Multiple NEXRAD flock indicators

The biologist who is searching for Purple Martin roost sites in the west is also interested in the Barn Swallow flocks that are visible on NEXRAD weather radar when they leave their roosts in the morning. He has identified, in addition to the large Yamhill Co. flock, at least five other roosting sites in the central and northern part of the Willamette Valley, and is hoping that birders in the area will try to find these sites as they go to roost for the night at about 7:30 p.m. Are they all using cornfields as is the flock in Yamhill County? We don't know, but we'd sure like to find out. And it would be very exciting to find Purple Martins roosting with the Barn Swallows. (Click on the image to see a larger version.)