This is a "dovecot", a building built many years ago to house pigeons or doves. People used them to provide doves and eggs for meals. There is a door in the back (you have to climb up the ladder to get to it) to get in to gather eggs etc. These are considered historic buildings and you aren't allowed to tear them down or remove them. This one at the mission home is much nicer than the standard simple stone ones in the country.