(noun) A sudden, dramatic and rapid increase in a bird population. Bird species will irrupt for different reasons and factors include the availability of food, suitability of climate and amount of predatory activity. Irruptions can be of two types; the first is caused by a dramatic increase in breeding activity that results in a larger bird population, while the second and more common irruption is caused by mass migrations, typically to follow food sources during winter months.
Some bird species have irruptions regularly, including pine siskins, red crossbills, common redpolls, evening grosbeaks and other winter finches. Regular irruptions may occur every 2-4 years for some species, while other irruptions are irregular and unpredictable.