A pale powder-blue-and-white seabird with a faint grayish dark “M” across the upperwings. Note the dark blue “thumb mark” collar, the blue eye-stripe, the white eyebrow, the white throat, and the grayish cap. Restricted to two known breeding islands, Saint Paul in the South Indian Ocean, and Gough in the South Atlantic. Almost identical to Antarctic Prion, but bulkier and more heavy-chested, with a slightly wider bill. All but inseparable from Salvin’s Prion, with which it shares the breeding island of Gough, and with which it was once considered one species; MacGillivray’s is incrementally broader-billed, but birds seen at sea may not be identifiable.