Why Do They Come?

By Bob Engel
Marlboro College Professor Emeritus of Biology and Environmental Science

      Long-distance migration is a miraculous achievement. Lobsters, some insects, whales, and even white sharks do it. But it's the birds that we really notice. Some fly thousands of miles, and then they arrive like clock work, one after another, often right into our own back yards. We know when they come, and we love it - spring is finally here - but we rarely consider why they come.
        That's a tough question. It is, after all, hard to ask them, especially through experimentation. We can and do ask them how they come, a physiological question, but why is the harder, evolutionary question. Let's muddle with that.
        Consider the bounty that is New England for a few precious months. During our summer,
a Vermont tree will outperform its analogue in Brazil when a warm Vermont day is followed by a cool night!  And so does everything else down the food chain, especially insect prey. This simply notion is due to the fact that plant growth is driven by the difference between photosynthetic gain and respiratory loss (plants use oxygen to do the same things we do, they just don't move much). Respiration is more temperature sensitive than photosynthesis, so a warm night consumes much of the photosynthetic gain of a warm day. Yes, tropical plants grow all year, but often slowly because of the warm days AND nights.  Here plants really spurt for a few months, especially with a large diurnal temperature difference. The food chain fairly pops and it's easier to feed a brood, especially because there are fewer competing species in the system.
        Another idea: life is just safer up here. A recent study with shore birds showed higher fledgling success rates in high latitude birds than ones that nested further south. There were fewer nest predators up north.
        We humans routinely hear that a rural locale is a better place to bring up our kids. Geography works for migratory tropical birds, too. In a place like Panama, a nest is likely to attract a suite of predators including ants, several birds, a few mammals, and snakes. The predators are abundant and the threat is incessant.  Many or even most chicks die in the nest. To compensate, tropical adult songbirds are often relatively long-lived (6+ years).
        Up here the story is different. If a bird nests in a tree, there are only a few birds (crows and jays, mostly) and red squirrels to worry about. Because more kids make it out of the nest, evolution has not resulted in adult longevity, so our birds average maybe 3 or 4 years of life. But that's enough time for a Vermont migrant to replace itself, especially if the fledglings avoid windows and marauding house cats.  Don't, however, forget that perilous, long trip, the one that takes everything a bird's got, and, often enough, more.
        Natural selection, usually the primary agent of evolution, is a merciless, unblinking score keeper.  It methodically adds up successes and failures and can create and maintain a complex behavior like migration.  As long as the winners outnumber the losers, our migrants will continue to come.  But as we relentlessly chew away at habitat both here and there, degrade necessary stopover points, and change the climate, we imperil this wonderful annual infusion.