Just a side note:
The last I knew, you can't keep any bird of prey (like an owl, hawk, eagle, kestrel, or falcon) legally in the United States without a rehab permit (as in the bird is hurt and can never be returned to the wild) or a falconer's license. Even owning a feather from a raptor is an offense. The only people allowed to own found plumage from birds of prey are Native Americans.
Truth. It's a federal offense. (and btw, kestrels are a type of falcon; add in accipiters, kites, vultures and harriers for the full list of 'groups' of raptors

) Normal people in the US can't have any raptor part without a permit, and normal people can't get permits. Native Americans are supposed to get permits as well, and they have to be for ceremonial/religious/whatever-the-right-word-is purpose. Rehabbers can only have a bird while it's not a candidate for release; certain institutions can acquire permits for individual birds for educational purposes (e.g., rehab centers and zoos) but there has to be some kind of proof that the bird can't be released, and you can't do anything purposefully to the bird to make it non-releasable (e.g., you can't imprint a baby on purpose, and if a baby at your facility gets imprinted, you have to send it to a different facility to live out its life)
Though I'm told that one can have a non-native raptor without a license, it'd be difficult and expensive to acquire one. Someone told me that in England, it's possible to acquire a raptor as a pet. Which IMO isn't a good idea...
They're great birds, and I really enjoy working with them, but they're not something the average person could have easily. They are pure carnivores, and the majority tolerate humans for food only. Yes, they can be trained to do things, but they're not the most intelligent birds. Harris Hawks are sort of an exception to most truisms about raptors (they are social, with a structure sort of like a wolf pack, have a lot of personality, and at least the one imprinted bird I've met seems to enjoy humans), but again, you need to be a falconer or rehabber to have one in your posession. Their housing requirements are a little different from other birds, though not too different, I guess. You can't be squeamish about feeding small furry or feathered creatures to them. Especially the larger ones can injure you if you don't know what you're doing and/or not paying attention. Most are fairly good-sized birds... bigger than most parrots people keep... so they need correspondingly more space and more food (unless you're a falconer who prefers to keep his birds tied +/- hood, rather than free lofting... both are valid methods, but I have more experience with free lofted birds)
Anyway. That's probably fairly offtopic... but raptors are what got me interested in birds in general, and they're still my favorite group by far.
Siren, thanks for that link, I'll probably start browsing that forum at least occasionally
Ummm, original question. It's hard to disinfect trees and dirt/rocks/whatever is going to be on the ground. You need to have good drainage so you can hose down the rocks/whatever under the tree to wash off the bird poop. Most zoos replace the top however many inches of dirt in their enclosures at least once a year because of that disinfection nightmare. At a minimum, keep the birds in a conventional cage and ensure that they're disease and parasite free before turning them loose in the tree area.