The nocturnal habits and remote breeding location for Manx Shearwater make it a difficult species to monitor. The global population of Manx Shearwaters likely exceeds 1 million individuals (Brooke 2004), with more than 390,000 breeding pairs (BirdLife International 2018k). Middle Lawn Island, Newfoundland, is the only known Canadian breeding colony, though a recent study showed that the species occurs on multiple islands along the south coast of the Burin Peninsula, Newfoundland (Roul 2010). Annual observations (e.g., female call rates, bird banding, and visual observations) of the Middle Lawn colony by Environment and Climate Change Canada and other researchers suggest that the colony size (< 100 birds in 2009) has likely declined since its discovery in 1977 (Robertson 2002, Roul 2010, Fraser et al. 2013), when it was estimated at around 360 birds (Storey and Lien 1985). Few if any birds have been detected at the colony over the past few years; they now appear to be breeding on Grand Colombier Island, which is just off the Newfoundland coast but part of the French colony of Saint Pierre et Miquelon. On the Pacific coast of Canada, Manx Shearwater is a rare but regular component of the marine avifauna (Force et al. 2006), and appears to have increased dramatically since the 1990s (Force et al. 2006, Kenyon et al. 2009). The status of Manx Shearwater in Canada prior to 1977 is not well known, but the species was likely never an abundant breeder in North America. Overall, these observations suggest a large decrease in the breeding population in Canada since 1970. However, because information from some breeding locations is unclear or unknown, and the populations on the two coasts have potentially different trends, the reliability of this assessment is considered low. The species is below the lowest acceptable level relative to its national population goal, which is the mean abundance of the early 1970s.
