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From: 76627.3473 at compuserve.com (Marianne Hickson)
To: obol at gaia.ucs.orst.edu (obol)
Subject: Snowy Owl Bio-Ecology 101 (long)
Date: Tue, 26 Nov


These spectacular birds moved me to crack a few books to see what they are all
about. I found some interesting tidbits, and invite others to share also.
There seems to be some dispute about the relative ranking of this year's
irruption, with some of our neighbors to the north calling it the biggest in
nearly 20 years, with others looking back to the 1966-7 winter for similar
numbers During an irruption the winter of 1926-7, 5000 snowy owls were
reportedly shot in Michigan & Maine.

Irruptions are usually attributed to a crash in the lemming (look like large
meadow mice, but with short tails) population. Others point to huge increases
in the snowy owl population during peak lemming years; snowy owl clutch sizes
can boom from 3 up to 10 or 15 when lemming populations explode, with way too
many left over when prey populations decline in following years. By comparison,
great horned owls average less than 2 young/yr.

Lemming populations do crash regularly, with accounts varing from every 4 yrs.
to a range of 4-7, and some reports of an even more dramatic crash every 10-12
yrs. Such declines reportedly often coincide with low snowfall years, possibly
reducing breeding activity of lemmings, and also subjecting them to increased
predation. Additionally, lemmings frequently die off during years of sparse
snow cover from malnutrition and exposure, especially in genetically weakened
populations resulting from rapid population buildup. A popular theory is that
the overcrowding halts breeding for several years at a time.

Rough-legged hawks often irrupt in the same years because both feed largely on
rodents. In contrast, northern goshawks tend to irrupt in concert with the 10
year cycle of hares & rabbits Snowys are so tuned to the lemming population
that they forgo breeding when prey numbers are down.

Just to mix us up, one B.C. study of 5 snowy winter territories found that
grebes & ducks comprised an estimated 90% (!) of the prey intake by wt. On the
coast, they also take shorebirds & gulls, tho the casting I examined on Bay
Ocean Spit contained rodent parts. Ring-necked pheasants are the #1 bird
species taken by snowys wintering on the northern prairies, but grouse, quail,
domestric poultry, & rock dove have been reported. Seattle Audubon
reports their irruptives feasting on wintering ducks, with fondness for the
small, plump harlequins & buffleheads. Has anyone noted such preferences here?

They usually nest on the ground, on small rises or on the mounds created by
frost heaves. Geese & eiders are smart enough to nest nearby & take advantage
of the protection from arctic foxes. I found it interesting that in large
clutches, the 1st owlet may be about to fledge by the time the asynchronously
hatched youngest is out of the shell!

Averaging some 20% bigger than the great horned, snowys are our largest N.A.
owl. Large females range up to 6 lbs., with some reports of an incredible 8
lbs.--almost as heavy as the male golden eagle. Apparently it is typical for
the imm. males to winter furthest south on average, and the adult females
furthest north; the imm. males are reported to arrive 1st in wintering areas of
southern Canada & northern U.S., only to be evicted soon after as the larger,
socially dominant females arrive, forcing the males to move further south. I'd
be interested to know if this pattern has been confirmed in the PNW.

Evidently Alaskan snowys are considered to be more or less residents, while
those of high arctic Canada regularly migrate to the southern provinces. Can we
be sure our irruptives are U.S. birds? And will they survive to return to their
breeding grounds? Most think not. Many arrive emaciated & fall prey to local
predators. Personnally, I want to believe that the magnificent snowy owl whose
eyes locked onto mine that morning on the beach will return to the high arctic,
fatten up on rodents, raise some young, and come back in time to help us usher
in the next millenium.

Bill Price (mhickson)
76627.3473 at compuserve.com




--
*********************************** I was of three minds
* Mike Patterson, Astoria, OR * like a tree
* mpatters at orednet.org * in which there are three blackbirds.
*http://www.pacifier.com/~mpatters* -Wallace Stevens