Before you decide to set up nestboxes for a purple martin colony, you might want to consider that none of the published statements appearing in the popular or ethnological literature that attribute a mosquito-feeding habit to the Purple Martin are based on a factual study. The oft-quoted statement "a martin eats 2,000 mosquitoes per day" has no evident means of support.
When scientists analyzed the stomach contents of purple martins, they've found very few mosquitoes. Since mosquitoes mostly fly at night 15-20 feet off the ground, and purple martins during the day at around 300 feet, this makes sense.
Only 2% of the purple martin's diet is mosquitoes. Insects on the menu are flying ants, beetles, butterflies, damselflies, dragonflies, mayflies, moths, stinkbugs, wasps, a few houseflies. Out of all of these, their favorite food is probably the dragonfly. During year end nest cleaning, the compartments are littered with dragonfly wings and skeletal parts.
Read scientific info on this topic.
Read general info on purple martins.