I have seen nothing written about what I have observed when collared doves bath, but as you can see here, a collared dove leaves white scum on the water's surface while bathing. I've seen this many times.
There is a powder in the bird's feathers that acts as a dry lubricant to stop feathers from tangling up. I can only assume that is what this white scum is.
There is a powder in the bird's feathers that acts as a dry lubricant to stop feathers from tangling up. I can only assume that is what this white scum is.