An irruption is when a large group of birds expands its normal range area because there isn´t enough food to feed them.

This normally happens after breeding season, when the young birds are becoming self sufficient. This normally happens to birds that are pretty picky about what they eat, and who eat things that tend to become available in cycles.

Some birds in north America that are prone to irruptions are the chickadee, nuthatch, blue jay, evening grosbeak, and red crossbill.

Birding Glossary