Irruption years are a good news, bad news, good news story. The first good news is masting in the northern boreal forests. Masting is when a tree species food source has an abundant+ crop. For example, conifers. This allows the bird species that rely on that food source to have a great breeding season – start early and have successful broods. For example, finches.
The bad news, and it’s not really that bad, is the numbers of birds surge, sometimes double or triple, but the available habitat in their normal range does not.
The response is the next good news. At least for birders that live further south. Young birds, in particular, are pushed further, and further, south.
Pine Siskins and Purple Finches are not uncommon in our yard in the winter. But this year has been exceptional and a real bonus for us sheltering-in-place birders.
There is one more bad news scenario. Masting years can be followed by sharp declines in seed production. Birds migrate further south for a different reason than in masting years. They have to search further and further south for food sources crowding in on normal winter residents.
Irruption years rotate in these boom or bust cycles. These cycles, like a lot of natural cycles, are being amplified by erratic, but trending warmer winters and hotter summers. While irruption years are fun for us in the south and we get to enjoy more variety of birds, there can be too much of a good thing if it happens more frequently than can be adapted by all species across the ecosystem.