Strange Markings

Strange Markings

Observation - Strange Markings. Description: Two Coal Tits visit my garden regularly that have unusual dark smudges, on both left and right hand sides, between the tibia and the wing. I have never seen anything like it. And it is the first time I have ever been able to identify individual Coal Tit

Two Coal Tits visit my garden regularly that have unusual dark smudges, on both left and right hand sides, between the tibia and the wing. I have never seen anything like it. And it is the first time I have ever been able to identify individual Coal Tits.

Does anyone know if this is at all unusual?