How to stop a metal bird bath from cracking in winter (and whether you even need to)
Quick answer: if your bird bath is cast iron or powder-coated steel with a lipped bowl, you do not need to move it, but you do need to stop water sitting in it through a hard freeze. A full bowl of ice expands roughly 9 percent by volume, and repeated freeze-thaw cycles open the seams at the base faster than any single cold night. Empty it, tip it face down, or drop in a length of closed-cell foam to take the expansion.
This applies to any hollow metal garden piece that holds water: a bird bath bowl, a hanging planter tray, a lantern base that collects rain. The failure mode is the same in all of them, and it is the water, not the metal, that does the damage.
Why a full bowl is worse than an empty one
Water is unusual in that it keeps contracting as it cools right down to about 4 C, then reverses and expands as it turns to ice. That reversal is the whole problem. A bowl that is completely full has nowhere to push the expanding ice, so the pressure goes into the bowl wall and the base seam. A bowl that is two-thirds full lets the ice dome upward instead.
Plastic and resin baths crack in a single winter. Cast iron and steel usually survive the ice itself, but the expansion works on the joint where the bowl meets the pedestal, and that joint is where they fail. On a pedestal bath the load is also levering the base, so a bowl that ices solid acts like a wedge.
Three options, ranked by effort
Easiest: tip the bowl. If the bath is a hanging or wall-mounted type, lift it off the bracket and store it in a shed or garage. This is the only option that removes every risk, and for a powder-coated steel bath it is what I would do in zone 5 or colder. A typical wall-mounted bowl is around 10 to 12 inches across and weighs 1 to 2 lb, so it comes off a bracket in seconds.
Middle: keep it out, drain it. For a heavy cast iron pedestal bath you are not moving, empty the bowl after every thaw and turn it face down or fit the bowl with a drain plug left open. The pedestal itself can stay outside; cast iron handles frost better than it handles standing water.
Least effort, still works: float a buffer. A 12-inch length of closed-cell foam pipe insulation, or even a partly deflated ball, floating in the bowl gives the ice somewhere to expand. This is a real method used on stock tanks, not a folk trick. It does not help if the bowl is already cracked, and it does nothing for the pedestal joint.
What if the bird bath is already cracked?
A hairline crack at the base seam can be repaired with a two-part epoxy putty rated for metal, applied to a surface you have cleaned back to bare metal with a wire brush. Key the edges out slightly with a file first so the putty has something to grip. Let it cure for the full 24 hours on the pack, not the minimum.
Two things to know before you bother: if the crack runs more than about a third of the way around the bowl, or if the pedestal itself is cracked, the repair will not hold through another winter and the bath is done. And never use silicone sealant on a bird bath, because birds will pick at it and it is not rated for potable water contact.
Does the bath need to come in if it is “frost proof”?
“Frost proof” on a garden product means the material will not shatter from a single freeze, not that it can hold water all winter. Apply the same logic to any hollow metal garden item with a water-holding bowl or tray. If you want to see the shapes this applies to, our own hanging planters and wall-mounted bowls all leave from the workshop drained and dry for this reason.
If you are in zone 7 or warmer and your winters mostly hover just under freezing rather than sitting at -10 C for weeks, draining after each thaw is usually enough and you can leave the bath standing.