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Escarpment Magaz ine Spr ing
2012
Among
the many joys of spring is the
arrival of waves of brightly-hued birds, full of
song and colour. Fresh from their winter
feeding grounds, birds of all kinds are eager
to mate and build nests, feed their young and ready them for the long
flight southwards before first frost.
Bird migration has long been a source of wonder and mystery. How do they
navigate? Do the same individual birds return to the same place each year?
And if they already have territories in the sunny south, why do they bother with
migrating thousands of kilometers to nest?
The last question can be answered in a single word: insects. The woods and
fields of Canada provide a bonanza of insect life – flying insects, crawling
caterpillars, grasshoppers and crickets and all their kin – all packed into a
brief period of time. And if you have a nest of hungry young constantly de-
manding food, nothing beats insects for a high-protein, nourishing diet. So
when our insect populations begin to dwindle in early August, some of our mi-
grants such as the wood warblers and shorebirds are already on their way
southwards again.
To understand the origins of bird migration, it is helpful to think of the Baltimore
Oriole in your yard not as one of “our” birds that flees south to escape the
winter, but rather as a tropical bird that has come north to take advantage of
that explosion of insect life. After all, the Oriole spends eight or nine months
of the year in the tropics, only making its excursion northwards for a few
months to breed.
How do migrating birds find their way? There appears to be no single answer
to that question. Many species migrate at night, using the stars as navigational
aids. Some species follow landscape features such as the Mississippi River.
Some can detect the low rumble of distant ocean surf to keep their bearings.
Others are able to sense the Earth’s magnetic field. There is lots of mystery yet
in the migration question. In some species, for example Arctic-nesting shore-
birds, the adults migrate first, leaving the young birds to find their own way a
few weeks later, which has to rank as an amazing feat of instinct.
Within the past few years, pioneering work by Dr. Bridget Stutchsbury and
her colleagues at York University has added greatly to our knowledge of mi-
gration. Her work on Wood Thrushes, a cousin of our familiar Robin which
nests in deep woods, shows that the majority of adults return to the same nest-
ing grounds year after year. More remarkably, her research also found that
thrushes who nest in the same general vicinity up here cluster together in the
same small bit of their wintering range in the tropics, even though they may
have followed quite different routes to get there. Biologists are now trying to
establish if this same pattern applies to other migrant songbirds as well.
escarpment
nature
FEATHERED FRIENDS
BY RON REID
Welcoming the Birds of Spring