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COMMENTARY
October 3, 2009
Going south
The cold rains come, night temperatures drop towards freezing; it is October and no question that the summer is gone. We are two weeks into "statutory fall," which at this latitude is no legal fiction. It is my favourite time of the year -- the spectacle of the leaves, the autumnal lighting, the coolness, and I would add the poignancy of the silences. Our songbirds have gone.Most clear out of here much earlier. This "trivial fact" came home to me a couple of weeks ago, when I suddenly realized my swallows had disappeared. This was a little urban flock I'd had the privilege to watch from an apartment balcony, with their extraordinary aerial acrobatics, cavorting each evening to the cost of urban insects. It was past Labour Day, the air was as warm as it had been all summer, but my swallows were on their way to Brazil. The mystery of where they go, and how they return, fed myth through time out of mind. From what I've read, the phenomena of bird migration still baffle the ornithological mind. The latest tracking methods -- miniature flight-recorders attached to them -- have revealed a very interesting clue about, e.g., purple martins, close relatives to my (barn) swallows. According to a study done out of Pennsylvania by Bridget Stutchbury and a team from Ontario's York University, these birds take their time flying south -- perhaps six weeks, with leisurely stopovers en route. But in spring, they are racing, and can make the "return journey" in two weeks flat. But mind that phrase: for I don't think it is the "return portion" of the trip. The birds of the world are overwhelmingly tropical, as are all other animals and vegetables. The "torrid zone" is where we find vast diversity of species. Up here so far north, we find a subset that appears mostly to have strayed from the reliable heat, and adapted to our climatic and environmental conditions over (a very long) time. We take a hemispherocentric view of our migratory birds. We tend to think of them as belonging here, because they breed here, and flying south to escape our winter, when their food sources disappear beneath the snows. But no: this doesn't make sense. Birds migrate from territories where winter food supplies would be quite plentiful. And even in Ontario, where the snows will surely come, they have gone long before the summer is ended. Some depart as early as July. The truth is more likely the opposite. Our migratory birds originate in the tropics, and come here to nest for the very reason that the tropics are so productive -- in order to escape all-species and all-niche population pressures, which include a wide array of specialized predators, to which their young would be exposed. They are not fleeing winter; they are instead fleeing perpetual summer. I would guess that from my swallows' point of view, Brazil is home, Canada is a summer vacation. They take their time flying home, because there is no rush. But in spring they race north, from their original territory, and will be delayed only by serious storm fronts, because their breeding season is approaching. They come here only to breed in relative safety, away from those tropical pressures. As soon as their fledglings can defend themselves, they fly home. It has nothing to do with the winter approaching: they've done what they wanted to do in Ontario -- breed -- and have no further use for the place. As I've confessed before, I simply don't believe microevolutionary adaptations drive macroevolutionary changes. But what I suspect we have here is a classic microevolutionary adaptation, "within the species," as it were. The distant ancestors of these birds migrated much shorter distances, from their tropical homes, in search of safe and food-plentiful breeding grounds. Over time the route was extended, like a string, until such grounds were found, farther and farther from home. I suspect flocks follow the same string back -- like cruise missiles following a topographical map. Birds have very big brains in proportion to their body mass (and with that, the trick of keeping them at a constant high temperature, in all weather, an amazing feat of pure macroevolutionary design). Here is something those brains could be used for: storing the whole route in detail. They migrate in flocks -- sensibly, to maintain a consensus on each point and turn. (A bird that migrates individually, may easily get lost, by missing just one turn.) There's an interesting Lamarckian aside here. Fledglings need much tutoring by their elders, but in many bird species the young then migrate separately from the old, and a few days later -- albeit from and to the same locations. They might therefore be born with a heritable route map imprinted. David Warren
© Ottawa Citizen
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