Snowy Owl on the Prowl

An afternoon visit to North Point Marina yielded a nice surprise:

Snowy Owl (Bubo scandiaca), North Point Marina,
 Lake County, IL, 1/18/2014

This is one of quite a few that have come down into the US this year. They breed on the Arctic tundra, in both Eurasia and North America (a distribution referred to as Holarctic). Some of them (mostly adult males) will spend the winter over much of that range, but many of them migrate south. In a typical year, a few will reach as far south as Chicago. In an atypical year like this one, there may be several dozen in Illinois. This year, one has been reported as far south as Jacksonville, Florida!

This sort of pattern is actually not uncommon for birds of prey from the Arctic -- Rough-legged Hawks, Gyrfalcon, and Snowy Owls all occasionally irrupt southwards in large numbers, as do Boreal and Great Gray Owls and Northern Goshawks from the taiga forest just south of the tundra. In some cases, (especially Northern Goshawks) smaller, localized irruptions can show a cyclical pattern -- every 10 years, for instance.

What's driving this? Well, this video holds the answer (keep an eye on the upper right at the base of the rocks):

As we often see, the answer is food! Specifically, the tendency of arctic rodents to undergo large, periodic swings in population size. Lemmings, of course, are famous for this, but perhaps the best data set showing these swings comes from Snowshoe Hares (Lepus americanus) in Canada. A century of trapping both hares and Canadian Lynx (Lynx canadensis) by the Hudson Bay Company means that we have a full century of population estimates for both species, and Lynx numbers follow hare numbers almost perfectly over a repeating ten-year cycle. (1) This cycle appears to be driven by a combination of Lynx predation on hares and the overbrowsing of food supplies by hares during the high parts of the cycle. (2)

Snowy Owls, on the other hand, don't show such a cycle -- when they come south, it tends to be in just one part of the country (West, Midwest, or East -- this year we're seeing a historic movement into the East spilling into the Great Lakes), but there's no apparent regularity to these irruptions. (3) Why the difference? Well, Snowy Owls moving this far south are just the vanguard of a larger movement. Any irruption similar to this year's means a lot of birds moving, which in turn means that they're coming from a very large area. So there must have been a collapse of Lemming and other rodent populations across a very large area. If there is any geographic structure to those populations, then in a normal year only small regions within that area would show a collapse, and we would only expect localized irruptions. Small enough ones would actually explain the normal annual variation that we see in southern Snowy Owl populations. Similarly, the more species the owls are able to prey upon, the more prey cycles would have to line up in order to create a poor enough year to drive them south.

So when you're out watching a Snowy Owl on the prowl, stop and think about the poor little lemmings that helped send that owl south to you.

(1) Elton, C. and M. Nicholson. 1942. The ten-year cycle in numbers of the lynx in Canada. Journal of Animal Ecology 11:215–244.

(2) Krebs, Charles J., Boonstra, R., Boutin, S., & Sinclair, A. R. 2001. "What Drives the 10-year Cycle of Snowshoe Hares?" BioScience 51.1: 25-35.

(3) Kerlinger, P., M. Ross Lein, and Brian J. Sevick. 1985. "Distribution and population fluctuations of wintering snowy owls (Nyctea scandiaca) in North America." Canadian journal of zoology 63.8: 1829-1834.