Bird foot warmers

On clear days In bleak icy weather road traffic often wears patches in the ice to expose the tarmac to a watery winter sun. You sometimes see a few large birds standing on the tarmac a if they are warming their feet.
I would like to give my garden birds the same foot warming treat.

Does anyone know what type of insulation board can be safely used?  Polystyrene tiles would seem to be the practical option but I fear birds pecking at seeds on the polystyrene could pick up tiny crumbs of polystyrene which I imagine could kill them if swallowed.  Any better ideas please?