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

I took a look on the Internet for you and had no success. Try posting on the bird watching forum. I'm not sure, but I think you'll be able to get an answer. -- Lori Here is a link that might be useful: GW Bird Watching Forum

Try checking out the PMCA's forum. Most folks there increase the size of the cavities by knocking out walls between two compartments in the smaller units. I've read that the six inch compartments are too small. I think you will find really great information on how to do that there. Here is a link that might be useful: PMCA

I was afraid that might be the case, they all look around the same size (martins) and info I found today said that the martins usually start to arrive here in February and look for nesting spots. In addition to All the other potential dangers/problems with the house/location where these martins are possibly planning to nest and raise babies, same neighbors also have a HAMM RADIO tower in their back yard and every year around late spring/ summer a great horned owl perches up there on and off and and hoots, I've heard him and seen him up there several nights in a row before. Is there anything I can do at this point, like put up a safe Martin house, or will they stay there now regardless? I'm hoping that if I talk to neighbors and explain the situation, they will do something about it. I feel sick at the thought of the martins going through whole process and end up losing babies or being harmed themselves. If I thought there was a possibility that they would relocate this late in the game, I'd gladly put up a safe place for them to nest and make sure it stays that wAy, although I'd be learning as I go, ughh, whole situation makes me nervous. I'm going to post the pics of the house and the birds tonight, I'm pretty sure the house is not the ideal layout either, it has little porches with metal railings on both floors and another railing across top and from what I've read so far, that makes it easy for owls to get to them. Thanks