I just saw this thread, and all great answers have been given already. It's really nice to have a second cage that is able to be kept outside for your birds, my mom has a huge, old, stainless-steel cage outside for her Grey, it's MASSIVE and he absolutely loves being outside in it, he can almost fly in it it's so large. I'm pretty sure she got it pre-owned on Craigslist, which is exactly where you should look for one, especially if you want to get a Stainless-Steel or Galvonized-Steel one so that it won't rust, as they are extremely expensive brand-new. It's amazing but I am constantly on Craigslist looking for good deals on bird cages, stands, toys, etc., and I see tons and tons of massive, huge cages that basically cost nothing compared to what they cost new, usually they are between $100-$200, but I hardly ever see any small/medium size bird cages at all. i think that people just want to get the massive cages out of their homes/garages/basements, they do take-up a lot of space. The other way to go is to build an outdoor Aviary of sorts, which costs little to do and are easy to build with some some 2x4's and some galvonized hardware-cloth, if you're at all handy with a saw, a drill, and a staple-gun.
***Just one thing I wanted to mention to you if you do decide to get a cage that is kept outdoors (meaning actually outside and not on a sun-porch or other enclosed area that is protected from the elements)...Do be sure that you check the bottom of the cage each time before you put your bird inside the cage, as bugs tend to crawl into the bottom of my mom's cage all the time, some nasty bugs too...She just keeps newspaper at the bottom of the cage as a liner, and knowing my mom I'm sure that she changes the newspaper out every single day, as she quite-literally and seriously, no joke, suffers from OCD when it comes to cleaning. Yet insects still crawl into the outside cage on a regular basis and she has to be sure that she checks every time before putting him inside the cage. Just an FYI. I'm sure it has to do with the lingering scent of either the bird poop or the bird pellets/seed/veggies/fruit/nuts she puts in the cage for him which attracts the insects, even though she removes all of the food from the cage each time she brings him back inside and cleans all poop that is inside the cage, along with the newspaper, the insects can still smell it.