A razorbill, Alca torda, in winter/non-breeding plumage.
The winds and tide had brought small fish in near the shore. The gulls were diving for them when they came too close to the surface. They had also attracted more serious fishers from the auk family.
I was at Pettycur harbour this afternoon and surprised to see two razorbills and two guillemots fairly close to the harbour wall. I spotted a single razorbill yesterday as well.
Their main distinguishing feature is their bill and I have included an 'extra' to illustrate this (guillemot on the left, razorbill on the right).