FREE ESSAY ON MAPPING MIGRATIONS

MAPPING MIGRATIONS

Mapping migrations
By Candace Savage
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 Paci?c 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 identi?cation 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.