Block the route up from underneath first
Most aviary rat entries happen from below. Rats tunnel readily, and a finch aviary floor of bare earth, loose slabs, or gravel is an open invitation to emerge inside at night exactly where ground-feeding finches roost low. The reliable fix is a solid base: a concrete slab, or welded mesh laid across the whole footprint and fixed to the frame before flooring goes over it, so a tunnelling rat meets steel in every direction. Extend the barrier slightly beyond the walls or turn its edge downward, because rats test the perimeter first. If your aviary already stands on earth, retrofit mesh panels section by section rather than accepting the risk for another season.
Choose mesh by the gap a young rat cannot use
Walls and roof need welded mesh with openings small enough that even a juvenile rat cannot squeeze through — rats exploit gaps far smaller than their body size suggests, and they gnaw at any edge that gives them purchase. Avoid chicken wire entirely: it keeps birds in but a rat bites through it, and avoid plastic netting for the same reason. Check the roof as carefully as the walls, since rats climb well and overhanging branches or a fence against the aviary give them a bridge to the top. Fit a mesh skirt or flashing where the walls meet the base, the junction where gaps most often open as timber shifts.
Seal the honest gaps: doors, pipes and corners
Walk the aviary slowly with a pencil and test every gap it will not pass through: door frames that have dropped, the space under the safety-porch door, cable and water-pipe entries, and corners where panels meet. Fill structural gaps with folded mesh, metal plate, or cement — never expanding foam or timber alone, both of which rats gnaw out in a night. Doors deserve special attention because they move daily: a self-closing fit, a bolt top and bottom, and a mesh-covered threshold strip will stop the classic under-door entry that leaves finches dead at dawn with no obvious hole anywhere.
Remove food and cover, the two things rats come for
Proofing works far better when the aviary stops advertising itself. Lift seed hoppers and ground trays at dusk or switch to feeders rats cannot sit in, sweep spilled seed from the floor daily, and store all feed in lidded metal bins outside the aviary, never in bags inside it. Clear the outside perimeter too: stacked pots, log piles, dense shrubs, and compost against the walls give rats hidden runs right up to the mesh. Keep a bare, visible strip around the structure so any new digging or gnawing shows at once on your morning round.
Check like a rat, regularly, and act on the first sign
Inspect monthly and after storms: fresh soil pushed up at the base, greasy rub marks along a run, droppings, or a single chewed feather pile demands action that day, not at the weekend. Ground-roosting finches are at their most vulnerable on cold nights when they sit low and still. If one bird is taken, assume the route remains open until you find it. For a thorough assessment of an older structure, a builder who knows aviaries, found through the KinsLe Pet Services Directory, can spot the weak junctions a keeper has stopped seeing.