Sachika

Hi, recently I have noticed my bird sitting on the bottom of the cage instead of the perch. Took look to find that she has no feathers around her vent area and she seems to peck on it. It’s red and possible sore. There was also lots of poop stuck there too which I was able to remove. I was wondering is she sick? What should I do? Is this contagious? ( I have two other birds too)