Sidney Island Shorebird Surveys, British Columbia

The surveys are conducted along the sandspit and within a 96 ha lagoon that encompasses mudflats, eelgrass beds, and saltmarsh at the northwest end of Sidney Island, located in the Strait of Georgia, British Columbia. The survey counts numerate two species, Western Sandpiper (Calidris mauri) and Least Sandpiper (Calidris minutilla), during a portion of the southern migration period (July, August, and early September), and have been conducted intermittently since 1990.

Sidney Island (48°37’39’N, 123°19’30”W) is located within the Salish Sea (Strait of Georgia), 4 km off the coast of Vancouver Island in southwestern British Columbia, Canada. Southbound Western and Least Sandpipers stop over within Sidney Spit Marine Park (part of the Gulf Islands National Park Reserve), roosting and feeding along the sandspit and within a 96 ha lagoon that encompasses mudflats, eelgrass beds, and saltmarsh at the northwest end of the island. These species are the most numerous shorebird species using the area during southern migration. Adults precede juveniles, arriving at the end of June and throughout July. Juveniles reach the site in early August, with their numbers trailing off in early September. As a result, the site experiences a transition from purely adult to purely juvenile flocks over the course of the season.

Daily counts, beginning in early July and ending in early September, were conducted in 1990 and from 1992-2001 (no counts occurred in 1991). Effort was reduced to weekly surveys between 2002 and 2013. Over the entire monitoring period median survey effort was 9 counts annually. All counts were conducted at the low tide of the day, when shorebirds were feeding in the exposed mudflat of the lagoon. Observers walked along the shore of the lagoon stopping periodically at vantage points to look for birds. For ease of data recording and to keep track of individual flocks, the survey area was divided into separate units demarcated by prominent geographical features.

Counts were made with the unaided eye, through binoculars, and with a 20 – 60x zoom spotting scope, depending on the proximity of the birds. All individuals in small flocks were counted and individuals in large flocks were estimated by counting in groups of 5, 10, 50 or 100 according to flock size in each successive field of view across a scan of the entire flock. Between 1990 and 2001, when daily counts were conducted, birds were occasionally counted more than once in a day. The largest count value obtained was used as the daily estimate for these days.

For smaller flocks, we were able to identify all individual birds to species and age-class. Sub-samples from larger flocks were also aged (adult or juvenile) and identified to species. Birds were aged by plumage characteristics. Adult Western Sandpipers are distinguished from juveniles by the dark chevron markings present along the sides and breast. Juvenile Least Sandpipers have a buffy breast compared to the distinct, darker one of the adult, and juveniles have bright rufous scapulars compared to the drab feather-edges of the adults. In both species, juvenile plumage appears brighter and cleaner than adult plumage, which is more worn and tattered.