According to Lusitania (1972):
Inglefield's other brain child was to attempt to train seagulls to defecate on periscopes, and for a short while a remote corner of Poole harbour in Dorset was littered with dummy periscopes and hopefully incontinent seagulls.
"Inglefield" means Admiral Frederick Inglefield.
Another 1972 book, The killing time: the U-boat war, 1914-18, says:
Another suggestion actually investigated by the Board of Invention and Research involved training seagulls to follow periscopes
The 1943 book The Death Ray Man: The Biography of Grindell Matthews, Inventor and Pioneer says:
The Admiralty advisers, barren of ideas, grasped frantically even at the feathers of the seabirds' tails, and could see nothing incongruous in teaching seagulls that periscopes meant food. Their plan was to release the birds, who, conscious of their great part in Britain's anti-submarine defence, would immediately gather around the periscope of any enemy underwater craft which broke through the waves of the English Channel!
There is a detailed February 1918 article To Train Seagulls as "Spotters" for Submarines The Guide to Nature, volume 10, pages 276-277 which instead credits Dr. A. D. Pentz, Jr. of Long Island, New York, with the idea. So it seems this idea was considered for US and UK waters.