Tuesday, June 2, 2015
Spring Bird Migration
Note: This program first aired on May 16, 2015.
Spring in Maine is such a delight. Every day brings a new sound, a new smell, a new sensation to be experienced. As I write this, yesterday I heard my first Black and White Warbler of the season, and today my first Black Throated Green Warbler. Each one of these little birds migrates to Maine from geographies further south. Black and Whites winter from Florida, through Mexico and Central America to Columbia and Ecuador. The Black Throated Greens winter in Cuba and the island of Hispanola, along the Yucataan peninsula and Honduran highlands, and in the northern most part of South America. Both of these birds, like so many others, make their way from their warm wintering grounds to mid and high latitude forests, for the express purpose of breeding. The mid and high latitude forests, just awakening from winter’s dormancy, offer a bonanza of resources to these migrants, from an explosion of nutritious insects, to nesting sites and lowered nest predation pressure. So these little birds arrive as ready for spring as you or I.
The difficulties of migration are many. These birds oxidize their body fat, muscles and internal organs for energy, and breathe so rapidly that they easily dehydrate, something that can become the limiting factor in length of migratory flight. The means by which birds navigate are complex, numerous and not entirely understood. What we know for certain is that human infrastructure confuses some migrating birds; bright lights and tall buildings have killed thousands of them. Migration is hard and dangerous and birds can suffer high mortality if they are in the wrong place at the wrong time.
Which brings me to my main point. When I am roused from sleep by the sound of a new bird song coming in my window on a spring morning I have to remember not just the excitement of hearing a new bird, but also deep humility in the face of that bird’s accomplishment. That new warbler I hear flew all night to get here. All of these song birds are nocturnal migrants, so not only did they fly all the way from Florida or further, they did so at night, while I was sleeping. During the day they rest and forage, and follow environmental cues as to whether it is warm enough to keep going (the cold gray period of a couple of weeks ago surely slowed down the East coast migration into New England, now that the sun is out and the weather warm, the birds have started showing up again). They fly at night for a number of good physical reasons; they get all day to eat before they have to fly again which can shorten their overall migration time. There is less wind and turbulence at night, so the flying is easier. The air is cooler so the birds lose less water, and because it is cooler, the relative humidity is higher which leads to higher density air. The higher the air density the easier it is to generate lift, a key component of flight. Night flight has many benefits going for it.
The most important reason for birds to migrate at night though, at least in my mind, is because they are trying to avoid predation. And what preys on little birds flying thousands of feet up in the sky you might ask? Other birds. Yes, the dirty secret of the avian world is that big birds eat small birds, and not just occasionally. The Accipiters, a group of hawks that includes Copper’s Hawks, Sharp Shinned hawks and Goshawks, and the Falcons a group that includes Merlins, Kestrels and peregrine falcons, are all birds that prey primarily on other birds. This predation pressure was strong enough, that when combined with the all the other positive attributes of night migration, yielded enough selective pressure to cause songbirds to evolve to migrate in the dark. Perhaps this impresses me because I can’t imagine running the Appalachian Trail twice a year, only at night, trying to avoid a man eating Bigfoot. If we all had to do that every year, it would quickly sort out the wheat from the chaff, if you know what I mean. And that means that the birds you hear at sunrise, the new voices that join the chorus each day, those aren’t just any birds, those are the survivors. So yes, winter was hard for us, I won’t deny it, but with each new warbler that arrives on a spring wind, take a moment to appreciate just how arduous and therefore amazing their journey to this season of plenty is.
Black and White Warblers: http://www.allaboutbirds.org/guide/Black-and-white_Warbler/id
Black Throated Green Warblers: http://www.allaboutbirds.org/guide/Black-throated_Green_Warbler/lifehistory
About Migration from the Cornell Lab of Ornithology http://www.birds.cornell.edu/AllAboutBirds/studying/migration/
From the Eastern Kentucky University, a marvelously in depth and referenced website about birds: http://people.eku.edu/ritchisong/avian_biology.htm
Migration altitude from Cornell: http://www.birds.cornell.edu/AllAboutBirds/studying/migration/pathways