Isotopes Link Birds to Breeding and Moulting Areas

The saying "you are what you eat" has new meaning since the discovery that certain chemical elements in animal tissues are linked to diet. One such element is deuterium, a stable isotope of hydrogen that occurs naturally in rainwater and enters the food chain through the tissues of plants. Concentrations of deuterium vary predictably across the continent, with lower amounts occurring in northwestern regions of North America and higher amounts in southeastern regions.

Scientists at Environment Canada's National Water Research Institute and Prairie and Northern Wildlife Research Centre in Saskatoon are world leaders in stable-isotope analysis. They have developed a method of measuring stable-hydrogen isotopes in the wing tissue of butterflies and birds as a means of determining origins. Experts at these facilities have recently expanded the application of this new technique to several projects related to the conservation and protection of bird species. The technique is being adopted by American and European researchers in studying animal migration.

One recent project was to determine the breeding and moulting areas, also known as catchment areas, being sampled by two migration monitoring stations. Scientists analyzed stable-hydrogen isotope values in feathers of Swainson's Thrush moving through monitoring stations at Delta Marsh in Manitoba and Long Point in southern Ontario to verify that population counts at the two locations cover individuals from a large geographic area. Such information is vital to linking population declines to specific regions, and thereby focusing conservation efforts on these areas.

Stable-hydrogen isotope signatures provided useful catchment area information. Results showed both monitoring stations sampled birds from broad regions of the Canadian Boreal forest, but that birds moving through Delta Marsh were clearly from more northwesterly origins compared to Long Point. The catchment area of the Delta station was northwestern Manitoba to northwestern Alberta, while the catchment area of the Long Point station extended from north-central Ontario and Quebec into western Canada.

Since the isotope composition of birds' feathers reflects their diet at the location where feathers are grown, feathers sampled from young-of-the-year as they migrated southward indicated breeding ground location, while those taken from after-second-year adults heading north indicated their moulting region from the previous year. Stable-hydrogen isotope values for adult feathers differed from those of the young, supporting results from previous studies that noted delayed moults in migrating adult Swainson's Thrushes during the fall. This indicates that southern portions of the Boreal forest represent important stopover habitat for this species.

The isotope study confirmed that dedicated avian migration monitoring stations in southern Canada are an appropriate way to quantify relative changes in population sizes for Swainson's Thrush and, likely, for other Boreal-nesting species as well.

A second project involving stable-hydrogen isotopes is being conducted in collaboration with the U.S. Fish and Wildlife Service and Ducks Unlimited Canada. Since 1999, scientists with Environment Canada have been analyzing isotopes in wings of hatching-year Lesser Scaup, a diving duck species killed by hunters in the United States during the ducks' migration south and on wintering areas. Lesser Scaup are on the decline in many parts of their core breeding range in the Boreal forest of western Canada. Determining hatching origins of harvested Scaup will enable scientists to tell whether or not the problem is linked to differential rates of harvest in certain areas or environmental changes in breeding areas. The data will also enable scientists to test the theory that Scaup breeding farther north generally travel farther south to winter.

In another stable-isotope project, Environment Canada scientists examined stable-nitrogen isotopes in feathers from flightless Mallard ducklings at 17 locations in western Canada. High nitrogen isotope values were found in the feathers of ducklings born in agricultural areas, reflecting the entry of excess nitrogen into local water-bodies. This is the first evidence that nitrogen isotope values in duckling feathers may record long-term nitrogen loading of surface waters in agricultural areas, and thereby provide another means of monitoring non-point-source nitrogen inputs into landscapes and food webs.

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