Abstract: The device (30) comprises a triangular hollow section having a apertures (5) in its mutually perpendicular sides. It is mounted on a ledge (L) of a building and has an upper surface which is inclined sufficiently to prevent birds, paticularly pigeons, from roosting thereon and depositing droppings on the building and surrounding pavement. It can be attached to the ledge by applying adhesive foam filler to the ledge surfaces (L, V) and pushing the device against the foam so as to force the foam through the apertures, whereupon it expands within the interior of the section and grips the inner surface thereof. The device can also be mounted on the exterior horizontal surfaces of beams and girders, e.g. under railway bridges.