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.
References:
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
