Two small silhouettes are floating on the tide. With apparent casualness, these “Bangor penguins”, as they’re known locally, dip their heads underwater to scan for butterfish. Their sleek bodies and monochrome plumage lend a little resemblance to their distantly related namesake, but the black guillemot is in fact an Arctic species of auk. Over a century ago, its presence here was sufficiently unusual for the Irish Naturalist science journal to remark that it was “a most extraordinary thing that such wild birds … should nest in such frequented places”. Times have changed; they are now a fixture here. Cliffs and boulder scree are the traditional nesting habitat but, after adopting crevices and drainage outlets in the harbour walls and jetties, this colony has accepted the nest boxes that have been planted among the marina’s piers. Local affection for the species extends to the harbourmaster’s refusal to allow boats to moor at Eisenhower Pier in case nesting black guillemots are disturbed. This tolerance is returned in kind – as I join their squatting line at the Long Hole, the smaller harbour here, they scarcely stir. Such handsome birds. Sunlight warms their velvet-black bodies to the darkest chocolate. Wings whirring, a newcomer comes in to land, its coral-red legs splayed. The disruption incites strident piping from the others, revealing the equally startling red of their gapes. One plunges into the waves. Flashes of white trace how their wings morph into flippers, thrusting smoothly through grey-green water with the legs’ rear-engine kicks. The seabird expert Shane Wolsey takes me to check on the housing stock under Commercial Pier. Seawater flickers from jade to black as our small inflatable putters along. The route is marked out by thick concrete columns topped with massive buttresses that support the ceiling’s concrete beams. Almost every ledge is furnished either with a wooden box or, in keeping with the architecture’s brutalist style, with a cellular breeze-block offering a one-up, one-down form of accommodation. So far, there are only dark-plumaged adults, huddling in their empty nests. Then a pied head eyes us nervously from a downstairs cell. We’re just in time. It’s a late fledgling that looks ready to fly.