The Azores do have a tendancy to hold vagrants. Becuase the birds are inevitably so far out of range a few individuals 'decide' they were lucky enough to get here in the first instance so 'decide' reluctantly or not at all to risk another journey. The White-winged Tern might have had a go but found it too difficult so reverse migrated back to somewhere it knew there was good feeding. I guess this is how these islands are colonized by birds in the first place. If these birds find a mate- it could be happy days with a new colonizing population starting up. Then over time (a lot of time) they begin to evolve in a new direction. That makes these birds inadvertent pioneers.
Also Roseate Tern in the harbour, 1st summer Great Black-backed Gull, good numbers of Kentish Plover in the quarry, two Sanderling, 1 Grey Plover and 6 Turnstone.