Wilson Cady, a regular contributor here, probably has some data on this.? I volunteer with him at the Steigerwald NWR near Washougal, and he has managed a purple martin nest box program there for many years.? I have been volunteering at Steigerwald for about 3 years and I would say the number of purple martins using natural cavities has gone up, though I have not been keeping data (but Wilson has).?

Here is a photo I took?in July of one of the dead trees that the martins are using at Steigerwald:

http://farm4.static.flickr.com/3220/3003865018_a757a91b44_b.jpg

Based on casual observation I would say the martins are using cavities in just about every large dead tree within about 1/2 mile of the nest boxes.

Here is a photo of one of the purple martin "bird bottle" poles, taken during a rain shower.? This was taken about 3000 feet from the previous photo.

http://farm4.static.flickr.com/3019/3003077473_971f729d46_b.jpg