Why do birds fly south for winter?

by Doug Morin

I opened my backdoor and stepped into the yard to a flash of red and buzz of wings – a hummingbird.  Maybe the last of his kind I will see this year, he perched on a small branch, tilted his head to either side, then flew off down the road.

Here in Vermont, hummingbirds disappear in late September and reappear in late April.  We know the story well: birds fly south for winter.  Of course they do.  But, have you ever wondered why?

First, let’s turn the clock back a few thousand years.  It turns out, most migratory birds in North America trace back to ancestors that lived in the tropics.  Over time, these birds expanded their ranges until a small proportion eventually made it to North America.  Even today, most birds arrive in late spring and leave in early fall – spending less than half their year in North America.  The real question then isn’t, why do birds fly south for winter? but, why do birds fly north for the summer?

This is a particularly important question because migration carries a deep cost.  It’s easy to discount the effort required to fly to and from the tropics, given the convenience of modern air travel (though I’d still like more leg room), but the journey for a bird takes huge amounts of time and energy as well as exposing the bird to unfamiliar environments and predators.  The hummingbird in my yard, for instance, weighed only as much as small handful of paperclips, yet over the next weeks, it will first fly to the southern coast of the U.S., then across the Gulf of Mexico to the Yucatan Peninsula in a single, non-stop flight lasting nearly 24 hours, and finally overland to southern Central America.

The time, energy, and risk involved in migration have severe impacts: migratory birds are twice as likely to die in any given year, compared to tropical non-migratory birds.  So, why in the world do they do it?

The answer is that the benefits outweigh even these high costs.  Since relatively few birds come to North America, migrants have easy access to abundant insects, plants, and nesting grounds.  With plentiful food and territory, migratory birds produce many more offspring each summer than their non-migratory counterparts.  As winter arrives, however, insects and plants disappear, and the diminished food supplies (rather than dropping temperatures per se) drive migrants south.

Overall, migratory birds do not live as long as non-migratory tropical birds, but produce more offspring each year – resulting in nearly the same number of over their lives.  Since the number of offspring determines how many birds will be in the next generation, these two strategies are roughly equivalent in evolutionary success.

So, the birds that grace our summers with color and song do so for windfall payoffs, but at immense cost.  As you see the last of our migratory birds leaving over the next few weeks, wish them well on their way.

Also, look out for a more in-depth post by Emily Brodsky on Raptor Migration in the next few days!

Note: For excellent maps of where birds spend their summers and winters, see the Cornell Lab of Ornithology’s website All About Birds.