Donna Kauffman says:

I saw some interesting behavior at the feeder the other day. What looked to me to be two different species of bird, was feeding the other. The “feeder” was a tall thin bird, sparrow size, while the “feeder” was a plumper, shorter bird. The smaller one would shake his wings and chirp then the taller bird would pick some seed out of the feeder and stick it in the open mouth of the chubby bird. Both birds were flying so the “feedee” was not a fledgling. The birds were in southwest Michigan. Anyone have any theories?