Cavity nesting birds would rather have a clean space, The only Sparrow that would nest in a cavity is one you dont want and I would be carefull about what Wrens you have. Go to www.sialis.org to find what birds you want to provide housing for and how to do it properly.
Blessings.
Lisa

It happens that I was recently reading "A Guide to Bird Homes" by Scott Shalaway, and he says:

"For years ... biologists argued that used nests harbor parasites that can infest subsequent broods of nestlings.
Recent research, however, suggests that when given a choice, bluebirds and other cavity nesters often choose a nest box that contains an old nest instead of an empty box. Apparently, a tiny parasitic wasp that controls blowfly populations lives and overwinters in old nests. Nests bulit on top of old nests may be more likely to succeed because the parasitic wasps are present and effectively control the blowflies. More field work is needed to confirm this hypothesis."

Interesting, but it may not apply in all situations. I'd bet there are other factors to consider.

I know that multiple birds will use established outside nests sometimes, and even across species. This summer we had a dove nest in an old robin's nest. We also had a catbird nest in another old robin's nest. So, I think it depends on the bird, and the suitability of the abandoned nest.

Yes, and they will sometimes build on top of another nest, and sometimes dis-assemble an old nest. But if you want to know which species does what, you'll need to consult an expert, and I'm not one of those.

Thanks for weighing in, everyone. Since I know absolutely nothing about nesting habits, all information was very welcome. I guess I'll leave the nests that are in the Pyracantha against the fence. Somebody can reuse them or there's plenty of room to build more if they want to. The one in the hanging birdhouse remains a question, tho. I never got a good look at who was going in and out except that it was very small. I have lots of Chickadees at the feeder so I assume that's who it was and I'd love them to nest again next year.