Western Sandpiper an indicator of wildlife sustainability in the Georgia Basin

What is Happening?

The small sparrow-sized Western Sandpiper (Calidris mauri) is the most numerous shorebird on the Pacific Coast of North America. Its abundance in southwestern British Columbia has been monitored by Environment Canada since 1992. In the Georgia Basin, annual counts are made on the mud and sand flats of the Fraser River delta during their spring migration and on Sidney Island during their southbound fall migration (see map). These census data show that declines have been occurring in both spring and fall migrants. As can be seen in the graphs below, these declines have been significant for spring migrants and for the juvenile southbound fall migrants. Declines have also occurred among the fall migrating adult birds but they are not statistically significant.

Figure 1. Peak number of Western Sandpipers counted on Roberts Bank, Fraser River delta (source Butler and Lemon, 2001). Numbers reported represent the peak single day counts. Counts were made using three methods. When flocks numbering into the hundreds of thousands of birds are counted, unknown but likely large estimation errors can be introduced.

Figure 2. Total number of juvenile Western Sandpipers divided by observer days on Sidney Island in August (source Butler and Lemon, 2001). Counts have high precision as flocks were generally small enough to count individual birds.

GRAPH DATA

Other studies by the Canadian Wildlife Service dating back to the 1960’s reveal that many of the North American continental shorebird populations have fallen off in numbers (Morrison, 2001). 

Why is it Happening?

There are several possible reasons why Western Sandpipers are declining. The survival and sustainability of Western Sandpiper populations depends on the quality of a chain of important migratory stop over sites linking arctic breeding grounds in western Alaska and eastern Siberia with tropical wintering locations in the southern USA and Central and South America. These long distance migratory flights require rapid accumulations of fat at each stop over site. In North America, major migratory stop over sites include San Francisco Bay, Grays Harbor, the Fraser River delta, the Stikine River and the Copper River delta. The important mud and sand flat habitat at some of these sites has been degraded by sewage (Panama) and industrial contaminants (San Francisco Bay). Western Sandpipers feed on invertebrates and a nutritious micro-thin layer on the surface of the inter-tidal mud flats.

The changes to ocean currents during the El Nino years of 1991 and 1997 may also have affected food supplies at winter sites and hence shorebird survival. Scientists at the Canadian Wildlife Service (CWS) used a computer model of the migration to estimate the effect of food shortages on sandpipers. The model calculated the number of young that an adult sandpiper would be expected to raise each year depending on when it began to migrate. The results showed that a well fed sandpiper could delay its migration by as much as a month without showing much of an effect. However, a not-so-well fed sandpiper could only delay migration by 20 days before it could expect to have an affect on the number of young it might raise. The implications are that we might expect to see declines in the number of sandpipers being raised each year if El Nino or climate change affects food supplies on the winter quarters which then stalls the migration by a few weeks. CWS began to notice declines soon after the 1991 El Nino event. It is also possible that breeding failure resulted in low recruitment of first time breeders, a documented cause for population declines in shorebirds (Gratto-Trevor et al., 1998).

Within the Georgia Basin, the decline in juvenile fall migrants on Sidney Island would lend support to breeding failure as a possible cause. Other potential factors include the three fold increase in predatory falcons within the Fraser River delta area since the mid 1980’s. Predation risk may also affect the shorebirds' use of key feeding areas. Fraser River delta changes have included habitat loss due to port facilities on Roberts Bank and urbanization of farmland. Roberts Bank has also been affected by ferry and port causeways which have altered nutrient distribution to the mud flats from the Fraser River. Increases in the estuary’s human population have also likely contributed to inter-tidal habitat degradation including the contamination of food supplies. The uptake of toxic chemicals may effect sandpiper navigation and breeding success.

Why is it Significant?

The entire world population of 3.6 million Western Sandpipers migrate along the British Columbia coast and hundreds of thousands briefly stop in the Georgia Basin. The Fraser River delta is one of the six major links in the chain of migration stop over/refueling sites in western North America and is crucial to the survival and sustainability of this shorebird. On single days, up to 500,000 of these birds use the Fraser delta. Monitoring their population numbers can help determine the long-term changes that may be related to loss and degradation of habitat, toxic chemical pollution, birds of prey and global climate change.

What is Being Done?

The Canadian Wildlife Service and the Centre of Wildlife Ecology at Simon Fraser University (SFU) have been studying the Western Sandpiper throughout its range. Mark-recapture techniques have also been developed for Western Sandpipers at SFU. They are being applied by University of Panama researchers on winter quarters in Panama and Mexico to help provide a means of detecting shifts in survival by various age classes. Studies of migration site use and breeding ground recruitment are ongoing in collaboration with US Fish and Wildlife biologists in Alaska and at the Point Reyes Bird Observatory in California. Results of this work can help point to where in the annual cycle and possibly why the impacts are occurring.

Other collaborative research at SFU is looking more closely at how the sandpipers refuel at each stop over site and how the sites are used. By employing computer modeling, the importance of atmospheric conditions on migration and the costs to reproductive success of late arrival at breeding grounds is being evaluated. For example, there is evidence to indicate migrating small birds are strongly influenced by upper atmospheric wind conditions. The models are examining survival estimates by factoring in things like refueling rates, predation risk and sex and age class.

Most key migration stop over sites have been identified and a level of protection has been put in place to help secure some of them. In the Georgia Basin, the BC Ministry of Environment, Lands and Parks has declared Boundary Bay and Sturgeon Bank Wildlife Management Areas. The western tip of Sidney Island is designated as a 400 hectare provincial park (Sidney Spit Marine Provincial Park). It provides protection for the key habitats of sand beaches, tidal flats and salt marshes used by migrating Western sandpipers. Although exceptionally large number of sandpipers gather each spring at Roberts Bank on the south shore of the Fraser River delta, it has not yet been secured as a wildlife area.

Because no simple answer for the declines is evident, multidisciplinary research is continuing on the interaction of the many complex factors that might be involved. Duration of stay at migratory sites, energetic requirements, interaction with weather, variable wind speeds,  predators and quality of habitat are all important elements in understanding whether the Western Sandpiper’s population is merely fluctuating or is experiencing long term decline.

For more information contact: Rob.Butler@ec.gc.ca

Check the following sites for additional information on this indicator:

• Hinterland's Who's Who - Shorebirds
• Canadian Shorebird Conservation Plan 
• Canadian Bird Trends Database for Western Sandpiper
• Butler on Birds
• Environment Canada's Science & Environment Bulletin - Sandpipers Feeling Squeeze in Strait

The following Web sites are not under the control of Environment Canada (EC) and they are provided solely for the convenience of users. Environment Canada is not responsible for the accuracy, currency or the reliability of the content. Environment Canada does not offer any guarantee in that regard and is not responsible for the information found through these links, nor does it endorse the sites and their content. Users should be aware that information offered by non Government of Canada sites that are not subject to the Official Languages Act, and to which Environment Canada links, may be available only in the language(s) used by the sites in question.

• North American  Bird Conservation Initiative
• US Fish & Wildlife - Shorebird Site 

For references used in this indicator click here.