Bird

The situation is this, I sit like that at work ,at the window, where the window sat a bird, then flew away, and so 2-3 times a day or a couple of days, I go in the other room at work and there flies a bird by their appearance the exact same before this, but to get it into the room could not, the door and Windows were closed, but that's not all, the house opened a window, and yesterday the same bird( on external signs) first flew in the window, раз5 and chirp, but today, even hall's body flew in the room and flew away, what is it?