Shorebird surveys
Surveys were conducted as part of the Arctic PRISM shorebird monitoring program (Bart and Johnston 2012). These surveys were designed to estimate population sizes and habitat relationships for shorebirds breeding across the whole of the North American Arctic, only recently completing the first round of surveys that covers each of the 19 PRISM survey regions. Here, we include data from the first two regions to be surveyed for a second time in the Canadian Arctic. We surveyed 25,000 km2 in the Rasmussen Lowlands, a coastal plain wetland complex located at the base of the Boothia Peninsula, designated as a Ramsar Wetland of International Importance (Carp 1980) and a Canadian Important Bird Area (Aguilar Mugica et al. 2009). We also surveyed a 25,000 km2 area in Foxe Basin including the coastal wetlands and inland dry areas on Prince Charles Island, several nearby islands, and a portion of western Baffin Island (Figure 2). Both regions, approximately 750km apart, include a wide variety of land cover types, such as intertidal flats, low-lying salt marshes, flat marshy tundra, heath tundra, dry grasslands, beach ridge complexes and unvegetated broken shale (Bart and Johnston 2012). Each region was stratified by habitat type, and plots were randomly sampled within each habitat type, with a greater proportion of plots located in wetland habitats (Bart and Johnston 2012). We included habitat type as a factor in our models, but did not use a design-based approach for calculating the means and variances. These two regions are well suited for the current study because they are high quality shorebird habitat, containing a good diversity and abundance of shorebirds, and because they are located at mid-Arctic latitudes, therefore hosting both colder- and warmer-breeding species.
Each region was surveyed twice, 22-25 years apart. The Rasmussen Lowlands region was surveyed in 1994-95 and 2019, while the Foxe Basin region was surveyed in 1996-97 and 2019. While the earlier surveys of each region were completed over sequential two-year periods, the later surveys in both regions were completed simultaneously in one season owing to increased funding and resources. We excluded the 1996 data because of unusually poor weather and flooding that disrupted the normal behaviour of the birds (Bart and Johnston 2012). Each plot was surveyed once per survey period (1994-1997 and 2019). Surveys were conducted between June 18 and July 15, during late courtship and early incubation, when breeding territories could be identified from the birds’ territorial displays. The detectability of birds is believed to change throughout the season, as birds settle on their nests and incubation progresses.  The late-courtship to early incubation period maximizes detectability.  Detectability could vary slightly even within this time window, and most certainly varies due to weather and other factors.  This variability in detection could introduce imprecision into the estimates.  However, we do not expect that this would contribute to bias because surveys in both time periods occurred over a number of days during late courtship-early incubation. In both time periods, 28 plots were surveyed in the Rasmussen Lowlands and 36 plots were surveyed in Foxe Basin (64 plots total). Plots were 16 ha (400m by 400m). Surveyors recorded the proportion of each plot covered by upland habitat (e.g. mesic grasslands or heath habitats, sparsely vegetated xeric habitats) or lowland habitat (e.g., hydric areas of grass/sedge, polygonal wetlands, saltmarsh). In our analyses, we categorized a plot as upland or lowland depending on which habitat type was predominant.
Following PRISM protocols (Bart and Johnston 2012), surveyors searched the plot walking straight-line transects, covering a breadth of 50m with the observers situated 25m apart, using a GPS to ensure complete coverage of the entire plot. Surveys took approximately 90 mins. Observers recorded the number and species of all birds observed within each plot. Altogether we observed 12 species of shorebirds: American Golden-Plover (Pluvialus dominica ), Baird’s Sandpiper (Calidris bairdii ), Black-bellied Plover (Pluvialis squatrola ), Buff-breasted Sandpiper (Tryngites subruficollis ), Dunlin (Calidris alpina ), Pectoral Sandpiper (Calidris melanotos ), Red Knot (Calidris cantus ), Red Phalarope (Phalaropus fulicarius ), Ruddy Turnstone (Arenaria interpres ), Semipalmated Sandpiper (Calidris pusilla) , Stilt Sandpiper (Calidris himantopus ), and White-rumped Sandpiper (Calidris fuscicollis ).