On the move

Migrating birds are on the move — as weather fronts (and winds) conducive to long distance flight move the birds northward through the midwestern U.S.  I saw numerous flocks of migrating geese and ducks while driving back roads to various sites along the Mississippi River yesterday.

Some flocks exhibit that highly organized “V” flight pattern behind a leader.

Some flocks were less well organized, milling about in the sky, changing direction continuously.

Some highly organized, long streams of birds flew rapidly over the agricultural expanse of Dakota County toward the Mississippi River.

Seeing natural phenomena like this makes one wonder:  Why do they fly in a “V”?  Why do they migrate along the river?  Why are so many flocks of birds seen on this particular day?

Why fly in a “V”?  A study of flying endangered Bald Ibis (published recently in the journal Nature) showed that individuals position their wing tips just downstream from the bird ahead of them, and synchronize their flapping to maximize the lift created by the updraft of that preceding bird’s flapping motion — finding a sort of aerodynamic “sweet spot”.

Wing beats are fairly well synchronized in this shot of Canada Geese flying overhead. Only the bird in the middle of this flock is unsynchronized with the others, with its wings in the upstroke instead of downstroke position.

Why fly along the river?  Rivers are natural migratory flyways, because they provide not only sustenance for hungry migrants, but are topographical markers that are easy to follow as birds move north or south along a migratory route.  Mixed flocks of migratory waterfowl seek protection in numbers resting on the water, while songbirds and raptors seek food and protection in the forests along the river’s edge.

A large flock of Canada Geese, as well as a dozen different species of ducks rested on this stretch of the Mississippi River, waiting for the next weather front to move them north.

Why fly today?  Changing weather patterns are key signals for bird migration.  In the spring, as warm air heats up in a low pressure area in the southeastern U.S., it pushes northward, and birds follow this warm air current as it rises over the cold air below. Where warm and cold fronts meet, precipitation might force birds to halt their migration, until the next warm front moves them in another wave north.

Restless geese on the move again.