Where did Golden Plovers (Pluvialis dominica) Breed during Ice Ages?

 

 

The golden plover has an incredible migratory route.  This tireless bird breeds near the arctic circle in northern Canada and Alaska, flies south over the Atlantic Ocean off the coast of North America during fall, spends its winters on the Argentine pampas, and travels north up the Mississippi River Valley back toward its breeding grounds in spring.  Golden plovers used to be abundant–along with Eskimo curlews they were the great flocks of birds Columbus saw that called his attention to the Caribbean Islands.  But market hunters, looking to replace the passenger pigeon with another edible bird, turned their shotguns toward golden plovers, and their population has yet to recover from this late 19th century slaughter.

Golden plovers prefer open habitat and breed on rocky slopes with some vegetative cover.  Birders see them on beaches, golf courses, pastures, airports, and tilled land.  Formerly, when this species was abundant, storms would blow tens of thousands of migrating golden plovers inland as far as the “hills.”

Pluvialis dominica (American golden plover)

Golden plover.

American Golden-Plover Range Map

Golden plover range map.  The breed in Alaska and northern Canada, winter on grasslands in Argentina, and migrate through the vast regions between.

Golden plover remains, dating to the Pleistocene, have been found at just 3 sites–2 in South America and 1 in Clark’s Cave, Virginia.  The specimen found in Virginia’s Appalachian Mountains must have been from a migrating flock blown far inland by a storm.  (The South American specimens were from winter populations.)  When I learned about the golden plover specimen found in Clark’s Cave, I wondered where this species bred during the Ice Age.  Their present day breeding grounds were under glacial ice then.  I thought maybe they had a compressed migratory route like ducks and geese probably had.  Though much of the present day summer breeding grounds for ducks and geese were under glacial ice during Ice Ages, there were still plenty of lakes along the Ohio River impounded by glacial advance.  These lakes served as an acceptable substitute for present day Canadian wetlands.  I assumed golden plovers found suitable barren ground favorable for breeding along the edge of the ice sheet.  However, upon further research, I learned of an ornithologist who had already speculated about this mystery.  He believes golden plovers bred on nunataks located in northern Canada that the ice sheet advanced past but never covered.

A nunatak is a rocky highland surrounded by glaciers.  Nunataks serve as a refuge for plants and animals that are surrounded by uninhabitable ice.  Geologists determined Banks Island and Victoria Island in northern Canada were never covered by ice, though they were completely surrounded by the glacier.  The islands enjoyed a brief 2 month summer–long enough for a few species of plants and insects to flourish and provide food for golden plovers and their nestlings.  Golden plovers eat insects, berries, seeds, and plant matter including seaweed.  There would have been little competition for this limited food supply.  The amount of land available for nesting was greatly reduced during Ice Ages, but golden plovers were probably safe from nest predators then.  Nunataks likely supported no predators because they were barren of prey for 10 months of the year.  Therefore, populations of golden plovers during Ice Ages may have been equal to interglacial numbers–less breeding space but the space they had was devoid of predators.

Banks Island, Canada.svg

Banks Island, Canada.  This and neighboring Victoria Island were nunataks during the last Ice Age.  They were rocky sparsely vegetated habitat surrounded by ice during the Ice Age.  They served as refugia for breeding golden plovers.  Golden plovers  diverged into 3 species due to Ice Age isolation of their breeding grounds.

Diagram of a nunatak.  Glaciers advanced past rocky highlands which remained free of the ice sheet.  They supported limited flora and fauna.

Ice Ages probably caused the divergence of the 3 species of golden plovers from a single Holarctic population.  Glaciers isolated breeding populations.  Pluvialis dominica bred on Banks Island and Victoria Island and migrated south along the Atlantic coast.  The Pacific golden plover (P. fulva) bred in Beringia and migrated south to Australasia and Hawaii.  The European golden plover (P. apricaria) bred in northern Europe and migrated south to north Africa.  After glacial retreat breeding populations of golden plovers expanded their ranges and came into contact with each other, but hybridization between species is rare.  Hybrids between species are less likely to survive because the genes that control where they migrate south are not in sync.  The speciation of the golden plovers is a good example of how evolution works.  Other species of arctic-breeding birds likely have a similar history of Ice Age isolation including 2 subspecies of white-fronted geese and western and semipalmated sandpipers.

The golden plover’s migratory route over the Atlantic Ocean was dry land during the Ice Ages because the continental shelf was exposed then.  A route over dry land makes more sense for survival.  Dry land habitat provides forage and rest for weary birds.  Yet, the instinct to fly over the inundated continental shelf remains strong.  I propose the instinct to fly this route, despite the greater risk, is a relic of the Ice Age when the continental shelf was dry land habitat.  The stubborn instinct to use the same migratory route year after year served the species well when they continued flying over the vast Laurentide Glacier during the Ice Age to reach their breeding grounds.  This amazing bird flew over hundreds of miles of ice then and now fly over hundreds of miles of open water.  The perseverance is literally a part of their DNA.

References:

Connors, Peter

“Taxonomy, Distribution, and Evolution of Golden Plovers (Pluvialis dominica and Pluvialis fulva)”

The Auk 100 July 1983

Tretten, H.P.

Geology of the Innutian Orogene and Arctic Platform of Canada and Greenland

Geological Survey of Canada 1991

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