This year in our area, common irruptive species such as red-breasted nuthatches, pine siskins, and purple finches are being reported. Evening grosbeaks, which are rarely seen in our area, are also being reported.
Bird irruptions follow some type of boom-and-bust cycle of food sources. The most common cause is a phenomenon called masting, which occurs when a single tree species produces a large number of seeds across thousands of miles of forest in the same year. The true cause of masting is unknown, but it typically occurs one to two years after a warm, dry spring. Often a masting year is followed by a poor seed production year.
When the conifers in the boreal forests of Canada and the northern United States experience a masting year, the abundance of seeds gives some species of boreal songbirds a boost. The birds can begin breeding and produce more offspring, resulting in a bird population boom. When fall arrives, the bird population has doubled or even tripled, but the available habitat hasn’t. Many birds move south, and young birds, in particular, may be pushed farther and farther, so people outside the boreal forests may start seeing unusual winter visitors at their feeders. Each year ornithologists make predictions about which species may wander south during the upcoming winter. You can read more about the yearly winter finch forecast here.
Source: Theconversation.com; When birds go roaming: The mystery of avian irruptions
Photo of a Purple Finch and an Evening Grosbeak by Luc Jacobs