Attached is a good fact sheet from a purple martin conservation organization. The section on competition addresses your question specifically, but the whole article in very interesting. Here is a link that might be useful: Click here for Fact Sheet

If I recall, March is the month! Mine is still on the ground because the telescoping pole is not telescoping. I have sprayed with lubricant but don't know if I can get it tall enough at the recommended 15 feet.

I did not know any of that about Purple Martins!! Congratulations on your full house(es)!! That is a lot of eggs to watch and a lot of baby birds coming up. Cool picture!! Thanks for sharing that. I had a late (to me) nesting of a pair of Carolina Chickadees. They had 3 youngsters and they have now flown the coop! Birdwatching is so much fun.

And now I interrupt the "end of the season" blues... ... to report about 20 Purple Martins are over my front yard and gourd rack right now. They are zooming around, soaring, circling, calling and chattering, and all-in-all being absolutely delightful. Even without grabbing the binoculars I could make out a few newly-fledged PMs bumbling along. I don't think these are "my" birds. They've been overhead for about an hour. Then they began dive-bombing and zooming by the gourds as fast as they could. After 15 minutes of that, two got brave enough to land and look inside a few of the gourds. Now there is a new fledge perched on the top pulley, as high up as he can get. He's chattering and bobbing, puffing up and looking all around in a very proprietorial way. Fledges all look the same, but I'm certain this is male. He's going to be a real (but delightful) pest next season as a SY, and an awesome beauty the next. Could he be the one to help produce my first 8-egg nest? It's August 13, three days past their latest departure date ever, and they are still being very good medicine.