Hi -
Many of our wintering raptors, including Kestrels, Red-tailed Hawks,
Rough-legged Hawks, White-tailed Kites, Harriers, Short-eared Owls, and maybe
Red-shouldered Hawks, feed mainly on voles in winter. For them, vole
availability is the default explanation for year-to-year differences in
abundance (perhaps not always the only explanation, but the default.) These
voles have cyclic population booms and busts, but it is not clear (at least to
me) how much of Western Oregon has these cycles synchronized. Vole
availability is affected by abundance, of course, but also by cover, farming
practices, snow cover, flooding etc.
The Rough-legs are all migrants from the north. The number that reach Oregon
is likely determined by productivity and food availability further north. The
number remaining to winter will be affected by vole availability. The
Red-shoulders are near the northern limits of their range and not very
migratory, but I see evidence here in the coast of immatures moving in for the
winter. The Kites are near the northern limits of their range and are kind of
nomadic, chasing vole population peaks. The more common species, Red-tails,
Harriers, and Kestrels, and also the Short-eared Owls, have both local breeding
birds and numerous migrants from the north. It is not very clear how many of
their breeders stay here vs migrate further south.
So in any given area, fluctuation in vole abundance is probably the main factor
affecting winter raptor abundance, unless local changes in farming practices
(e.g. switching crops, or going to no-till) affect vole availability. Snow
cover and flooding can change availability and cause raptors to relocate, but
both usually occur after the wintering birds have arrived, and movements are
often pretty local.
Wayne
On 1/22/2018 11:06:56 AM, Lars Per Norgren <larspernorgren@xxxxxxxxx> wrote:
*
I am not aware of a universal, killing frost yet. Our house is typically 10-15
degrees Fahrenheit cooler than Portland, 40km southeast of us and 270m lower.
It finally got cold enough to kill the Nasturtiums some night in December, but
there must be two dozen other species of plant in the garden that are quite
perky, including celery. Last year we had snow on the ground for 90-100 days,
many times longer than any of the previous 25 years. As I recall there was
enough snow in downtown Portland last year to make restaurants close
repeatedly. I don't know how widespread snowcover was on the valley floor to
the south.
This year seems spotty to me, with no raptors in some traditional
spots, but many in other small areas. Perhaps the biggest concentration of
Rough-legged Hawks I've ever seen was in a wheat-stubble field on De Jonghe
Road in Yamhill County. The practice of no-till farming presumably creates a
new opportunity for both the voles and raptors. The winter raptor situation is
probably too complex to fully explain. A good breeding season can generate lots
of young
Rough-legged Hawks in the sub-arctic, or Red-tailed Hawks further south. Their
numbers here then might be high despite unfavorable local conditions. The
lowlands of western Oregon have lots of resident kestrels and Red-tails. It has
never crossed my mind that they might leave in the winter. If the Lorane raptor
route had snow cover for a week long might it make mouse eating birds leave? I
don't know how far they would travel, nor how soon they might come back.lpn
On Jan 22, 2018, at 8:33 AM, Nathaniel Wander wrote:
U.S. Fish & Wildlife biologists appear to be attributing long-lingering raptors
to the mild winter. At least one has said that kestrels have hardly migrated
away at all. They were kestrel banding over by Corvallis last weekend.
Nathaniel Wander
Portland, OR
Max Planck is supposed to have said:
A new scientific truth does not triumph by convincing its opponents and
making them see the light, but rather because its opponents eventually
die
and a new generation grows up that is familiar with it.
Andreas Wagner observed of Planck's remark:
Science, like nature, advances one funeral at a time. (Arrival of the Fittest,
p.197)
