It only takes having his glasses stolen by a flock of magpies once to make Giles wary of birds forever. He's absolutely certain the ones back home weren't this evil.

"Right. Um." He glances around a bit helplessly, before noticing something on the outside of the aviary dome. "Just hang on. I'll be back in a minute." He hurries off.

Presumably back when this castle was occupied, the birds were actually taken care of by a groundskeeper. The groundskeeper is long gone, but he left a shed behind, and in that shed is a variety of equipment one might need in order to take care of birds. Including a ladder.

It's a bit awkward, but Giles manages to retrieve it and bring it back to the tree.