Hello,
In the species treatment of Snowy Owls, by David Parmelee, 1992, In The Birds
of North America, No. 10, Philadelphia: The Academy of Natural Sciences, the
relevant paragraph out of 20 pages:

"Many Snowy Owls that move southward from arctic regions are mistakenly
assumed to die from starvation. Although this may prove to be the case during
irruptive migrations of young in western and eastern sections of North
America, there is no evidence that this is so in the N. Great Plains. In
Alberta, 45% of the specimens examined had moderate to heavy fat deposits,
and traumatic injuries were the major cause of mortality (kerlinger and Lein
1988a). Causes of death or injury were collisions with unknown objects
(45.5%), automobiles (14.1%), utility lines (4.2%) and airplanes (1.4%); also
gunshot wounds (12.7), electrocution (5.6%), fishing tackle (1.4%). Only
14.15 was believed due to starvation. Even as far south as Kansas, a Snowy
Owl fed on rodents at a lumberyard for nearly a month before being accidently
electrocuted (Parmelee 1972). Gross (1947) inferred that individuals seen far
from land at sea never live to return, but this is a moot question, difficult
to resolve."

So, I hope we're seeing mostly adult birds that will survive and return to
their far north home.

For info re the Birds of North America, some 40 species treatments per
volume, call 1-800-345-8112 or fax 215-586-3232. I believe several volumes
are complete & total cost of 18 volumes will be around $3000.
Thanks and good birding.
Sue