Good-luck birds?

These two little birds, all of a sudden one day, started to come to our front door and sleep there. One day, I opened the door and thought that a bird flew out of somewhere by the door. How can they have a nest on an inch wide space...?? I thought it couldn't be... until it happened again. Another time, Wally noticed two little birds right above the front door. Every evening they came, and every early morning they left. It happened for about a month. Then one left. After a few days, the other left too, and they never came back.....