We have purple Martins with a total of 4 houses, with 16 apartments in each. They are all full this year. Early each year normally in April a few scouts show up to check the housing. Within a few weeks they start arriving, cleaning and building the nests. Once the little ones start flying they will take extended days out preparing for the migration. Some times they will be gone a day or two then return. In Arkansas by the end of July they have all left for the winter migration.