By Tina Yerkes, Ph.D.
As the old saying goes, you are what you eat. Of course, the same is true for waterfowl. But the bodies of ducks and geese also contain hidden clues about where they eat. By analyzing feathers, blood, and other tissue samples, researchers are able to learn much about the habitat preferences and movement patterns of waterfowl.
The science behind this fascinating and relatively new field of waterfowl research is similar to what you might see on an episode of the popular TV series CSI. But instead of analyzing evidence to track down a murderer, waterfowl researchers use an intriguing combination of chemistry and ecology to trace bird movements over large geographic areas.
Stable isotope analysis helps researchers unravel patterns of movement at geographic scales not previously possible. The inability to track birds as they migrate from breeding grounds to wintering areas and back again has left large gaps in our knowledge of many species. Traditional radio telemetry or leg band returns have limited utility because of the large distances covered by many birds and their use of remote habitat. And some species are rarely banded or harvested. Although satellite telemetry is filling some of those gaps, stable isotope techniques are the newest tool in the waterfowl CSI toolbox.
Stable isotopes are naturally occurring forms of elements such as nitrogen, carbon, and hydrogen, which combined with oxygen produce water. As birds and other animals feed, stable isotopes are incorporated into their body tissues. Isotopes are specific to geographic areas or feeding conditions. Therefore, isotope “signatures” in body tissues can place a bird in the location where it grew a specific body part, such as a feather or toenail. For example, ducklings grow their flight feathers soon after hatching and then fly south. If you sampled a flight feather from a hatch-year bird on the wintering grounds, the isotope signature in that feather would indicate where the duck hatched—the Prairie Pothole Region, the boreal forest, or elsewhere.
Researchers use stable isotope techniques to delineate populations, determine diet, identify habitat preferences, and examine important connections among breeding, wintering, and migration areas. And they are able to get this information without having to follow individual birds from place to place over long periods. This helps conservation planners and habitat managers determine landscape-level conservation practices that will most benefit waterfowl. Because researchers can now track birds throughout their entire life cycle, conservation planners can begin to unravel how events in one season, such as lack of food on spring migration routes, affect events in another season, such as reproductive success on the breeding grounds.
Waterfowl researchers have used isotope analysis to answer several interesting questions. For example, we know little about some species of ducks because their habitat is so remote. Sea ducks, for instance, remain in northern areas throughout most of their life cycle. King eiders winter on the northeast and northwest coasts and nest in the high Arctic. Their numbers are declining on both coasts, but wintering birds likely encounter different environmental influences on each coast. Biologists wanted to know whether king eiders wintering on different coasts mixed on the breeding grounds, as well as whether specific wintering areas were affecting reproduction differently. Using band recoveries to delineate these wintering populations was not possible because few king eiders have been banded. On the breeding grounds, researchers wanted to be able to determine if an individual hen king eider wintered on the east or west coast. They collected hens’ head feathers, which are grown on wintering areas, and examined them for stable isotope differences. The results allowed biologists to delineate eastern and western wintering populations on the breeding grounds.
Another waterfowl mystery has been whether the locations selected by hens in winter and spring have an effect on their condition when they arrive on the breeding grounds. Hens that arrive on the breeding grounds with good fat reserves are more likely to nest and successfully raise ducklings. Using isotope analysis, waterfowl managers can identify wintering and migration areas where hens fail to build adequate fat reserves and can then focus conservation efforts on improving habitat conditions and food availability in those areas.
One hypothesis for the decline in the northern pintail population is that the arrival condition of breeding hens may be limiting their reproductive success. DU researchers studied pintail hens arriving in Alaska during spring by analyzing breast feathers (grown in winter) and blood (which reveals the bird’s activities over a small window of time during spring migration). The results showed that pintails that wintered or staged during spring in coastal habitats with more agriculture nearby arrived with less fat than those wintering and staging in freshwater habitats with less agriculture nearby. As a result, researchers might focus efforts on the Gulf Coast and in western Mexico and the Pacific Northwest to determine if food resources are inadequate in these vital coastal areas.
Advances in stable isotope analysis have resulted in large gains in our understanding of waterfowl and other wildlife, but this fascinating field of science has also been used to track sources of pollution and oil spills, trace contaminants in food, identify the origin of illegal drugs, and even determine the diet of Neanderthals.
Dr. Tina Yerkes is director of conservation planning at DU’s Great Lakes/Atlantic Regional Office in Ann Arbor, Michigan.