At a site without an established martin colony, a single pair of Tree Swallows can monopolize an entire martin house or gourd rack and you will not get martins. You also will not get more Tree Swallows. (There have been a few reports of more than one pair of Tree Swallows nesting in a martin house, but don't count on it - that is an extremely rare occurrence!) Tree Swallows set up a territory of 30-50 feet around their compartment and defend it very, very fiercely against all other birds, including their own species.
Obviously, a Tree Swallow pair does not need a martin house. They are perfectly happy with a single gourd or Bluebird-style house. If you follow Dan Drew's instructions, then you have a good chance of attracting both Tree Swallows and a colony of Purple Martins. Click on:
http://www.drugfreeworkplace.com/~Dan/T ... GENCY.html
On my property, I have a colony of Purple Martins in multi-compartment housing (47 breeding pairs last season) and 5 pairs of Tree Swallows in single-compartment housing. Heaven on earth.
Purple Martins are our largest swallow - almost twice the size of Barnies and Tree Swallows (and they weigh 2 1/2 times more than other swallows). Martins are special in many ways. For more info, click on: http://www.purplemartin.org/update/ThirtWays.html
From that article:
The Purple Martin is the only secondary-cavity nesting Passerine in North America that is also a colonial nester. A "secondary-cavity nester" is an animal that nests in a pre-existing cavity because it doesn't have the equipment to excavate its own like a woodpecker does. A "colonial nester" is a bird that nests in dense groups. Bank Swallows are colonial cavity nesters, but they aren't secondary cavities; they excavate them. Barn, Cliff, and Cave Swallows are also colonial nesters, but they aren't secondary-cavity nesters - they construct their nests of mud.