Migration takes a lot of energy, so when possible, birds use weather fronts to give them a tail wind and help them along the way. Earlier this week we had a small front come through bringing a little rain (1/4 inch) and a cool front (100's before, low 90's after). When I drove into Midwestern State University, I saw a kettle of turkey vultures (TV) forming overhead, probably 25 or so when I first saw it, gathering more as it moved away. Turkey vultures are usually here in large numbers until late September--this could have been the first of our locals heading out or a group from further north passing through. I was reading up on Birds of North America Online and noted that TV migration is not well studied. It always amazes me what we don't know about fairly common birds.
Later that afternoon, I saw a group of about 30 common nighthawks forming up. This is about the time the vast majority leave the area--they are mostly gone by mid-September, although a few linger to the beginning of October. We received a newly hatched common nighthawk this week at Wild Bird Rescue, so I hope at least some adults linger longer--adults have to feed on the wing and handfeeding a bird for months over the winter is not a pleasant prospect.