Strangers In The Night

We all know that birds (and many other animals) migrate.  It’s a survival phenomenon which, above all, allows them to utilize their mobility to translocate to a climate which provides an advantage for obtaining food, enduring seasonal weather, and raising offspring.

In the northern hemisphere, most migratory birds fly north in the spring to latitudes with progressively greater hours of daylight to breed, nest, and provide for their young.  In the southern hemisphere there are similar movements, these to the south during their spring (our autumn).  The goal is the same, procreation, though the landmass offering sustenance for species other than seabirds is limited “down under”.  Interestingly, there are some seabirds that breed in the southern hemisphere during our winter and spend our summer (their winter) feeding on the abundant food sources of the northern oceans.

Each autumn, migratory breeding birds leave their nesting grounds as the hours of sunlight slowly recede with each passing day.  They fly to lower latitudes where the nights aren’t so long and the climate is less brutal.  There, they pass their winter season.

Food supply, weather, the start/finish of the nesting cycle, and other factors motivate some birds to begin their spring and autumn journeys.  But overall, the hours of daylight and the angle of the sun prompt most species to get going.

But what happens after birds begin their trips to favorable habitats?  Do they follow true north and south routes?  Do they fly continuously, day and night?  Do they ease their way from point to point, stopping to feed along the way?  Do they all migrate in flocks?  Well, the tactics of migration differ widely from bird species to species, from population to population, and sometimes from individual to individual.  The variables encountered when examining the dynamics of bird migration are seemingly endless, but fascinatingly so.  Bird migration is well-studied, but most of its intricacies and details remain a mystery.

Consider for a moment that just 10,000 years ago, an Ice Age was coming to an end, with the southernmost edge of the most recent glaciers already withdrawn into present-day Canada from points as near as the upper Susquehanna River watershed.  Back then, the birds migrating to the lower portion of the drainage basin each spring probably weren’t forest-dwelling tropical warblers, orioles, and other songbirds.  The migratory birds that nested in the lower Susquehanna River valley tens of millennia ago were probably those species found nesting today in taiga and tundra much closer to the Arctic Circle.  And the ancestors of most of the tropical migrants that nest here now surely spent their entire lives much closer to the Equator, finding no advantage by journeying to the frigid Susquehanna valley to nest.  It’s safe to say that since those times, and probably prior to them, migration patterns have been in a state of flux.

During the intervening years since the great ice sheets, birds have been able to adapt to the shifts in their environment on a gradual basis, often using their unmatched mobility to exploit new opportunities.  Migration patterns change slowly, but continuously, resulting in differences that can be substantial over time.  If the natural transformations of habitat and climate have kept bird migration evolving, then man’s impact on the planet shows great potential to expedite future changes, for better or worse.

Now, let’s look at two different bird migration strategies, that of day-fliers or diurnal migrants, and that of night-fliers, the nocturnal migrants.

Diurnal migrants are the most familiar to people who notice birds on the move.  The majority of these species have one thing in common, some form of defense to lessen the threat of becoming the victim of a predator while flying in daylight.  Of course the vultures, hawks, and eagles fly during the day.  Swallows and swifts employ speed and agility on the wing to avoid becoming prey, as do hummingbirds.  Finches have an undulating flight, never flying on a horizontal plane, which makes their capture more difficult.  Other songbirds seen migrating by day, Red-winged Blackbirds for example, congregate into flocks soon after breeding season to avoid being alone.  Defense flocks change shape constantly as birds position themselves toward the center and away from the vulnerable fringes of the swarm.  The larger the flock, the safer the individual.  For a lone bird, large size can be a form of protection against all but the biggest of predators.  Among the more unusual defenses is that of birds like Indigo Buntings and other tropical migrants that fly across the Gulf of Mexico each autumn (often completing a portion of the flight during the day), risking exhaustion at sea to avoid the daylight hazards, including numerous predators, found in the coastal and arid lands of south Texas.  Above all, diurnal migrants capture our attention and provide a spectacle which fascinates us.  Perhaps diurnal migrants attract our favor because we can just stand or sit somewhere and watch them go by.  We can see, identify, and even count them.  It’s fantastic.

What about a bird like the Canada Goose (Branta canadensis)?  It is often seen migrating in flocks during the day (the truly migratory ones flying much higher than the local year-round resident “transplants”), but then, during the big peak movements of spring and fall, they can be heard overhead all through the night.  Perhaps the Canada Goose and related waterfowl bridge the gap between day and night, introducing us to the secretive starlight and moonshine commuters, the nocturnal migrants.

The high-flying migratory Canada Goose can be seen during the daytime and heard at night when passing over the lower Susquehanna River valley.  A large flight exiting from Chesapeake Bay in late February or early March often results in a 12 to 24 hour-long stream of northbound flocks.

The skies are sometimes filled with thousands of them, mostly small perching birds and waders.  These strangers in the night fly inconspicuously in small groups or individually, and most can be detected when passing above us only when heard making short calls to remain in contact with their travel partners.  They need not worry about predators, but instead must have a method of finding their way.  Many, like the Indigo Bunting, can navigate by the stars, a capability which certainly required many generations to refine.  The nocturnal migrants begin moving just after darkness falls and ascend without delay to establish a safe flight path void of obstacles (though lights and tall structures can create a deadly counter to this tactic).  Often, the only clue we have that a big overnight flight has occurred is the sudden appearance of new bird species or individuals, on occasion in great numbers, in a place where we observe regularly.  Just days ago, the arrival of various warbler species at Conewago Falls indicated that there was at least a small to moderate movement of these birds during previous nights.

In recent years, the availability of National Weather Service radar has brought the capability to observe nocturnal migrants into easy reach.  Through the night, you can log on to your local National Oceanic and Atmospheric Administration’s National Weather Service radar page (State College for the Conewago Falls area) and watch on the map as the masses of migrating bird pass through the sweep of the radar beam.  As they lift off just after nightfall, rising birds will create an echo as they enter the sweeping beam close to the radar site.  Then, due to the incline of the transmitted signal and the curvature of the earth, migrants will be displayed as an expanding donut-like ring around the radar’s map location as returns from climbing birds are received from progressively higher altitudes at increasing distances from the center of the site’s coverage area.  On a night with a local or regional flight, several radar locations may show signs of birds in the air.  On nights with a widespread flight, an exodus of sorts, the entire eastern half of the United States may display birds around the sites.  You’ll find the terrain in the east allows it to be well-covered while radars in the west are less effective due to the large mountains.  At daybreak, the donut-shaped displays around each radar site location on the map contract as birds descend out of the transmitted beam and are no longer detected.

Weather systems sometimes seem to motivate some flights and stifle others.  The first example seen below is a northbound spring exodus, the majority of which is probably migrants from the tropics, the Neotropical migrants, including our two dozen species of warblers.  A cold front passing into the northeastern United States appears to have stifled any flight behind it, while favorable winds from the southwest are motivating a heavy concentration ahead of the front.

National Oceanic and Atmospheric Administration/National Weather Service radar image from May 5, 2010, at 11:18 PM EDT, shows rain associated with a cold front moving east from the border of Ontario, Canada, and the United States into New England and the Mid-Atlantic region.  The heavy blue and green reflections surrounding the radar locations ahead of the front are nocturnal migrating birds taking advantage of favorable conditions for flight including a tail wind from the southwest.  Note the lighter migration behind the advancing front.  Heavy radar echoes on the gulf coast, particularly in Texas, indicate dense bird concentrations exiting the tropics to fan out into North American breeding areas.  The westward progression of expanding echoes surrounding individual radar sites indicates birds rising into the radar beam at local nightfall.

The second and third examples seen below are an autumn nocturnal migration movement, probably composed of many of the same tropics-bound species which were on the way north in the previous example.  Note that during autumn, the cold front seems to motivate the flight following its passage.  Ahead of the front, there is a reduced and, in places, undetectable volume of birds.  The two images below are separated by about 42 hours.

National Oceanic and Atmospheric Administration/National Weather Service radar image (still) from September 5, 2017, at 2:38 AM EDT, shows rain in the northeast associated with a slow-moving cold front stretching from Maine southwest to New Mexico.  Heaviest nocturnal bird flights can be seen behind the advancing front where there are favorable tail winds from the north or northwest.
Nearly two full days later, the slow-moving cold front from the previous image has crossed Pennsylvania.  As nightfall progresses from east to west, ascending nocturnal migrants enter NEXRAD radar beams, their echoes creating expanding rings around individual sites.  Concentrations of southbound birds can be seen along the gulf coast.  Many will follow the Texas coastline into Mexico.  The Neotropical migrants that try to cross the Gulf of Mexico this night could be in for a perilous voyage.  Hurricane Katia is churning in the southern gulf and a much stronger storm, Hurricane Irma, is rolling toward the Bahamas and Florida from the southeast.  Masses of birds that follow learned routes or instinct to venture offshore and cross seas under such circumstances could suffer catastrophic losses.  (NOAA/National Weather Service image)

You can easily learn much more about birds (and insects and bats) on radar, including both diurnal and nocturnal migrants, by visiting the Clemson University Radar Ornithology Laboratory (CUROL) website.  There you’ll find information on using the various mode settings on NEXRAD (Next-Generation Radar) to differentiate between birds, other flying animals, and inanimate airborne or grounded objects.  It’s superbly done and you’ll be glad you gave it a try.

SOURCES

Clemson University Radar Ornithology Laboratory (CUROL) website:   http://virtual.clemson.edu/groups/birdrad/    as accessed September 6, 2017.

 

The Wall

It was one of the very first of my memories.  From the lawn of our home I could look across the road and down the hill through a gap in the woodlands.  There I could see water, sometimes still with numerous boulders exposed, other times rushing, muddy, and roaring.  Behind these waters was a great stone wall and beyond that a wooded hillside.  I recall my dad asking me if I could see the dam down there.  I couldn’t see a dam, just fascinating water and the gray wall behind it.  I looked and searched but not a trace of a structure spanning the near to far shore was to be seen.  Finally, at some point, I answered in the affirmative to his query; I could see the dam…but I couldn’t.

We lived in a small house in the village of Falmouth along the Susquehanna River in the northwest corner of Lancaster County over fifty years ago.  A few years after we had left our riverside domicile and moved to a larger town, the little house was relocated to make way for an electric distribution sub-station and a second set of electric transmission wires in the gap in the woodlands.  The Brunner Island coal-fired electric generating station was being upgraded downstream and, just upstream, a new nuclear-powered generating station was being constructed on Three Mile Island.  To make way for the expanding energy grid, our former residence was trucked to a nearby boat landing where there were numerous other river shacks and cabins.  Because it was placed in the floodplain, the building was raised onto a set of wooden stilts to escape high water.  It didn’t help.  The recording-breaking floods of Hurricane Agnes in June of 1972 swept the house away.

The view through the cut in the woodland, a little wider than in the early 1960s with the addition of the newer electric transmission wire towers. The “Wall” is the same.

During the time we lived along the Susquehanna, the river experienced record-low flow rates, particularly in the autumn of 1963 and again in 1964.  My dad was a dedicated 8mm home-movie photographer.  Among his reels was film of buses parked haphazardly along the road (PA Route 441 today) near our home.  Sightseers were coming to explore the widely publicized dry riverbed and a curious moon-like landscape of cratered rocks and boulders.  It’s hard to fathom, but people did things like that during their weekends before football was invented.  Scores of visitors climbed through the rocks and truck-size boulders inspecting this peculiar scene.  My dad, his friends, and so many others with camera in hand were experiencing the amazing geological feature known as the Pothole Rocks of Conewago Falls.

Conewago Falls on the Susquehanna River and several exposed York Haven Diabase Pothole Rocks.  Lancaster (foreground) and Dauphin (center) Counties meet along a southwest to northeast borderline through the rapids.  Lands on the west shoreline in the background are in York County.  Three Mile Island is seen in the upper right.

The river here meets serious resistance as it pushes its way through the complex geology of south-central Pennsylvania.  These hard dark-gray rocks, York Haven Diabase, are igneous in origin.  Diabase sheets and sills intruded the Triassic sediments of the Gettysburg Formation here over 190 million years ago.  It may be difficult to visualize, but these sediments were eroded from surrounding mountains into the opening rift valley we call the Gettysburg Basin.  This rift and others in a line from Nova Scotia to Georgia formed as the supercontinent Pangaea began dividing into the continents we know today.  Eventually the Atlantic Ocean rift would dominate as the active dynamic force and open to separate Africa from North America.  The inactive Gettysburg Basin, filled with sediments and intruded by igneous diabase, would henceforth, like the mountainous highlands surrounding it, be subjected to millions of years of erosion.  Of the regional rocks, the formations of Triassic redbeds, sandstones, and particularly diabase in the Gettysburg Basin are among the more resistant to the forces of erosion.  Many less resistant older rocks, particularly those of surrounding mountains, are gone.  Today, the remains of the Gettysburg Basin’s rock formations stand as rolling highlands in the Piedmont Province.

Flooded from the heavy rains of Tropical Storm Lee, the sediment-laden Susquehanna River flows through the Gettysburg Basin just south of Harrisburg, PA, September 10, 2011.  The “Wall” as seen from space.  (NASA Earth Observatory Image)

The weekend visitors in 1963 and 1964 marveled at evidence of the river’s fight to break down the hard York Haven Diabase.   Scoured bedrock traced the water’s turbulent flow patterns through the topography of the falls.  Meltwater from the receding glaciers of the Pleistocene Ice Ages thousands to tens of thousands of years ago raged in high volume abrasive-loaded torrents to sculpt the Pothole Rocks into the forms we see today.  Our modern floodwaters with ice and fine suspended sediments continue to wear at the smooth rocks and boulders, yet few are broken or crumbled to be swept away.  It’s a very slow process.  The river elevation here drops approximately 19 feet in a quarter of a mile, a testament to the bedrock’s persisting resistance to erosion.  Conewago Falls stands as a natural anomaly on a predominantly uniform gradient along the lower Susquehanna’s downhill path from the Appalachian Mountains to the Chesapeake Bay.

The scene of dangerous tumbling rapids during high flows, the drought and low water of 1963 and 1964 had left the falls to resemble a placid scene; a moonscape during a time when people were obsessed with mankind’s effort to visit earth’s satellite.  Visitors saw the falls as few others had during the twentieth century.  Much of it was due to the presence of the wall.  I had to be a bit older than four years old to grasp it.  You see the wall and the dam are one and the same.  The wall is the York Haven Dam.

The initial segment, a crib dam constructed in 1885 by the York Haven Paper Company to supply water power to their mill, took advantage of the geomorphic features of the diabase bedrock of Conewago Falls to divert additional river flow into the abandoned Conewago Canal.  The former canal, opened in 1797 to allow passage around the rapids along the west shore, was being used as a headrace to channel water into the grinding mill’s turbines.  Strategic placement of this first wall directed as much water as possible toward the mill with the smallest dam practicable.  The York Haven Power Company incorporated the paper mill’s crib dam into the “run-of-the-river” dam built through the falls from the electric turbine powerhouse they constructed on the west shore to the southern portion of Three Mile Island more than a mile away.   The facility began electric generation in 1904.  The construction of the “Red Hill Dam” from the east shore of Three Mile Island to the river’s east shore made York Haven Dam a complete impoundment on the Susquehanna.  The pool, “Lake Frederic”, thus floods that portion of the Pothole Rocks of Conewago Falls located behind the dam.   On the downstream side, water spilling over or through the dam often inundates the rocks or renders them inaccessible.

During the droughts of the early 1960s, diversion of nearly all river flow to the York Haven Dam powerhouse cleared the way for weekend explorers to see the Pothole Rocks in detail.  Void of water, the intriguing bedrock of Conewago Falls below the dam greeted the curious with its ripples, cavities, and oddity.  It was an opportunity nature alone would not provide.  It was all because of the wall.

York Haven Dam and powerhouse. The “Wall” traverses Conewago Falls upstream to Three Mile Island to direct water to the powerhouse on the west shore of the Susquehanna River.

SOURCES

Smith, Stephen H.  2015.  #6 York Haven Paper Company; on the Site of One of the Earliest Canals in America.  York Past website www.yorkblog.com/yorkpast/2015/02/17/6-york-haven-paper-company-on-the-site-of-one-of-the-earliest-canals-in-america/  as accessed July 17, 2017.

Stranahan, Susan Q.  1993.  Susquehanna, River of Dreams.  The Johns Hopkins University Press.  Baltimore, Maryland.

Van Diver, Bradford B.  1990.  Roadside Geology of Pennsylvania.  Mountain Press Publishing Company.  Missoula, Montana.