We saw a similar situation with hawks on the Tillamook CBC last December, and
it probably wasn't related to the vole population. The CBC was held about two
days after floodwaters of the local rivers had barely subsided below stream
bank level. All the pastures were so saturated with water that the voles
apparently were on or near the top of the ground. Lots of hawks, herons, and
egrets were walking around in the pastures, no doubt finding voles with ease.
Darrel & Laura
From: "larspernorgren" <larspernorgren@xxxxxxxxx>
To: obol@xxxxxxxxxxxxx
Sent: Monday, March 21, 2016 10:44:15 PM
Subject: [obol] winter Butéos and voles
There are two species of voles in the Willamette Valley. Microtus canicaudis is
prone to wild fluctuations in numbers, sometimes more than one per square foot.
Under these circumstances hawks can hunt without flying. There was white clover
field southwest of McMinnville a few years back that attracted many birders due
to the presence of a Ferruginous Hawk. There the raptors were often walking,
not running, from mouse to mouse. According to Werts and Carraway (Land Mammals
of Oregon) M. townsendi has much more stable populations.
Rough-legged hawks are arctic nesters, prone to population swings similar to
Snowy Owls. There must have been plenty of food up north last summer as I saw
more Rough-legs this fall and winter than any previous season I can remember.
Weather to our north must be involved as well. Extensive snow would force some
ground feeding hawks further south than they would otherwise travel. Resident
Red=tails are greatly augmented by migrants from various points to our north.
They too surely have variable breeding success from year to year. We tend to
put a lot of emphasis on phenomena we can observe directly.
Lars
