The world’s largest breeding colony of Leach’s Storm-petrel Hydrobates leucorhous has declined

Abstract

SummaryDespite the global significance of the Leach’s Storm-petrel Hydrobates leucorhous colony on Baccalieu Island, Newfoundland and Labrador, Canada, the estimate of 3.36 million breeding pairs reported for 1984 by Sklepkovych and Montevecchi stands as the single published population estimate for the world’s largest colony. This study increases knowledge of this population by analysing data from additional independent surveys conducted in 1984 and 1985, and by updating the population status with a survey conducted in 2013. Population estimates were derived by extrapolating occupied burrow densities to the estimated occupied area of four main habitat types (heath, forest, grass and fern), which in turn were based on proportions of habitats observed in plots (1984 and 1985) or by using a Geographic Information System approach (2013). Based on these surveys, the Leach’s Storm-petrel breeding population size on Baccalieu Island was estimated at 5.12 ± 0.73 (SE) and 4.60 ± 0.42 (SE) million pairs in 1984 and 1985 respectively, representing estimates 37–51% greater than the original 1984 survey. While discrepancies among these estimates were largely driven by the way occupied areas were estimated, our study confirms that Baccalieu Island hosts the largest Leach’s Storm-petrel colony in the world. Results from the 2013 survey estimate the current breeding Leach’s Storm-petrel population at 1.95 ± 0.14 (SE) million pairs, representing a 42% decline over 29 years (-1.4% per year), relative to the original published estimate of 3.36 ± 0.12 (SE) million pairs. The most prominent change has occurred in the density of storm-petrel burrows found in forest habitat which dropped by 70% despite forest remaining the second most abundant habitat available to nesting storm-petrels on Baccalieu Island. The cause of this decline remains unknown and is likely multi-faceted. Future research focusing on demographic studies is required to understand what is driving the population decline of this internationally important colony.

Breeding population decline and associations with nest site use of Leach’s Storm-Petrels on Kent Island, New Brunswick from 2001 to 2018

“…Despite the decline in the north, the highest occupied burrow densities (0.091 ± 0.02 burrows/m²) on the island were observed in the open habitat (fern, grass, shrub/bramble) of the north. This shift in preference of habitat from forest to open found in our study contrasts with other research showing that storm-petrels nest at greater densities in forested habitat as compared to open habitat , Robertson et al 2006, Wilhelm et al 2015, although recent census work on Baccalieu Island has shown dramatic declines in the number of storm-petrels nesting in forest as well (Wilhelm et al 2019). As burrow densities are directly correlated to the suitability of habitat for storm-petrel nesting (Robertson et al 2006), these results suggest that changes in suitable habitat may have occurred.…”

“…The annual rate of decline observed in the predominantly forested northern region aligns closely with annual rates of change observed in open habitat in a major Newfoundland colony Montevecchi 1999, Wilhelm et al 2015). Moreover, declines observed over the past ~30 years at the largest colony for this species (Baccalieu Island, Newfoundland and Labrador) show annual rates of decline of 1.4% per year and the largest declines were observed in forested habitats (Wilhelm et al 2019), similar to our results on Kent Island. Given these similar declines among multiple colonies and the very high nest-site fidelity observed with this species (Pollet et al 2019), dispersal is an unlikely mechanism for the decreasing population on Kent Island.…”

“…Though small in comparison with more northern storm-petrel colonies in Atlantic Canada (Wilhelm et al 2015(Wilhelm et al , 2019, the Kent Island population currently represents one of the two largest colonies (Pollet and Shutler 2018) south of Newfoundland, and the largest colony in the province of New Brunswick. This study documents a substantial decline across a span of 17 years when many other colonies across Atlantic Canada have observed precipitous declines (Robertson et al 2006, Wilhelm et al 2015.…”

“…The Leach's Storm-Petrel (Hydrobates leucorhous; hereafter storm-petrel) is an abundant pelagic seabird belonging to the Procellariform order that nests in burrows on islands throughout the North Pacific and North Atlantic oceans, with the majority breeding in Atlantic Canada (Pollet et al 2019). Population declines have been observed at many major colonies throughout this region in the past 20-30 years (Robertson et al 2006, Wilhelm et al 2015, including the largest colony in the world on Baccalieu Island, Newfoundland and Labrador (Wilhelm et al 2019). These declines have resulted in storm-petrels being uplisted globally from "Least Concern" to "Vulnerable" status (BirdLife International 2018), and because the reasons for these declines are largely unknown, monitoring population trends at colonies provides vital information for the management and conservation of this species.…”

Linking 19th century European settlement to the disruption of a seabird’s natural population dynamics

“…We also acknowledge that the distributions of occupied burrow densities within habitats tend to show a right skew, and therefore the Delta method may not fully capture the true shape of the underlying distributions. In general, we followed the approach in Wilhelm et al (68), in which the number of occupied burrows in each plot is estimated from the data, and randomization procedures are used to propagate uncertainty through to the final estimates of colony size. We only reestimated surveys from 2004, 2008, and 2011 because there was sufficient data for a meaningful population estimate.…”