Irruption of the great gray owl in last 40 years. Mostly a 4-year cycle with the occasional 5 year interval.

Winter 1978-79
Winter 1983-1984 (this was a 5 year interval)
Winter 1987-88
Winter 1991-92
Winter 1996-97 (this was a 5 year interval)
Winter 2000-01
Winter 2004-05
Winter 2008-09
Winter 2012-13
Winter 2016-17
Winter 2020-21 (next up if 4 year cycle holds)