the name suggests, a bird guard is used to prevent birds, vermin
and leaves or debris from entering a chimney or flue while allowing
the flue to function to its full efficiency.
They are available in many forms, but the two most popular are
the mesh type that normally fasten to the outside of the chimney
pot or the wigwag style that fits snugly inside the pot. The
bird guard must provide good ventilation and can also give added
protection against wind and rain.