Whole populations of birds sometimes move suddenly to a new place outside their normal range, a phenomenon known as irruption. People have been noticing bird irruptions for centuries: an account from thirteenth century England describes an irruption of crossbills where so many birds invaded apple orchards that the entire crop was damaged.
The Difference Between Migration and Irruption
Bird migration is a familiar and predictable seasonal movement of birds. Species that migrate do so every year at approximately the same time, traveling in a predetermined pattern, often to exactly the same destination time after time. Migrations are closely related to the breeding season and the arrival of spring in the breeding range.
In contrast, irruptions are unpredictable. Some species have a fairly regular irruption pattern: Snowy Owls irrupt roughly every four years; crossbills in Europe, roughly every seventeen years. Species that irrupt don’t always go to the same place or even in the same direction, and they don’t move because the season changes: ornithologists believe that insufficient food triggers the exodus.
What Bird Species Irrupt?
Irruptions occur primarily in the north and involve bird species that have a restricted diet: when their particular food is in short supply, they can’t turn to eating something else, so they must move to a new location where food is available.
For example, the Great Tit of Europe, the Middle East, and Asia, feeds on beech nuts, which are usually abundant in any given region about every second year. In years when there is plenty of food, the birds breed successfully and the population of Great Tits goes up. The following year, there are more birds but food is scarce, triggering irruption to another location where beech trees are producing more generously.
Some raptors of the north also regularly irrupt, though their food source is small animals rather than seeds or nuts. The Snowy Owl feeds on voles and lemmings in the north. Lemmings are particularly well known for their population fluctuations, with a crash about every four years. When the food source dwindles, large numbers of Snowy Owls move, sometimes as far south as the Gulf of Mexico in North America, and the Mediterranean Sea in Europe.
Other bird species are known to irrupt, including finches, the Great Spotted Woodpecker, and Pallas’ Sandgrouse.
Mistaken Beliefs About Bird Irruptions
The unexpected appearance, or disappearance, of a bird species sometimes causes misunderstandings:
- People may interpret the sudden appearance of many birds, such as finches, at feeders as a sign of a harsh winter to come. In fact, the event that caused the birds to move has already passed.
- When familiar feeder birds all disappear, it may inspire the belief that they have irrupted and gone elsewhere. In fact, a plentiful supply of food in the wild may have simply made them less dependent on feeders.
The first people to notice an irruption are usually birders in places where unfamiliar birds are arriving in significant numbers. If you live in the Northern Hemisphere, keep your eyes open.
Sources:
Atlas of Bird Migration. Elphick, Jonathan ed. Buffalo: Firefly Books, 2007.
Firefly Encyclopedia of Birds. Perrins, Christopher ed. Buffalo: Firefly Books, 2003
“Irruptive Bird Survey.” BirdSource
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