Winter habitat associations of Purple Sandpiper (Calidris maritima) and Harlequin Duck (Histrionicus histrionicus) in Atlantic Canada

Abstract

Canada's coasts provide vital habitat for congregations of birds throughout the year, including during the nonbreeding period. On the Atlantic coast, the Purple Sandpiper (Calidris maritima) and Harlequin Duck (Histrionicus histrionicus) are both known for their hardiness through the winter, electing to rest and forage along some of the most turbulent, rocky coastlines and nearshore waters. Despite their resilient nature, both species also face threats on the overwintering grounds from human activities and associated habitat degradation. The aim of this study was to identify overwinter habitat characteristics for Purple Sandpiper and Harlequin Duck in Atlantic Canada, with the ultimate objective of improving survey designs whereby both species can be efficiently targeted within their overlapping winter range. Using helicopter-based survey data covering >13,000 km of coastline between 2011 and 2016, winter habitat was characterized based on both traditional (i.e. publicly available science-quality products) and non-traditional (i.e. Atlantic Canada Shore Zone Character dataset) data sources. In the southerly extent of the Canadian winter range, densities of Purple Sandpiper were 0.045 and 0.2 groups·km−1 surveyed, respectively. Similarly, for Harlequin Duck, densities were 0.11 and 0.06 groups·km−1 surveyed, respectively. Both species were found there primarily in areas with lower intertidal zones classified as bedrock platforms (89% and 88% of observed locations, respectively) with the remainder as beach or tidal flats, while the backshore was mostly classified as cliff (67% and 83%, respectively). In the northern extent, Harlequin Duck were generally uncommon, while Purple Sandpipers were detected at densities of 0.05 groups·km−1, using a greater diversity of lower intertidal zone forms (46% platform/ledge, 22% anthropogenic structure, 19% cliff, 7% low islets). Probability of Purple Sandpiper presence was highest when natural barriers (e.g. headlands) and human activity were both present, increasing with higher mean tide height and higher coastline ruggedness. For Harlequin Duck, probability of presence was most strongly influenced by oceanography, increasing with greater maximum water depth within 1.5 km of shore and lower mean monthly sea surface temperature. Notwithstanding the coarse spatial scale of the data and in turn our analyses, these results confirm the importance of winter onshore habitat associations shared between the Purple Sandpiper and Harlequin Duck in Atlantic Canada, as well as revealing differences related to their foraging ecology. Continued and improved collection of baseline survey information on habitat associations for both species will be crucial to managing population health, habitat needs, and range shifts in the face of future habitat and climate uncertainty.