Mapping Migrations 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 Pacic 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 hillsides in orange drifts.
But once the insects have landed, they all look the same to us, and we have no way of knowing their precise origins. Which ones came from Ontario? Which from Ohio? If one of the wintering sites were logged, how would this affect the breeding stock? The tried-and-true technique of tagging, which has taught us so much about the migratory movement of birds, has been disappointing with monarch migration. Over the past 50 years, hundreds of thousands have been marked with tiny identication stickers, yet fewer than 130 have ever been recovered in Mexico. “The tag recoveries are really appalling,” Wassenaar laments. The beauty of the new technique is its directness. By gathering dead butterflies from the wintering sites and analyzing them in the lab, Wassenaar and Hobson were able to read each individual’s hydrogen signature.
This in turn revealed where the butterflies had grown up. As a result, we now know that the monarchs at the winter roosts are of mixed origins (Ontarians and Ohioans crammed in wing by wing) and that most of the overwintering flocks come from the midwestern U.S. The discovery of the midwest’s crucial importance in maintaining the breeding stock will provide an added focus for conservationists. Gratified by this success, Wassenaar purrs with confidence. “The sky’s the limit with this new tool,” he says. Rather than spend years on banding projects, with uncertain results, why not head for the isotope lab and an immediate outcome? Certainly, that prospect appeals to Bob Clark, also of the CWS, who has urgent concerns about the welfare of the lesser scaup, a diving duck. (That’s “scawp,” an imitation of the bird’s characteristic squawk.) Cute as a rubber ducky with its upturned blue bill, the scaup has traditionally been among the most plentiful of waterfowl, with an estimated population of six million.
But its numbers took a downturn in the mid-1980s, a trend that has recently intensified into a seven-year sequence of record lows. Two-and-a-half million birds have vanished. The losses seem to be worst for scaups that nest in the boreal forest of northern Alberta and the southwest Northwest Territories. Is “something funny going on” in the north woods, as Clark suspects, or does the source of the problem lie farther south, along the birds’ migration route or on their wintering grounds in Mexico and the U.S.? These perplexities would be easier to cope with if we knew precisely where scaups from the boreal forest go for the winter. Clark thinks the answers may lie in the scaup wings that are submitted for the annual bees. Scaups grow new feathers before leaving their breeding range, so their hydrogen signature should tell him where each bird spent the summer, be it on the plains or in the forest. By mapping this location and the spot where the duck was shot, he expects to build a detailed picture of scaup migrations and wintering grounds. Similar information is required for a growing number of migratory species, including many of our favourite songbirds.
Since population declines tend to affect particular subpopulations (like the boreal forest scaup), we can no longer get by with a broad-brush sketch of migratory movements. The hydrogen-isotope technology offers to fill in the details at a moment when this knowledge is urgently needed. Candace Savage is a Saskatoon-based writer and author of 18 books on wildlife, environmental issues and other subjects.