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
