Unidentified Flying Object

Unidentified, because I simply don't know what it's called. Please, if someone out there knows what it is, do let me know. I should know but cannot remember and can only find one picture of him on the internet. Naturally the picture has no caption.
I was down by the sea shooting crashing waves and spray this morning, down by where these little guys live. After lots of waves I turned my attention to the birds. Three frames later I knew I had my photo for the day. Came home. Job done.