Regular estimates of breeding populations are important for detecting declines and for implementing appropriate conservation measures in a timely manner. In Atlantic colonies, Leach’s Storm Petrels Oceanodroma leucorhoa are in decline at most colonies that have been surveyed. Consequently, the species has recently been uplisted from ‘Least Concern’ to ‘Vulnerable’ by the International Union for the Conservation of Nature. On Bon Portage Island, the largest Leach’s Storm Petrel colony in Nova Scotia, the last survey was completed in 2001. The aim of this study was to update the population estimate for this colony. Our results suggested that the current population is 38,916 ± 8,749 pairs, a 20% decline in 16 years. Several factors are most likely responsible for this decline, but loss of breeding habitat may be the principle cause on this island.

Leach’s Storm Petrels Oceanodroma leucorhoa are seabirds with a broad geographic range that breed in the northern hemisphere, primarily in the North Atlantic (Huntington et al. 1996). Recent surveys have detected sharp population declines at important breeding colonies in Newfoundland, including the world’s largest colony on Baccalieu Island (Robertson et al. 2006; Wilhelm et al. 2015; S. Wilhelm, pers. comm.). Furthermore, this trend is also occurring across the Atlantic, in some United Kingdom breeding colonies (Newson et al. 2008; Bicknell et al. 2009) and in Iceland (E. Hansen, pers. comm.). Consequently, in December 2016, the International Union for the Conservation of Nature (IUCN) uplisted Leach’s Storm Petrel from ‘Least Concern’ to ‘Vulnerable’ (IUCN 2016). Given alarming declines in important parts of the species’ range, it has become a priority to obtain up to date information about population sizes at larger colonies to monitor global population trends and inform conservation measures.

The largest colony in Nova Scotia, Canada, is on Bon Portage Island (43°28’N, 65°44’W; Huntington et al. 1996). Breeding surveys of Leach’s Storm Petrel were completed on Bon Portage in 1983 (MacKinnon 1988), 1997/8 (Oxley 1999), and 2001 (DS, unpubl. data) with population estimates ranging from 47,379 (95% confidence interval [CI] ±11,169) to 57,603 (95% CI ± 12,434). Large confidence intervals are common for population estimates of burrow-nesting species, because accurate assessment of nocturnal burrowing species is a challenge (Oppel et al. 2014; Rexer-Hubert et al. 2014).

Breeding Leach’s Storm Petrels mostly excavate nesting burrows in forested (spruce/fir) and meadow (fern, grass-herb) habitats (Stenhouse & Montevecchi 2000; Wilhelm et al. 2015) and the configuration of these vegetation types partly determines distributions of Leach’s Storm Petrel burrows on any given island. As such, a change in vegetation type over time may change the distribution of potential habitat for burrows and may influence breeding population size. For a survey to be representative, it requires either systematic sampling across all habitats, or stratified random sampling, and incorporation of ratios of each vegetation type in population estimates (Gregory et al. 2004).

The aims of this study were to repeat an island-wide survey of Leach’s Storm Petrels on Bon Portage Island, and compare results to previous survey efforts in 1983, 1997/8 and 2001. We used the same methods as Oxley (1999) and Shutler (2001) to facilitate comparisons and estimate population trends.

We are grateful to Nathan Brouwer, Andrew Collins, Nicole Cooper, Kyle d’Entremont, Ryan Fisk, Ali Gladwell, Kylee Graham, Karlie Maki, Jarrod Myers, Jessica Oakley, Tanya Pelerine, and Jenna Priest for field assistance. We thank Lee and Carlene Adams for logistic support getting to and while on Bon Portage Island. Funding was provided by The Seabird Group through a small research grant and Environment and Climate Change Canada through an Atlantic Ecosystems Initiatives grant to Bird Studies Canada. We thank Laura Tranquilla (Bird Studies Canada) and Sabina Wilhelm (Environment and Climate Change Canada) for comments during the preparation of this manuscript, Erpur Hansen (South Iceland Nature Research Centre) for his insights on Leach’s Storm Petrel population trends in Iceland, and two anonymous reviewers for valuable input.

Ambagis, J. 2004. A comparison of census and monitoring techniques for Leach’s Stormpetrel. Waterbirds 27: 211–215. [Crossref]

Bicknell, T. W. J., Reid, J. B. & Votier, S. C. 2009. Probable predation of Leach’s Storm-petrel Oceanodroma leucorhoa eggs by St Kilda field mice Apodemus sylvaticus hirtensis. Bird Study 56: 419–422. [Crossref]

Blackmer, A. L., Ackerman, J. T. & Nevitt, G. A. 2004. Effects of investigator disturbance on hatching success and nest-site fidelity in a long-lived seabird, Leach’s Storm-petrel. Biologocal Conservation 116: 141–148. [Crossref]

Blackmer, A. L., Mauck, R. A., Ackerman, J. T., Huntington, C. E., Nevitt, G. A. & Williams, J. B. 2005. Exploring individual quality: basal metabolic rate and reproductive performance in storm-petrels. Behavioral Ecology 16: 906–913. [Crossref]

Catry, P., Campos, A., Segurado, P., Silva, M. & Strange, I. 2003. Population census and nesting habitat selection of thin-billed prion Pachyptila belcheri on New Island, Falkland Islands. Polar Biology 26: 202–207. [Crossref]

Elliot, L. 1993. Bird Sounds from the Borror Laboratory of Bioacoustics: Leach’s Stormpetrel. Ohio Link (https://drc.ohiolink.edu/handle/2374.OX/46235?submit=Go&query=leachsstormpetrel&focusscope=2374.OX/30658&mode=search). Accessed 18 July 2017.

Fife, D. T., Pollet, I. L., Robertson, G. J., Mallory, M. L. & Shutler, D. 2015. Apparent survival of adults Leach’s Storm-petrels (Oceanodroma leucorhoa) breeding on Bon Portage Island, Nova Scotia. Avian Conservation and Ecology 10(2): 1. [Crossref]

Google Earth. 2017. Bon Portage Island 43.465 N, 65.7511 W. Accessed 19 September 2017.

Gregory, R. D., Gibbons, D. W. & Donald, P. F. 2004. Bird census and survey techniques. In: Sutherland, W. J., Newton I., & Green, R. (eds.) Bird Ecology and Conservation: A Handbook of Techniques. Oxford University Press, Oxford. pp. 17–55. [Crossref]

Huntington, C. E., Butler R. G. & Mauck, R. A. 1996. Leach’s Storm-petrel (Oceanodroma leucorhoa). In: Poole A. & Gills, F. (eds.) The Birds of North America, No. 233. The Birds of North America, Inc., Philadelphia, PA. [Crossref]

IUCN. 2016. Leach’s Storm-petrel. The IUCN Red List of Threatened Species (http://www.iucnredlist.org/details/22698511/0). Accessed 2 December 2016.

Lawton, K., Robertson, G., Kirkwood, R., Valencia, J., Schlatter, R. & Smith, D. 2006. An estimate of population sizes of burrowing seabirds at the Diego Ramirez archipelago, Chile, using distance sampling and burrow-scoping. Polar Biology 29: 229–238. [Crossref]

MacKinnon, C. M. 1988. Population size, habitat preferences and breeding ecology of the Leach’s Storm-petrel (Oceanodroma leucorhoa Viellot) on Bon Portage Island, Nova Scotia. MSc Thesis, Acadia University, Wolfville, Nova Scotia, Canada.

Mitchell, P. I., Newton, S. F., Ratcliffe, N. & Dunn, T. E. 2004. Seabird populations of Britain and Ireland: results of the Seabird 2000 census (1998–2002). T and A.D. Poyser, London.

Nettleship, D. N. 1976. Census techniques for seabirds of Arctic and Eastern Canada. Occasional Paper 25, Canadian Wildlife Service. 33 pp. (http://publications.gc.ca/collections/collection_2018/eccc/CW69-1-25-eng.pdf) Accessed 22 November 2018.

Newson, S. E., Mitchell, P. I., Parsons, M., O’Brien, S. H., Austin, G. E., Benn, S., Black, J., Blackburn, J., Brodie, B., Humphreys, E., Leech, D., Prior, M. & Webster, M. 2008. Population decline of Leach’s Storm-petrel Oceanodroma leucorhoa within the largest colony in Britain and Ireland. Seabird 21: 77–84. [Crossref]

Oppel, S., Hervías, S., Oliveira, N., Pipa, T., Silva, C., Geraldes, P., Goh, M., Immler, E. & McKown, M. 2014. Estimating population size of a nocturnal burrow-nesting seabird using acoustic monitoring and habitat mapping. Nature Conservation 7: 1–13. [Crossref]

Oxley, J. R. 1999. Nesting distribution and abundance of Leach’s Storm-petrel (Oceanodroma leucorhoa) on Bon Portage Island, Nova Scotia. M.Sc Thesis, Acadia University.

Parker, G. C. & Rexer-Huber, K. 2016. Guidelines for designing burrowing petrel surveys to improve population estimate precision. Agreement on the Conservation of Albatross and Petrels (http://www.acap.aq/en/resources/acap-conservation-guidelines). Accessed 6 January 2017.

Peterson, C. H., McDonald, L. L. Green, R. H. & Erickson, W. P. 2001. Sampling design begets conclusions: the statistical basis for detection of injury and recovery of shore-line communities after ‘Exxon Valdez’ oil spill. Marine Ecology Progress Series 210: 255–283. [Crossref]

Pollet, I. L., & Shutler, D. 2019. Effects of Great Horned Owls on a Leach’s Stormpetrel population. Wilson Journal of Ornithology. https://doi.org/10.1676/18-13.1

Pollet, I. L., Ronconi, R. A., Jonsen, I. D., Leonard, M. L., Taylor, P. D. & Shutler, D. 2014a. Foraging movements of Leach’s Storm-petrels Oceanodroma leucorhoa during incubation. Journal of Avian Biology 45: 305–314 [Crossref]

Pollet, I. L., Hedd, A., Taylor, P. D., Montevecchi, W. A. & Shutler, D. 2014b. Migratory movements and wintering areas of Leach’s Storm-petrels tracked using geolocators. Journal of Field Ornithology 85: 321–328. [Crossref]

Pollet, I. L., Leonard, M. L., O’Driscoll, N. J., Burgess, N. M., & Shutler, D. 2017. Relationships between blood mercury levels, reproduction, and return rate in a small seabird. Ecotoxicology 26: 97–103. [Crossref]

Ratcliffe, N., Vaughan, D., Whyte, C. & Shepherd, M. 1998. Development of playback census methods for storm petrels Hydrobates pelagicus. Bird Study 45: 302–312. [Crossref]

Rexer-Hubert, K., Parker, G. C., Ryan, P. G. & Cuthbert, R. J. 2014. Burrow occupancy and population estimate in the Atlantic petrel Pterodroma incerta: a comparison of methods. Marine Ornithology 42: 137–141.

Robertson, G. J., Russell, J., Bryant, R., Fifield, D. A. & Stenhouse, I. 2006. Size and trends of Leach’s storm-petrel Oceanodroma leucorhoa breeding populations in Newfoundland. Atlantic Seabirds 8: 41–50.

Ronconi, R. A., Allard, K. A. & Taylor, P. D. 2015. Bird interactions with offshore oil and gas platforms: review of impacts and monitoring techniques. Journal of Environmental Management 147: 34–45. [Crossref]

Sklepkovych, B. O. & Montevecchi, W. A. 1989. The world’s largest known nesting colony of Leach’s storm-petrels on Baccalieu Island, Newfoundland. American Birds 43: 38–42.

Stenhouse, I. J. & Montevecchi, W. A. 1999. Indirect effects of the availability of capelin and fishery discards: gull predation on breeding storm-petrels. Marine Ecology Progress Series 184: 303–307. [Crossref]

Stenhouse, I. J. & Montevecchi, W. A. 2000. Habitat utilization and breeding success in Leach’s storm-petrel: the importance of sociality. Canadian Journal of Zoology 78: 1267–1274. [Crossref]

Stenhouse, I. J., Robertson, G. J. & Montevecchi, W. A. 2000. Herring gull Larus argentatus predation on Leach’s storm-petrels Oceanodroma leucorhoa breeding on Great Island, Newfoundland. Atlantic Seabirds 2: 35–44.

Wilhelm, S. L., Mailhiot, J., Arany, J., Chardine, J. W., Robertson, G. J. & Ryan, P. C. 2015. Update and trends of three important seabird populations in the Western North Atlantic using a geographic information system approach. Marine Ornithology 43: 211–222.