During spring migration this year my friend Steve Bodio commented on the number of "eastern" birds we were finding in this corner of Montana. I promised I would return to the subject in a future post. I figured it was finally time for the post on Steve's comment now that those same birds are moving back through our area in the other direction!
The species Steve mentioned were the Rose-breasted Grosbeak and Broad-winged Hawk but there are many more "eastern" species that move through here. To illustrate this discussion I would like to first mention Westby, MT, a small town as far northeast as you can get in MT without venturing into Canada or North Dakota. My friend Ted Nordhagen has lived and birded here since he was little and it has become a hotspot for Montana birders. Here is a list of species found in Westby this fall (spring is similar):
Black and White Warbler
Cape May Warbler
Black-throated Green Warbler
Many of the birds on this list may seem like "eastern" birds to us living in the United States but in reality they are boreal birds that breed as far west as Alaska (see figure 1 below).
Figure 1. Boreal Forests of North America (found here)
The reason we consider them eastern birds is because most of these species migrate through the eastern portion of the United States to reach the boreal forests, even the forests in Alaska.
So why are these birds being observed in Montana?
There are known migration routes in North America known as flyways. These are pathways that many birds migrate along when returning from or heading to their breeding areas in North America. Figure 2 below illustrates those routes.
Figure 2. Flyway routes in North America (from here).
In particular, I would like to discuss the Mississippi and Central Flyways. The maps below illustrate each of these flyways individually.
Figure 3. Central Flyway (from here)
Figure 4. Mississippi Flyway (from here).
Notice that the eastern portion of the Mississippi Flyway just cuts across the extreme northeastern corner of Montana. The main portion of the Central Flyway also comes through far eastern Montana. The boreal breeding birds that migrate up the Mississippi Flyway cut across northeastern Montana on their way to and from the far western reaches of the North American boreal forests. These are the same birds that migrate across the Gulf of Mexico to and from South America. Westby, Montana is right in the path of this river of birds and Fort Peck, just 125 miles to the southwest, lies in the backwater eddies of the river. We still find many of these species, but no where as regular or in the numbers found in Westby. Although this line appears to be curved in reality it is nearly a straight line between the northern part of the Yucatan Peninsula and the western reach of the boreal forest in Alaska (Figure 5).
There are still a few boreal species that we do not find in either place with as much regularity as their breeding distribution would suggest. These include the Swamp Sparrow, Connecticut Warbler, Eastern Phoebe, and Great Crested Flycatcher.
Figure 5. Straight line between the Yucatan Peninsula and the boreal forests of Alaska.
A side note on bird migration in Montana concerns the Pacific Flyway (figure 6). Note the branch of this route that comes into Western Montana along the Rocky Mountain Front. The birds utilizing this route, most notably the Snow Goose, winter in the Central Valley of California and cross the Great Basin in Nevada to continue north over the mountain of southwestern Montana and then along the Rocky Mountain front through Canada and into central Alaska. This route is repeated in the fall when these birds return south. One of the primary stopping points for the Snow Geese is Freezout Lake near Choteau, Montana where thousands of birds can be found during migration.
Figure 6. Pacific Flyway (found here)