This cattle egret seems to be having some medical issues. Binkie at Sint Maarten Birding actually saw the same egret the other day (https://www.facebook.com/SintMaartenBirding). I don’t know what caused this, but it seems like some sort of infection. Perhaps someone who knows about these things could weigh in. Since they do forage in the water sometimes, I would imagine that a cut could become infected by bacteria in the water.
