Fw: [Maine-birds] Lemmings, Snowy Owl productivity

Meant to send the reply to the list, as I am sure many others have the same questions:

Hi Stella, 

I think the simple answer is we "don't completely know," but it is surely a combination of the two, and perhaps other factors.  Although the entry is now over 20 years old, the Birds of North America account has the best summary of the knowledge at the time.  I am sure someone with more time can find some more recent publications, but I am not aware of any uber-revelatory publications.

Here's the BNA account on the topic of irruptions:

"The classic explanation of the species' irruptions implied that the owl's principal prey, lemmings, crashed along a broad tundra front following a four-year peak, forcing the owls to wander in search of prey. The fact that many Snowy Owls appear in s. Canada usually one winter in four suggests that these irruptions are linked to the lemming cycle, although admittedly the flights sometimes occur in two successive winters, the second involving smaller numbers (Fig. 3; Godfrey 1986).

"Recent observations on wintering Snowy Owls challenge the implication of the classic hypothesis that a coordinated lemming-owl cycle is geographically synchronous continent-wide. Although these owls show unpredictable periodic irruptions in eastern and western North America, a large majority appear to be regular migrants over much of their winter range, notably in a central zone including the northern Great Plains where they are especially abundant (Figs. 1, 3; Kerlinger et al. 1985). Lacking hard evidence that lemmings fluctuate synchronously over vast areas (Maher 1970), Kerlinger and associates believed that the microtines occupied a mosaic of tundra patches varying in size. Owl dispersion even in the favored winter midlands is patchy, and the abundance of owls within patches may vary markedly between seasons. Dispersion is thus thought to be linked to local availability of their primary prey (Kerlinger and Lein 1988b).

"Looking at the geographic dispersion of lemming abundance and Snowy Owl breeding during any given year yields a mosaic. Peak numbers of varying lemmings (Dicrostonyx groenlandicus) at high latitudes in North America appear to be far less than peak numbers recorded at lower latitudes where both the varying lemming and brown lemming (Lemmus trimucronatus) occur in the same areas, or in those areas where extraordinary numbers of only brown lemmings occur at times (Parmelee 1972). Also, lemming fluctuations appear to be less regular at the higher latitudes where owl clutches average smaller, judging by a long-term study on Bathurst Island at 76°N in the Canadian Arctic. Compounding the problem of synchronizing owl breeding with lemming peaks is the fact that at least a few owls may breed during periods when lemmings are only moderately abundant. Nevertheless, the crux of the phenomenon is a mobile breeding population of owls that move nomadically, breeding where and when their prey is abundant.

"Not so easily explained are the large, geographically synchronous winter irruptions that are observed in eastern and western North America (Fig. 3). Breeding areas for the birds involved in these irruptions must necessarily be on the order of millions of hectares, far greater than can be attributed to lemming mosaics (Kerlinger et al. 1985). Other factors, possibly weather, i.e., the relationship between snowfall and temperature conditions on the breeding grounds, could be important. Herein lies a crucial, little known area in the species' makeup that merits additional research.

Reference:

 Parmelee, David F. 1992. Snowy Owl (Bubo scandiacus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:

http://bna.birds.cornell.edu.bnaproxy.birds.cornell.edu/bna/species/010doi:10.2173/bna.10

Hope that helps,

Derek

*****************************************
Derek and Jeannette Lovitch
Freeport Wild Bird Supply
541 Route One, Suite 10
Freeport, ME 04069
207-865-6000
www.freeportwildbirdsupply.com

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From: Stella Walsh <stellawalsh@earthlink.net>
To: Maine-Birds <maine-birds@googlegroups.com>
Sent: Monday, December 9, 2013 4:18 PM
Subject: [Maine-birds] Lemmings, Snowy Owl productivity

Snowy Owl irruptions are normally attached to high productivity or lemming population crashes.

So what is the 2013 status? And has the lemming cycle simply failed in North America and the species not recovered from its last collapse. Or did Snowies have a breeding year like  that preceding 2010-11.

I'm full of questions. Hopefully someone out there has better answers than Google...and/or a subscription to JESTOR.

Stella

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