

Range maps are indispensable to birders. But they also do not convey all the spatial information we need. Idaho is purple because there are some Cassin’s Finches around the state at any time of the year. But that does not tell us how many or whether or not the birds in Ada County in July are the same ones we see in January. We can get to the first question by examining eBird Bar Charts for all of Idaho’s counties. But we can’t get to the second question without marking individual birds and discovering where they go. My guess is that they mostly are not the same birds and that “our birds” are somewhere well south of Idaho in winter. Welcome back (?)

You can reach Terry at terryrichbrd@gmail.com