Rings on Blue-Tailed Bee-Eaters

Is the time of the year when migratory birds arrived from the north so as the Blue-Tailed Bee-Eater(Merops philippinus).

I saw a few of them this morning and what puzzle me was the rings on their wing. I can't find any info about the wing rings. I'm sure is for id purpose but usually the ringing are done on their leg instead of the wing. I took some of the photos when they fully extended their wings on flight. The feather arrangement seems affected by the ring.
Not too sure if their flight patterns affected as well? Anyone have any info about wing ringing?