Why is it that some years you see unusual birds at your feeder, while in other years, only the regulars show up? It's often because of a change in regular migration patterns among some bird species. What causes these changes? The answer can be found in distant northern, or boreal, forests, where an important food is grown.
The boreal forests are a huge tract of trees that grow across Canada. These forests produce enormous amounts of cones, which support all sorts of birds, small mammals, and their predators. When cone production fluctuates, bird populations grow or shift. So, during years when cone production is high, you won't see many unusual birds at your feeder, since they're happily munching away in their woods. When the cone crop is poor, they head south in large numbers, and sometimes visit your feeder.
Pine Siskins, Common Redpolls, Red-Breasted Nuthatches, and other northern seed-eaters experience irruptions. But they're not the only ones. When the cone crop is poor, small-mammal populations decline, which sets raptors on the move. That's why you might see a Rough-legged Hawk, a Northern Goshawk, or perhaps a Saw-Whet sitting in a nearby tree. Even Great Grey Owls have been seen way south of their normal range.
