Here’s a brief article about what an irruption is. I don’t think it’s a term many of us are familiar with.

Here’s what amounts to a TL;DR from the article, but it has some better details if you read the rest, and it touches on some of the irruptive owl species as well as some other irruptive birds.

While many people think of migration as a north-south or south-north phenomenon, irruptive species are nomadic with their movements linked to food supplies. Their migration can as likely be east or west. As a result the details of these nomadic movements vary from species to species and from year to year.

In some cases they involve nearly all the immature birds leaving the poorest territories, while in other cases virtually the entire population of a species may move. If they did not move they would suffer very high mortality rates. This may still occur if their travels bring them to areas which also lack sufficient food sources