Essay, Research Paper: Mapping Migrations


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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 Pacĩc 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 identĩcation 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.
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