I don't remember ever seeing the word irrupt, but I'm very familiar with the word irruption: it's used to describe the phenomenon when a lot of birds that usually hang out in the far north show up in the winter in southern areas. This year looks like a good one - there are already a number of snowy owls showing up in southern Canada and the States. It's frequently described as being linked to food supply - either not much food there, or such a successful breeding season that all the "extra" young birds have to move south to find food.
