Everyone knows what an eruption is. When a volcano blows its top and spews ash and lava from the cone, that’s an eruption on a grand scale. People prone to temper tantrums can erupt. So can shook up pop cans. But there is a homonym of eruption that’s not nearly as well known: irruption.

When applied to biology, irruption means a sharp increase in numbers. This term is probably most familiar to birders, many of whom eagerly anticipate irruptions of boreal songbirds. Boreal refers to the vast belt of northern forest that covers much of Canada, Alaska, and the northernmost United States. Sometimes referred to as taiga, the sprawling boreal forest