The mystery of avian irruptions

 When birds go roaming: The mystery of avian irruptions

Archer Larned and Sarah Luttrell

Red-breasted Nuthatches are irrupting this winter across North America. Heather Elaine Ritchie/Flickr, CC BY-NC-ND

 In North America when the leaves start to fall and the days get shorter, many birds head south for warmer weather. Only a small number of species stick it out for the winter. If you’re out in the woods in a typical February, it’s a pretty quiet place.

But every once in a while, something unexpected happens. Hundreds or thousands of birds of a single species leave Canada’s boreal forests and move far south of their typical ranges in a phenomenon called an irruption. These irregular movements are big events for birders and wildlife biologists.