Thursday, March 25, 2010

The Changing Face Of Bird Migration

On any given spring day here in the Adirondacks you are bound to hear an American robin calling from a treetop or front lawn. And then you ask, when did they get here? Later that day as you are paddling along the shoreline of a quiet lake you see an osprey circling overhead as it hunts for fish. Again you ask, When did they arrive?

Do these birds all arrive at once or is there some sort of bird-migration-schedule that our feathered friends use on their northward journeys?

A simple answer is yes, they do arrive on somewhat regular schedules. But as with all things in the world of biology, it is not that simple.

Let’s look at the big picture first. We know that birds arrive in the spring and we can kinda guess-within a week to 10 day margin-when they might arrive. Someone once told me that our Adirondack hummingbirds arrive within a week, either side, of Mother’s Day. That’s true.

Our summer visiting common loons are usually on lakes and ponds just at or near complete ice out. Come to think of it that’s pretty easy for a loon to determine as they fly 500-1000ft in the air getting a loons-eye view of what lakes are open.

But looking at the nuts and bolts of migration we see there are many biological signals that kick these birds into migration mode.

Deep within birds there are biochemicals, testosterone and estrogen, surging through veins and into cells. This in turn signals other processes to begin: reproductive parts start to increase in size; increasing day length signals readiness for migration; and there is a general restlessness that birds exhibit as they begin orienting their movements towards north or south, depending on the season.

As all this occurs, birds then get an uncontrollable urge to eat and fatten up as a result. This is good because the fat is the fuel for long distance migration.

But back to the main point-are birds on a schedule? One way to answer this is to look at the “waves” of different species that arrive in our woods, fields, or wetlands.

As March began we noticed bunches of Canada geese, and snow geese in flight far overhead. A week later we take note of our first red-winged blackbird down by the swamp or cat-tail marsh. About 2 days later the common grackles find their way into the fields and call to announce their arrival. Cardinals, brown creepers, and golden-crowned kinglets begin singing their songs for the early riser.

As I write this blog near the end of March we find the arrivals of song sparrows and fox sparrow to the neighborhood. My guess is that next week will bring tree swallows, northern flickers and white-throated sparrows.

April is when the migratory birds really make their presence known across the Adirondacks. Belted kingfishers will be seen along the shorelines of ponds, American kestrels(small falcons) will be hunting for the first crickets from telephone poles and wires. Great blue herons will be fishing among the grasses of the beaver swamps, and eastern phoebes will be scolding us from the roofs of backyard sheds.

Think of bird migration as a large conveyor belt cycling though the spring months and dropping off species of birds at timed intervals. But keep a sharp eye about you because the bird scene changes daily and then weekly.

Personally I’m looking forward to late April when the wild flowers have poked through the dead leaves and the sounds of the first warblers fill the woods and finally reach my winter-weary ears.

Photo Credit: Canada Geese-Wikipedia

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