[obol] Re: Wintering raptors
- From: "Robert O'Brien" <baro@xxxxxxx>
- To: "carole.hallett@xxxxxxxxx" <carole.hallett@xxxxxxxxx>
- Date: Mon, 22 Jan 2018 14:49:25 -0800
I'm very interested to hear that Carol. I have a nesting pair at my place
which i have long assumed to be year-round residents. Now i know for sure.
Bob O'Brien Carverwhen or
On Monday, January 22, 2018, Carole Hallett <carole.hallett@xxxxxxxxx>
wrote:
For what it's worth, I have monitored red-tailed hawks near Portland, OR
since 2000. Although not every bird has been banded or tagged enough of
them are for me to say with confidence that our breeding birds remain on
territory year round. Migrants, floaters and wintering birds come and go
but our birds are always around.
Carole Hallett
Portland, OR
On Jan 22, 2018 11:32 AM, "Wayne Hoffman" <whoffman@xxxxxxxx> wrote:
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)
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