When thinking about animals that migrate, some the first ones that come to mind are birds. Many bird species migrate in a similar way that involves breeding in more northern areas, then flying south for the winter. There are some species however, that migrate in a different way, which can lead to something called an Irruption.
Irruptions are times when a particular bird species suddenly moves into an area in great numbers. Not every species can have these irruptive migrations however. In general birds that breed and reside in the boreal forests are most known for these mass movements. Some of the species include finches such as redpolls and crossbills, as well as more common backyard birds like blue jays and red-breasted nuthatches. You may remember years when these species were more plentiful In your neck of the woods or maybe they were even downright abundant. This is because of an irruptive year.
While these events can seem random, there actually is a method to the madness. Some irruptive species rely on crops of different seeds for food. If these plants have poor crops, the birds that rely on them have no choice but to move, causing an influx of birds in other parts of their range, or even places they don’t normally live. Since different species rely on different types of cones, its’ possible to have one or two species with irruptive movements, while others remain very scarce in the same area. A larger irruption can occur if food during the summer breeding months was plentiful, thus allowing birds to raise many chicks, but then scarce in fall and winter, meaning there is high demand for food, but a low supply. This causes a massive movement of birds. If the food supply is high, and the demand is low, then the birds stay put. However, there are some other factors at play with these movements, such as sever weather, especially snow and freezing rain. If the food is available but covered in a layer of ice, this can also cause birds to move
Obviously, birders and nature enthusiasts get excited about irruptive species, especially when they get to see them in their home state. Fortunately, there’s a way to try and figure out which species will be on the move ahead of time. The winter Finch Forecast is a report originally created by Ron Pittaway that looks at information from northern cone and fruit crops, and predicts which birds will be on the move during the fall and winter. Tyler Hoar picked up the mantle and now leads the charge in creating the annual winter Finch Forecast along with help from Matt Young and the Finch Research Network as well as many other volunteers all going out to survey both birds and cone crops.
The forecast is then posted, usually in early fall, and birders can start gearing up for what to expect, although weather can cause the forecast to change without much prior notice.
It’s worth noting that it’s not just finches and other song birds that can have irruptive migrations, but also many raptor species too. Raptor irruptions work In a very similar way but instead of fruit and cone crops being the driving force its typically small mammals such as lemmings. Among the more iconic irruptive raptor species are owls, especially the Snowy Owl, which can move into the lower 48 states in larger or smaller numbers during the winter based on the lemming population in the north, as well as how many owl chicks were raised during the breeding season.
Overall, irruptive bird species are very unique in the way they move and can add a lot of excitement to the fall and winter due to their variability. If you’re interested in learning more about these species, check out our videos about them and be on the lookout for the Winter Finch forecast. As always, thanks for reading, and be sure to check out the Badgerland Birding YouTube channel.
