These are known as 'fat cages'.
You place a 'fat cake' in it for the birds to omnomnom on in winter.
You can make them yourself (as my dad does). All you need is an old foil pie tray. Pour in any cooking fat from grilling bacon, sausages etc into it and add dried fruit, breadcrumbs, bits of pastry, meat rinds and bits of bird seed/ seeds in it and wait for it to set (you can layer it up as you go along). Once you have a full tray, let it set completely and peel the 'fat cake' out of the tray and pop it in the cage
Birds love love love fat cakes
e: in before 'The birds around here love a fat cake wink, wink' etc etc
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