An irruption is the sudden change in the population density of an organism. In North American birds, irruptions often refer to the movement of northern-wintering species to the south in years of low food availability. You can recognize irruptive movement patterns at your feeders: some winters you may see a species at your feeders in great numbers, but in other winters they don’t show up at all.
Irruptive Species Gallery
Previous slideNext slide
Common RedpollThe Common Redpoll is a small, brown-streaked finch with a yellow bill and a red fore-crown. It breeds in the northernmost regions of Canada and Alaska. Erratic winter movements are generally thought to be associated with the food supply; when you notice lots of redpolls at your feeders, it is likely that their natural food supply in the boreal forests is running low that winter. Common and Hoary redpolls show a great diversity of plumage variation: some are rosy-hued all over, others have clean white breasts, others are heavily streaked with brown. One diagnostic feature, however, is their little black chin, which distinguishes them from other finches such as house and purple finches.
Photo © Yves Déry
Pine SiskinWhen seeing Pine Siskins for the first time, they can be hard to identify. Sometimes they mix with flocks of American Goldfinches, which are similar in size, shape, and behavior, making them hard to notice unless you look closely. When seen alone, they look very much like female House Finches. Pine Siskins have brown streaks like House Finches, but they are smaller and have small bills. Pine Siskins also have a touch of yellow in their wings, but that can be hard to see, especially at a distance.
Photo © Laura Erickson
Evening GrosbeakEvening Grosbeaks look a bit like giant goldfinches – yellow bodies, black wings, but they are many times the size of a goldfinch and they are becoming much less common. The Evening Grosbeak is an irruptive migrant, occasionally moving out of its boreal and montane breeding areas to winter at lower latitudes and elevations. These yearly fluctuations have been documented by participants of Project FeederWatch, allowing researchers at the Cornell Lab of Ornithology to track changes in the abundance and distribution of the species. According to a recent study, reports of Evening Grosbeaks at FeederWatch sites declined by fifty percent between 1988 and 2006.
Photo © Mike Wisnicki
Red-breasted NuthatchUnlike Evening Grosbeaks, Red-breasted nuthatches are doing just fine – their populations have been stable or increasing for the past several decades. Nonetheless, they may not be common at your feeders every winter, because in years of plenty, they may stay in their northern coniferous forest habitats instead of moving south. Red-breasted nuthatches can be distinguished from their white-breasted relative by their smaller size and conspicuous chestnut-colored breast and flanks.
Photo © Ron Kube
Monitor Irruptive Species!
Join Project FeederWatch and help scientists monitor irruptive species movements.