Pleistocene Robins (Turdus migratorius) Depended Upon Heavily-grazed Landscapes

Robins are year round residents throughout most of North America, excepting Canada and Alaska, but populations of this species tend to shift south during winter.  There is no organized migration.  Instead, flocks of robins congregate wherever they find food.  They eat insects and earthworms, but fruit makes up 60% of their diet. During winter robins can sometimes subsist entirely on overripe crabapples stubbornly clinging to leafless trees or even seemingly unpalatable juniper berries.  Robins are attracted to areas with no snow cover where they can hunt for insects and worms, and they will fly for many miles looking for snowless landscapes.  In Augusta, Georgia I see the highest number of robins in February and March when local populations are supplemented by an influx of “Yankee” robins.

Robins prefer to forage in landscapes with short grass, so they can observe and avoid potential predators.  Yet, they also like areas with shrubs and short trees for nesting, though if push comes to shove, they will nest on open ground.  Humans have created the ideal landscapes for robins by constructing and maintaining yard lawns, golf courses, cemeteries, and plowed fields.  Nevertheless, robins did exist in North America before humans ever colonized this continent.  I hypothesize Pleistocene megafauna shaped favorable habitat for robins.  Mammoths, bison, and horses heavily grazed grasslands, keeping the grass in many areas cropped short, so that a patchwork of short and tall grass habitats occurred.  The fruit seeds defecated by mastodons and ground sloths sprouted into natural orchards, providing winter and autumn food and nesting habitat for robins.  Perhaps robins were just as abundant during the Pleistocene as they are today, but there is no way of comparing Pleistocene populations with modern numbers.

https://i1.wp.com/i.telegraph.co.uk/multimedia/archive/02141/bird-worm-1_2141751i.jpg

Robins can hear earthworms moving underground.  This species of thrush forages on landscapes with short grass where they can observe potential threats.  During the Pleistocene the only landscapes with short grass were those heavily grazed by megafauna.

American Robin Range Map

Robin range map.

Pleistocene bird fossils are less common than mammal fossils because bird bones are hollow and easily crushed.  Some species of birds that were probably common during the Pleistocene are absent or nearly absent in the fossil record.  However,  I used a google search combined with data from the paleobiology database and found 8 Pleistocene-aged fossil sites that included the remains of robins in their list of species.  Specimens of Pleistocene-aged robins have been found at Hiscock in New York, the La Brae Tar Pits in California, Bell Cave in Alabama, Cheek Bend Cave in Tennessee, Clark’s Cave in Virginia, Natural Chimneys in Virginia, Dead Man’s Cave in Arizona, and Little Box Elder Cave in Wyoming.

Genetic evidence suggests robins have had as many as 5 transcontinental divergences, though they were shallow compared to those of other species of thrushes.  These divergences correspond to changes in the distribution of favorable habitats that occurred during Ice Ages.  Different populations of robins became isolated from other populations for thousands of years, but the haphazard migratory habits of this species allowed some occasional intermingling.  This is unlike many other species of North America birds that became completely separated into eastern and western clades and in some cases resulted in speciation.

Rufous-collared Thrush

Rufus-Collared Thrush (Turdus ruftorques).  This is the closest living relative of the robin.  The robin’s evolutionary ancestor probably looked similar to this species.  They are found in the highlands of Central America.

Genetic evidence also suggests robins are a relatively young species, having existed for about 320,000 years.  Other species of thrushes are thought to be much older–Hunt’s thrush has existed for 4 million years and Swainson’s thrush has existed for 2.6 million years.  Robins evolved from an unknown now extinct species of thrush.  Their closest living relative is the rufus-collared thrush found in the highlands of Central America.

Reference:

Topp, Carrie; et. al.

“How Migratory Thrushes Conquered North America: A Comparative Phylogeographic Approach”

PeerJ 2013

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One Response to “Pleistocene Robins (Turdus migratorius) Depended Upon Heavily-grazed Landscapes”

  1. Pinkney Says:

    The fields on Peter’s Point are loaded with Robins. Thanks for this. I wondered what the hell these flocks of Robins were doing in the short grass fields. Lots of birds back here, but February has clogged with Robins.

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