It looks like each brick is hollow, so think of them as birdhouses integrated into the wall. As for proximity there are many species that nest close to each oth

It looks like each brick is hollow, so think of them as birdhouses integrated into the wall. As for proximity there are many species that nest close to each other - it might seem like density would help predators, but there are defense benefits to having neighbors sound alerts and join you in putting a sharp beak into the back of a squirrel's skull.