Why birds are a problem
Jackdaws and other birds regularly nest in unguarded flues, and a nest can fully block a chimney — a serious fire and carbon monoxide risk if the fireplace is then used without checking.
How a bird guard works
A properly fitted mesh guard sits over the pot, stopping birds and larger debris getting in while still allowing the flue to draw and ventilate normally.
Fitting it correctly
The guard needs to be sized and fitted securely to the specific pot, since a loose or ill-fitting guard can itself blow off or let smaller birds through the gaps.