Purple martins are unique in that they are the only bird in North America dependant on humans for housing. So accustomed to having nesting boxes set out for them, they rarely return to their instinctive habit of nesting in old woodpecker cavities.
They are very tame around humans because they have become so accustomed to contact. Also, humans have never hunted them. Purple martins are the largest of the swallows, and so they generally concentrate on larger insects for food. Usually, they will ignore smaller insects like mosquitoes.
The belief that these affable birds will eat up to 2000 mosquitoes a day is untrue. Zealous birdhouse manufacturers inserted these claims in marketing materials, and so a myth was created out of a sales promotion campaign.
Usually, mosquitoes make up no more than 3% of their diet, and then only in areas where day feeding mosquitoes are present. Instead, they prefer larger insects like moths, katydids, grasshoppers, dragonflies, damselflies, cicadas and beetles. In reality, it is probably better to keep them out of the yard and keep the dragonflies in.
The truth is that these birds feed during the day, when most mosquitoes are resting under leaves. A martin’s foraging time only overlaps a mosquito’s for about 10 minutes a day. If you are looking for a bird that will reduce mosquito populations in your yard, they are not the answer. And, with purple martins, if you don’t build it, they won’t come. They will wait on an invitation to nest coming from a birdhouse set out for them.