Sometime this winter, waterfowl experts from across Canada will gather for their annual "wing bee." Their task will be to sort through a small mountain of duck wings obtained from a randomly selected group of hunters, and assign the wings to piles by species, age and sex. Together with statistics from similar shindigs held in the United States, this information will provide a picture of the year's kill and will also offer hints about the ups and downs of duck populations.
             That may seem like a lot to learn from a heap of dried-up remains but, to Len Wassenaar of the National Water Research Institute in Saskatoon, a room full of duck wings is like an archive that can be studied for clues about each bird's life history and movements. Wassenaar and his colleague Keith Hobson of the Canadian Wildlife Service have developed a technique for reading a feather's chemistry and tracing it onto a map.
             The story begins with rain, which always contains a minute percentage of heavy water. That's regular H2O burdened with deuterium, a rare isotope of hydrogen. In North America, the amount of deuterium in rainfall is greatest along the PaciÞc coast and decreases to the east and south, as weather systems sweep across the continent.
             Every region has a unique "hydrogen isotope signature" - a characteristic ratio of ordinary hydrogen to deuterium - imprinted onto the ecosystem, passing from the rain into soil, soil into plants, plants into birds and animals. When the hydrogen is incorporated into hard tissues, it provides a lasting clue to where those tissues were made.
             Last year, Wassenaar and Hobson used this fact to resolve a mystery that has troubled researchers for decades. Since the mid-1970s, we've known that monarch butterflies congregate for the winter in a dozen remote locations in central Mexico. Several hundred million monarchs from Eastern Canada and the U.S. settle onto the hills
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