Knocking at my windows

Almost every day guinefowl come to the glass doors, and sometimes fly up to the upstair windows, and tap loudly on the glass with their beaks. I don't know why they do that. I used to think that they were asking for food but they run far away as soon as they see me and they still do it even though I have not offered them food for years. Maybe they have some sort of inherited memory of the time that I used to feed their ancestors.
To get this close-up picture, I hid behind an arm chair and they did not see my little cellphone/camera.