The problem with oak is the tannins - mostly in the bark- and it is only a real issue if the birds chew on it. Since you have beautiful gouldian and zebra finches I don't think that this will be an issue, however if you built this for say a macaw I'd be worried.
Congrats, I'm very jealous, been trying to get hubby to build an indoor cage for my chickens for a LONG time. My greenwing just got a new all stainless cage... so he's set.
one question: any plans for build in filtration?
Thanks for the info, I looked into the Tannins issue all over net and basically came up with the same conclussion that it was mostly in the bark that became the issue (we also asked our Vet), also the only oak on the inside of the cage is the face frame, and the front door, and the fact that the inside of the front is smooth (no place for the birds to hang onto) they would have a hard time chewing on it if they did chew wood (they would have to do it while in flight to start with.
If you mean air filtration, I thought about it but the cages (3 seperate in the unit) are open on the tops, sides, and bottom with stainless wire mesh. Once I start the design for the top shelf light box combo unit I might add some air flow and filtration, I plan on having an antique brass wire mesh to allow for air movement, about 5" tall or more between the top of the cages and the bottom of the light box. That's one of next years projects.
We don't have Zebras any more they breed like crazy, the wife has Green Singers, Cordon Blues, Shaft Tails, and Owl finches, (might have missed some) along with the Gouldian's: she trying to get a good clutch of Blue back Gouldians going now.