Changing Times

Very soon, there will be a major upheaval in the bird populations in this area.  Dozens of birds that are abundant and widespread at the present will vanish entirely.  In their place, many previously absent species will swarm in, including large numbers of brightly colored creatures streaming north from the tropics.  Is it global warming?  The Mayan apocalypse?  Planetary ecosystem collapse?  Of course not.  It is the spring migration.

The annual movement of birds north, and then back south, throughout the northern hemisphere, is one of the grandest happenings in all of the natural world.  The feats accomplished by these little balls of fluff and air are mind-boggling.  Not only do many fly nonstop across the Gulf of Mexico from Yucatan to the U.S. Gulf Coast, if the weather is good they will often keep on flying without a break until they finally decide to land hundreds of miles inland.  Most of this traveling happens at night, when the birds steer by the moon and the stars.  Research has found that some birds are born with an instinctive star chart in their heads, allowing them to navigate the same way human mariners did for thousands of years.  The hoards of migrating birds are dense enough to fog the weather radars at night in good conditions.  This spectacle repeats every year, like the pendulum of a grand global clock, north, south, north, south, tick, tock.

Though the thickest part of this global avian pendulum doesn’t reach Tennessee until late April and early May, as we come into March its northward swing has already begun.  New arrivals begin to trickle in, and the abundance of the birds that winter here but nest farther north starts to noticeably wane.  Like many other plants and animals, birds mostly use two keys to keep track of the seasonal calendar:  the weather, and the changing lengths of the days.  While the weather will vary greatly between years, the day lengths are a fixed and precise indicator of exactly where you stand in the annual cycle.  This helps them not be confused by events such as the spring-like weather we have been experiencing in mid-winter.  So even though our weather seems to be running months ahead of schedule, I would not expect the birds’ movements this spring to be much more than a week or two earlier than is typical.

The northward march of spring warmth happens rather slowly; it takes months for it to travel from the Gulf Coast to central Canada.  In contrast, a bird can cover this same distance in only a couple of days.  Rather than just slowly creeping northwards with the spring, many of the birds that nest in the far north instead pass the time waiting in the tropics, not even heading north at all until late in spring when their breeding grounds will have finally warmed up.  Many woodland birds seem to time their arrivals to coincide with the leafing out of the trees in their nesting areas; in the far north woods this does not happen until late May.    So, paradoxically, the birds that have the farthest to go often are the ones that leave the wintering grounds last!

If you are sitting in a single location midway between Mexico and Canada, such as Tennessee, this creates a fascinating effect.  Watching the spring migration in progress can seem like a journey northwards.  Many of our local nesting species are among the first to arrive.  Birds like the Northern Parula, White-eyed Vireo and most of the swallows are usually here before the end of March, and many others such as Indigo Buntings and Common Yellowthroats are not far behind.  Species that nest in the Midwest, Great Lakes region, Appalachians and southern Canada arrive in large numbers in late April.  This group includes such handsome beasts as Rose-breasted Grosbeaks, Baltimore Orioles and many warblers.  In early and mid-May the flow shifts to birds that will be setting up their summer homes in central Canada, including Blackpoll, Magnolia and Tennessee Warblers.  Though these last two species have names that would make you think they are denizens of Dixie, in reality they neither nest nor overwinter here, being found in the land of sweet tea only in transit.  Finally, if you are near one of the wetland hotspots that attract them, many arctic-nesting sandpipers pass through in late May into early June, when the tundra is finally thawing.

As any trucker can tell you, Tennessee is the crossroads of eastern North America.  This applies to the birds as well, and it makes the spring and fall migration seasons dynamic and exciting times here.  Even from your own backyard, you can simply sit in one place and watch the whole continent fly by.

 

Bill Pulliam got started in birdwatching by his junior high science teacher in 1974, and has been an avid birder ever since in 48 U.S. states and 7 foreign countries.  He is currently the Tennessee editor for eBird, an online project that compiles millions of observations from tens of thousands of birders around the world.

 


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