[EBB Sightings] Cornell Website definition of irruption

>From the Cornell Website:

 "What is an irruption?"

"An irruption is the sudden change in the population
density of an organism. When speaking of birds,
irruptions refer to the movement of northern-wintering
species to the south in years of low food availability
[or of mountain birds moving down in elevation under
similar conditions, here in the West. DV]. Irruptive
species include redpolls, Evening Grosbeaks, and
Red-breasted Nuthatches, among others."

Who am I to argue?

Debbie Viess