Hi -
Rough-legged hawks are more vole specialists than are Red-tails. Many vole
species, including our grassland ones, are strongly cyclical. When I was at
OSU, we were taught that they were typically on a 4-year cycle. I think they
are now recognized to be a bit more variable in cycle length? In any case, this
strong and intense cycle (2+ orders of magnitude between high years and low
years) will make it very difficult to see and interpret habitat based trends in
vole predators.
An alternative hypothesis might be that the low periods of the cycles get lower
in the annual grass fields than in more natural grasslands, and so the
predators are more constrained in those years.
White-tailed Kites are even more specialized on voles (and ecological
equivalents - Hispid Cotton-Rats in the southeast) than are Rough-legged Hawks.
It would be interesting to see how similar their distributions are in the
Willamette Valley, and whether they expand and contract in synchrony? Are the
winter raptor survey data stored in formats that would allow that to be tested?
Wayne
From: "joel geier" <joel.geier@xxxxxxxx>
To: "obol" <obol@xxxxxxxxxxxxx>
Sent: Sunday, March 20, 2016 5:54:25 PM
Subject: [obol] Re: Rough-legged Hawks at Fern Ridge
Hi Priscilla and all,
Along with the prairie restoration areas on the west side of Eugene,
this has also been a good winter for Rough-legged Hawk sightings close
in to town around Corvallis, and also reasonably good at Finley NWR.
On the other hand, I've seen much fewer than usual on private farmland
around southern Polk County and northern Benton County. I also haven't
seen as many as I'm used to seeing, on a couple visits to the
Lebanon/Tangent/Brownsville area.
One idea that a few of us have been kicking around is that the long-term
shift from perennial grasses to annual grasses on grass-seed farms,
along with conversion of some fields to filbert orchards & vineyards,
might be reducing habitat for wintering Rough-legged Hawks, with the
result that more of them are crowding into prairie-restoration lands
where perennial native bunchgrasses have been established and provide
good habitat for voles.
Perhaps some of the same trends have been causing more Rough-legged
Hawks to push farther south into Lane County.
One winter probably isn't enough to judge, but this seems like something
to keep an eye on.
Good birding,
Joel
--
Joel Geier
Camp Adair area north of Corvallis
POST: Send your post to obol@xxxxxxxxxxxxx
JOIN OR QUIT: //www.freelists.org/list/obol ;
OBOL archives: www.freelists.org/archive/obol
Contact moderator: obol-moderators@xxxxxxxxxxxxx
