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Re: Web hints at more winter irruptors on the way...



On Mon, 18 Oct 1999 PJPHILIP@aol.com wrote:
> 
> JR - Irruptions r fascinating ... anybody speculating on the 
> whywithall of such events ??? R they all good indications ?? [these 
> kinds of questions are easy to ask; I admit] - Phil J [NHAS]. > 

Irruptions, as I understand them, are connected to seed and cone crops of 
boreal plants, to boreal snow cover, and to rodent population 
fluctuations. The irruptions of seed-eating birds are connected mainly to 
the seeds and cones; the early and abundant appearance of Red-breasted 
Nuthatches and Pine Siskins indicates that the seed and cone crop must be 
poor this year. Crossbills, Evening and Pine Grosbeaks, and Redpolls 
generally irrupt in the same years as Siskins, so be on the lookout for 
these species.

Raptors are tied to rodent populations and snow cover. Rodent populations 
in the arctic - mainly lemmings and voles - naturally go through boom and 
bust cycles. On top of this, snow cover varies from year to year; if the 
snows are deep up north, then even in a boom year for rodents the hawks 
and owls will have trouble locating food. I was at Whitefish Point, 
Michigan during a year of very deep snows; there was a major owl 
irruption that year, and many instances of starvation among owls later in 
the winter. The big Saw-whet movement and early southern Rough-legged Hawk 
sightings indicate that a raptor irruption may also happen this year. I 
think the two co-occurring is most likely a coincidence; there have been 
past years of finch irruptions without raptors and vice versa.

Good birding,

Josh



Joshua S. Rose
Duke University
Department of Zoology

jsr6@acpub.duke.edu
http://www.duke.edu/~jsr6/