Monday, December 9, 2013

Random(ish) Thoughts on Snowy Owls

I had a thought about this unpredicted Snowy Owl irruption.

Muskegon County, Michigan, November 30. Phonescoped by hand.

Let’s get the background that we’re all familiar with out of the way first.

The primary driver behind Snowy Owl (SNOW) irruptions well south of their range is an abundance of owls caused by a highly successful breeding summer. A good crop of lemmings to eat leads to lots of surviving hatching-year owls. When winter comes, the owls move to pack ice and islands to spend the winter eating ducks and waterbirds (primarily, not exclusively). As this territory is limited, the extra juveniles are often forced out of range and they head south. These are the owls (usually juvenile and usually in poor health) that we see in the United States during a SNOW irruption. (For purposes of clarity in this post, “United States” refers to the contiguous 48 states.)

The winter of 2011-12 saw a major irruption of Snowy Owls across much of the United States and Canada. It was widespread from coast to coast and particularly stunning in the interior where the invasion pushed all the way into Texas. Two places that did not see an irruption of SNOW in 2011-12 were Newfoundland and the Atlantic coast south of Long Island.

2011-12 Irruption. Note that Newfoundland and south of Long Island are sparse in owls.
The winter of 2012-13 saw a predictable echo irruption covering the core of the 2011-12 area minus the extremes of the range. Echo flights involve left over birds that survived their winter in the south, returned north, and head south again. It's common after a major irruption. 

2012-13 Echo Irruption

With those data in hand, we could predict that lemming numbers would not have recovered by now and most of the owl abundance from the productive breeding season of 2011 had died off. There shouldn’t be any “extra” owls to invade southward, therefore it would be a very weak year for SNOW in the United States.

2013 so far. Notice the purple in Bermuda!
Obviously that prediction went down in flames. For reference, the map below shows what an "off" year should look like.

2010-11 The winter before the huge irruption.
We’re seeing a massive irruption of SNOW again, with numbers rivaling or eclipsing 2011-12, except this time there is a significant irruption in both Newfoundland and the Atlantic coast as far south as Cape Romain, SC, but almost nothing is happening in the Plains or West.

As the action in Newfoundland heated up, I picked up this anecdote from a conversation online: SNOW irruptions in Newfoundland have not historically correlated with the widespread irruptions elsewhere in eastern North America.  Keep that in mind, and let’s get back to this current event.

My North Carolina birding friend (and ex-patriate Michigander) Cody Porter and I were dishing wild speculation about Snowy Owls at each other and he pointed me to the folks at eBird suggesting invasions from different parts of the arctic lead to different irruption patterns. In particular, they were wondering about Greenland, given all of the birds on the Atlantic coast and that one in Bermuda. (If you fly south from Greenland and miss Newfoundland, Bermuda might be the next interesting thing you see.)

The problem I saw with that was that if Greenland lemmings follow a different cycle than those in arctic Canada (which is a reasonable assumption), then why have SNOW irruptions always followed a very typical 4-5 year cycle with little variation. You’d think these back-to-back irruptions would happen more often.
One possibility is that oddly-timed irruptions DO happen frequently and it’s just legend that the irruption cycle is so regular. I read it plenty of times in 2011 and filed it away as gospel. Perhaps a dig through Christmas Bird Count records could color this picture up a little.

Snowy Owl in Gratiot County, Michigan, December 8, 2013. Photo by Lansing, Michigan birder Greg Smith. Check out his other stuff here.

There’s another possibility that I came up with which may be a brilliant idea, the rantings of an ignorant dolt, or something that’s already known.

Perhaps Greenland has an independent lemming cycle and the owls from there do fly south at regular intervals. Most of the time they all end up on Newfoundland, because that’s pretty much the first place they’d hit, and there are generally plenty of good ducks for eating there. That explains the Newfoundland irruptions that tend to not correlate with the continental irruptions. This year, perhaps, was such an exceptionally good season in Greenland, that Newfoundland has overflowed (200+ owls in a 40km strip on the Avalon Peninsula!) and they’re headed down the eastern seaboard. And thus, no owls in the Plains or West.

This would require several things for further investigation:

  • CBC data for a century’s worth of SNOW irruptions. (Time consuming, but it’s there.)

  • Knowledge of lemming populations in Greenland. (Someone has to know this.)

  • Whether a preponderance of the owls on the Atlantic coast right now are Greenlandish in origin. (No way of knowing this that I’m aware of.)

Still, speculation is fun and exciting. It gives you something to do while cruising ag fields looking for white bumps.

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