Black Forest CBC and Rosy-Finches:
We have 100-150 Grey-crowned Rosy-Finches that come to our feeders daily. A characteristic flight pattern is that which you describe: a leap-frogging, rolling-over each other pattern, during which those in back fly over those on the ground to land in front of the flock, then the ones in back fly over those who just landed and settle at the front and so forth.
At times, the whole flock rises in response to some stimulus and they can look like starlings, or cowbirds, even, at a distance, individuals can look like small falcons--just for a moment--until your eyes adjust to their distance from you.
The shimmer of their plumage also can be deceiving and help them look like cowbirds. The variation in plumage among the individuals no doubt has led to many mis-identifications. I have scanned the flock several times, each individual one by one, hoping to find one of the other Rosy-Finches, but no luck! We have had two Black Rosy-Finches. Those two came as single individuals to the feeders to the other side of the house and stayed just moments and clearly were different from anything among the flock of Grey-crowned.
Our neighbors across the ridge share the flock of Grey-crowned with us and it has been a delight to have them be so regular. Sharing them with our neighbors gives some security that they will stay close by. We also are lucky to be among tall grassland, for I believe the species would otherwise depend upon the seedheads of the grasses for food, if we were not there.
I hope this helps.
David Matson