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.”

Golden plover.

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.  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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A Pleistocene Great Horned Owl’s (Bubo virginianus) Roost in Virginia

Clark’s Cave is a rare site where remains of Ice Age birds have been found.  Bird bones are fragile and less commonly preserved than mammal fossils.  We know little about the distribution of bird species during the Pleistocene, even though many species must have been at least as abundant as they are today.  Yet, Clark’s Cave provides the only known Pleistocene occurrence for some bird species.  Scientists excavated these remains in the 1970’s, and an 80 page paper about them was published in 1977 by the Carnegie Museum of Natural History.  The paleobiology database lists the species recovered from this site.  I studied this list and concluded the cave must have served as a great horned owl’s roost for decades. Perhaps generations of owls roosted here.

Clark’s Cave is in Bath County, Virginia.  The remains of prey inside the cave suggest it was a great horned owl’s roost for generations, perhaps centuries.

Great horned owls prey on skunks so frequently, they often smell of skunk.  The spray doesn’t seem to bother them.

Great horned owls are a top predator of small animals wherever they occur, and they have been recorded taking a wide range of prey species.  I noticed the list of small species recovered from Clark’s Cave closely matched what one could find on a great horned owl’s menu.  This large owl carried striped skunks (Mephitis mephitis) and 5 kinds of weasels into the cave including pine marten (Martes americana), mink (Mustela vison), long-tailed (M. frenata), short-tailed (M. ermine), and least (M. nivalis).  Though some of these species may have established dens inside the cave, I think it’s highly unlikely every species that lived in the region would naturally inhabit and die in the cave.  This diverse assemblage could best be explained by the activities of a predator’s daily hunting.  Snowshoe hares (Lepus americana) and cottontail rabbits fell victim to the Pleistocene owls.  The paper identified the latter remains as a New England cottontail (Sylvilagus transitionalis), but this paper was written before the Appalachian cottontail (S. obscurus) was recognized as a distinct species.  I’m not sure if a distinction between the 2 cottontails can be made without a molecular study of the genes.  The owls brought 23 species of rodents into the cave.  Flying squirrels are active at night, so it’s no surprise both northern (Glaucomys sabrinus) and southern (G. volans) were eaten by nocturnal owls.  Even porcupines (Erithizon dorsatum)get killed by great horned owls.  Great horned owls probably also ate the smaller birds of prey found in the cave–sharp shinned hawk (Accipiter striatus), broad winged hawk (Buteo platyperus), kestrel (Falco sparverius), screech owl (Otus asio), short-eared owl (Asio flammeus), and saw whet owl (Agolus acadicus).  Great horned owls are notorious decimators of game birds such as turkey (Meleagris gallopavo), bobwhite (Colinus virginianus), and ruffed grouse (Bonasa umbellus).  Great horned owls destroy waterfowl–both ducks and wading birds.  The owls brought 5 species of ducks and 4 species of rails into the cave.  This is the best evidence the cave represents owl predation because neither duck nor rail would frequent cave habitat.  Some of the 29 other species of songbirds may have ended up in the cave, not as a victim of owls, but for some other reason.  Chimney swifts (Chaetura pelagica) are too fast for owls to catch but would nest inside a cave.  However, most of the species of birds found in the cave were likely killed by owls.  The great horned owls even caught slow-moving fish (eels, suckers, minnows, catfish, pickerel) that otherwise would have never been found in a cave environment.

Remains of the least chipmunk (Eutamias minimus) were the most surprising find at this site.  The least chipmunk no longer occurs this far east but is presently restricted to western Canada and the Rocky Mountains.

Remains of the least chipmunk were found in Clark’s Cave.  It no longer ranges this far east.

Modern day range map of the least chipmunk.  Over half of its present day range was under glacial ice during the Ice Age.  The uninhabitable ice sheet forced it to occupy range south at least as far as Virginia where it co-occurred with the eastern chipmunk.

An uninhabitable glacier covered about half of their present day range during the late Pleistocene.  The range of this species shifted south where it co-occurred with the eastern chipmunk (Tamias striatus).  Remains of the least chipmunk have also been excavated from New Trout Cave, West Virginia–a site also dated to the late Pleistocene.

Not every species recovered from Clark’s Cave was a victim of owl predation.  Black bears (Ursus americanus) likely used the cave as a winter den and dire wolves (Canis dirus) raised pups here.  These carnivores dragged white tail deer (Odocoileus virginiana) and elk (Cervus canadensis) into the cave.  None of the familiar now extinct Pleistocene megafauna (aside from the dire wolf) left remains in the cave.  Was it chance or did the deposition of the cave occur after most of the Pleistocene megafauna became extinct?  The cave was studied before carbon dating was refined.  Researcher should re-date this material.

7 species of bats roosted in the cave.  Most were probably not owl prey but a few might be.

The composition of animal species recovered from the cave tells us what kinds of environments existed within a great horned owl’s home range here during the Pleistocene.  The presence of northern flying squirrel, red squirrel, and pine marten indicates red spruce forest at higher elevations.  A mix of red spruce and deciduous forest must have occurred at lower elevations because gray squirrel, southern flying squirrel, and chipmunk lived here.  Direct evidence of oak, hickory, hackberry, and tupelo was found inside the cave.  Elk and woodchuck are evidence of mountain meadows.  Muskrat, duck, and rail prove grassy marshes contributed to the diversity of habitats.  Young dense forest and shrub covered some areas preferred by rabbit, hare, and grouse.  Meadowlark and 13-lined ground squirrel needed treeless prairies.  A Paleo-Indian could have walked across all 6 of these habitats in a day.  Thanks to the great horned owl, we can imagine the kinds of environments a Paleo-Indian may have experienced on a day’s walk.

Reference:

Guilday, J.E.; P.W. Parmalee and H.W. Hamilton

“The Clark’s Cave Bone Deposit and the Late Pleistocene Paleoecology of the Central Appalachian Mountains of Virginia”

Bulletin of the Carnegie Museum of Natural History 1977

The above reference is available at the Carnegie Museum website for $13 plus shipping.  I never read the paper but the following link lists every species found inside the cave including 6 orders of insects.  That was enough info for this blog entry.

http://fossilworks.org/bridge.pl?a=collectionSearch&last_collection=94426&collection_name=Clark%27s+Cave

A Good Narrative about the American Cheetah (Miracinonyx trumani) may be Ruined but maybe not

The close physical similarity between the extinct cheetah (Miracinonyx trumani) of Pleistocene North America, and the still extant cheetah (Acinonyx jubatus) of Africa and Asia caused confusion among paleontologists.  The anatomy of both species shared characteristics of a cat built for great speed.  Paleontologists thought cheetahs originally evolved in North America and later colonized Asia and Africa.  Then, based on a re-evaluation of the fossil evidence and new genetic studies, scientists realized the similarity between the Old World cheetah and the North American cheetah was just a case of convergent evolution that occurs when 2 unrelated species evolve similar traits to help them adapt to similar environments.  The 2 species were not as closely related as formerly thought.  Instead, the North American cheetah evolved from an extinct Asian cougar (Puma pardoides) that crossed the Bering Land Bridge over 6 million years ago.  After Puma pardoides colonized North America, the species diverged into 3 lineages.  One line led to an animal adapted for hunting on the grassy plains–M. trumani.  Another line evolved into the jaguarundi (Puma jagouaroundi), a small cat of tropical brush habitat.  The third line evolved into the modern cougar (Puma concolor), a generalist species well adapted for living in a wide range of environments.  Puma concolor doesn’t occur in the fossil record until ~500,000 years BP, but I believe its evolutionary predecessor was Miracinonyx inexpectus.  Temporally, fossil material of Puma concolor and M. inexpectus doesn’t overlap. The latter was likely the late Pliocene/early Pleistocene version of the cougar.  Miracinonyx studeri, a scientific name used in some studies, is merely a synonym for M. inexpectus.

Artist’s depiction of an American cheetah chasing a pronghorn.  Pronghorns can run up to 60 miles per hour.  No extant predator in North America even comes close to this.  An analysis of the anatomy of the extinct American cheetah suggests it was built for this kind of speed with long legs, flexible spine, and large nasal passages for rapid air intake.

Pronghorn antelopes (Antilocapra americana) reach speeds far exceeding any extant predator living in North America.  Scientists hypothesized they evolved this capability to outrun a predator that is now extinct.  They believe M. trumani was that predator.

A few years ago, paleontologists excavated fossil material they identified as M. trumani from several caves within the Grand Canyon.  This high elevation habitat was home to mountain goats (Oreamnos harrington and Oreamnos americanus) not pronghorns.  These scientists proposed the American cheetah, at least at this locality, occupied a niche like that of an alpine snow leopard (Uncia uncia), a big cat that hunts on steep rocky slopes.  It would seem the narrative about the American cheetah and pronghorn might be ruined.  However, Ross Barnett, author of a study referenced below, is not convinced the fossil material found in the Grand Canyon is from American cheetah.  These specimens were identified by comparing them with bones from modern cougars and other American cheetah remains.  M. trumani was somewhat larger than modern cougars, so it was assumed the Grand Canyon material represented American cheetah, not cougar.  Dr. Barnett suggests the material should have been compared with fossil remains of Pleistocene cougars which were on average larger than modern cougar.  The Grand Canyon material may actually be Pleistocene cougar.  Cougars are well adapted for living on steep slopes. So the narrative of the American cheetah and the pronghorn may not be ruined. Incidentally, the cougars that lived in North America were an extinct ectomorph–all modern North American cougars descend from a small population originating from eastern South America.

There’s no fossil evidence M. trumani ever lived east of the Mississippi River.  But M. inexpectus and Puma concolor are a common enough (for a large carnivore) find in fossil sites throughout southeastern North America.

Some now refer to the American cheetah as the “false cheetah.”  I don’t think the adjective “false” should be used to describe an animal, simply because humans were once confused about its evolutionary relationships.

References:

Barnett, Ross; et. al.

“Evolution of the Extinct Sabre-tooths and the American Cheetah-like Cat”

Current Biology 2005

Hodnett, Jean-Paul; et. al.

“Miracinonyx trumani (Carnivore: Felidae) from the Rancholabrean of Grand Canyon, Arizona and its Implications for the Ecology of the American Cheetah”

Programs and Abstracts, Journal of Vertebrate Paleontology 2010

Rapid Sea Level Rise on the Georgia Coast

Glaciers expanded and sea levels fell during Ice Ages.  20,000 years ago, the Georgia coast was located 90 miles to the east of the present day shoreline.  The continental shelf off the Georgia coast (known as the Georgia bight) was above sea level between 80,000 BP-7,000 BP.  Near the end of the Ice Age, glacial ice melted and sea level rose rapidly but in stages.  The partial dissolution of glacial Lake Agassiz in Canada 12,900 years ago caused a sudden rise in sea level.  The Atlantic Ocean likely advanced toward the present day shoreline many miles in a few short years.  But an ice dam reformed, containing the rest of Lake Agassiz’s water until 8200 BP when it completely collapsed, releasing the rest of this great lake’s water.  (Lake Agassiz contained more water than all of the modern Great Lakes combined.)  This caused another sudden rise in sea level along the Georgia coast.  By 4500 BP the Georgia coast consisted of just 2 barrier islands separated by the southeasterly flowing Altamaha River.  The Ogeechee, Satilla, and St. Mary’s Rivers were tributaries of the Altamaha and didn’t directly drain into the Atlantic Ocean as they do today.  The Altamaha drained into a huge sound located to the east of where Little Cumberland Island stands today.  Sea level rose again, flooding and eroding the areas where the Ogeechee, Satilla, and St. Mary’s flowed into the Altamaha and splitting the 2 barrier islands into 10 smaller ones.  All of these rivers formerly flowed southeasterly, but the high marine transgression caused them to straighten to a more easterly direction.

Map of the  modern day Georgia coast.  About 4500 years ago, there were only 2 barrier islands between the Savannah River and the Florida border, and they were bisected by the Altamaha River which then emptied into a huge sound located at the same latitude as modern day Little Cumberland Island.  The other rivers in this region that today flow into the ocean were tributaries of the Altamaha.

Aerial view of salt marsh in Glynn County, Georgia.  There are 4500 year old logs from a rapidly flooded forest underneath the surface.

Wood storks (Mycteria Americana) on a salt marsh behind Sapelo Island.  There are now 10 major barrier islands off the Georgia coast, but before 4500 years ago, there were just 2, separated by the Altamaha River.  A person using an excavator to dig here would probably find 4500-2000 year old logs in less than an hour.

The Holocene Climatic Optimum between 9000 BP-5000 BP was a warm climatic phase, especially at higher latitudes, resulting from both the northern hemispheric tilt of 24 degrees and the timing of the perihelion.  The earth was closest to the sun during the boreal summer.  Summers were much warmer but winters were cooler.  The effects of the Holocene Climatic Optimum were felt in different regions at different times.  The sea level rise on the Georgia coast occurred toward the end of this climatic phase.  The ocean’s response to the Holocene Climatic Optimum may be a factor explaining this marine transgression.  Tectonic processes (the shifting of the earth’s crust) and isostatic adjustment (the rebound and subsidence of the earth’s crust in response to the weight of glaciers) also probably played a role in this sea level rise.

Scientists studying the rapid rise of sea level on the Georgia coast find fossil trees beneath salt marshes, and they date to between 4400 BP-2000 BP.  St. Simon’s and Jekyll Islands were connected until 2000 BP.  The youngest fossil trees are found here.  This was one of the last areas of the coast to get inundated, though Ossabaw Sound was breached about the same time. There are abandoned river channels beneath salt marshes as well. Sea level is still rising but not at an unprecedented rate as global warming alarmists and political pundits claim.  Sea levels haven’t even reached the marine transgression high stand of the last interglacial known as the Sangamonian (~132,000 BP-~118,000 BP).

References:

Chowns, Timothy

“Drainage Changes at Ossabaw, St. Catherine’s, and Sapelo Islands and their Influence on Island Morphology and Spit building on St. Catherine’s Island”

American Museum of Natural History Anthropology Papers 94

Chowns, Timothy; and S. Hannah Hill

“Mid-Holocene Sea Level Rise on the Georgia Coast”

Georgia Journal of Science Abstracts 2014

Glaciers Shaped the Ohio River

Weak Ice Ages began occurring as early as 5 million years ago.  Gradually, they became more severe.  1.4 million years ago, for the first time, glaciers advanced through valleys incised by the Erigan River drainage.  This river system flowed through the present day sites of the Great Lakes which didn’t exist yet.  The Laurentide ice sheet obliterated the Erigan River system and advanced beyond another major, now extinct, river–the Teays.  The Teays River began in the North Carolina mountains and flowed in a northwesterly direction through what today is Virginia, West Virginia, Ohio, Indiana, and Illinois before emptying into the Mississippi River.  Glaciers formed a dam, blocking the northwesterly flow of the Teays River and creating the massive Lake Tight, a 7000 square mile body of water as deep as 800 feet in some spots.   Lake Tight must have been quite a sight–gray gravel and ice on the northwestern side and green boreal forests of spruce, pine, and northern hardwoods on the southeastern shore.  Many species of fish lived in the water, attracting great flocks of gulls; and it was a summer destination for duck, goose, and swan.  The churning waters spawned big waves like those of an ocean rather than a lake.  Overflow from the lake was captured by a minor tributary of the Cumberland River.  The ice forced the water to erode backward into bedrock, lengthening this tributary. This large creek/small river became the mighty Ohio river.  When the glacier retreated, the ice dam melted, releasing an incredible quantity of water into the Ohio river and incising a deeper valley toward its outlet, the Mississippi River.

The ancient Teays River was  a major regional drainage system during the Pliocene and early Pleistocene.  The advance of glaciers during Pleistocene Ice Ages dammed this river, allowing a minor tributary of the Cumberland River to capture the stream flow.  This small river became the mighty Ohio.

Map of Ohio River drainage. Glaciers pushed the water content of the Teays River south, creating the Ohio River instead.  Formerly, it was a small tributary.

Subsequent glacial advances during Ice Ages over the past 1.4 million years have had a major influence on the shape of the Ohio River.  The southern lobe of the Laurentide ice sheet frequently advanced far enough south to push sediment into the northern part of the Ohio River, damming tributaries and creating an extensive network of lakes.  During glacial maximums there were always a chain of lakes along the Ohio border with West Virginia and Kentucky.  The Illinois Ice Age was 1 of the most severe.  It lasted from ~240,000 BP-~135,000 BP.  The Laurentide ice sheet advanced as far south as northern Kentucky–its greatest extent ever.  This backed up lakes from the present day site of Louisville to the Pennsylvania border, forcing water into the Ohio River headwaters and incising 45 feet of bedrock.

Though the Wisconsin Ice Age (~114,000 BP-~11,000 BP) was not as severe as the previous glacial advance, the Ohio River valley was frequently incised by pulses of glacial meltwater.  A recent study of river sediment found that changes in the Ohio River were closely correlated with global climate change.  Warmer climate phases within the Ice Age were associated with greater incising and erosion, resulting from melting ice and large water discharge.  Colder climate phases and lower water discharge caused greater sediment build-up, known as aggradation.

Today, the Teays River valley is mostly hidden by sediment, but its descendent is clearly visible on maps.  Government officials used the Ohio River as a convenient demarcation to draw up borders between states.  Imagine how different a modern day map of the United States would look, if there had been no Ice Ages, and accordingly, no Ohio River worth noting.

Reference:

Counts, Ronald; et. al.

“Late Quaternary Chronostratigraphic Framework of Terraces and Alluvium along the lower Ohio River, Southwestern Indiana, and Western Kentucky”

Quaternary Science Reviews February 2015

 

Bird Songs of the Pleistocene

The other day, during my morning jog, I spotted a summer tanager, fluttering over a wooden railroad tie used to border a driveway.  This was the first summer tanager (Piranga rubra) I had ever seen, though decades ago I did see a scarlet tanager (P. olivicaea).   Summer tanagers are summer migrants, known in the south as summer redbirds, while cardinals (Cardinales cardinales) are known as winter redbirds because they are year round residents.  In 2009 North American tanagers were reclassified and are now considered part of the cardinal family.  This is surprising because cardinals primarily feed on seeds, but tanagers are insect and berry eaters.  Summer tanagers are heard more often than they are seen.  They inhabit high tree tops where they hunt bees and wasps.  Amazingly, tanagers catch, kill, and eat members of the Hymenoptera order without getting stung.  When I searched the Cornell University ornithology website to listen to summer tanager vocalizations, I discovered their song was a familiar sound that I’d been hearing every summer for years.  A few days later, I spotted a summer tanager again and was able to take the following photo.

Click on the photo to enlarge and see the summer tanager in my blueberry bush.

Here’s a better photo than the 1 I took.

In May during the peak nesting season I like to sit in my backyard and listen to birds calling.  It’s easiest to learn bird calls by witnessing each specific species make its call.  This is a better method than just listening to bird calls on the internet because after a while, they all kind of run together, and it’s hard to remember the distinctions between them.  Moreover, I’ve seen birds make certain calls that are not among those recorded on Cornell University’s website.  To complicate matters, some birds, such as mockingbirds, imitate vocalizations of other birds, animals, and even people.

Birds don’t sing for their own amusement.  Their vocalizations serve practical purposes.  Birds sing to establish mating territory and to maintain contact with members of their own species.  During nesting season the sound of bird calls near my house is almost constant. But I did notice the music stopped when a flock of predatory crows raided my neighbor’s tipped-over trash can a few days ago.  As soon as the crows left the area, the birds began singing again.  Perhaps, the birds didn’t want to give their nest locations away to the egg-eating crows.

I wonder how the bird songs I hear today near my house differ from those that could have been heard at this same location 36,000 years ago.  Many of the present day common species were likely uncommon during the late Pleistocene.  Then, some of these species probably depended on an unusual landscape niche.  Conversely, many presently rare and a few extinct species may have been common during the late Pleistocene. 7 species of birds nest on or near my property this spring including chimney swifts (Chaetura pelagica), Carolina wrens (Thryothorus ludovicianus), tufted titmouse (Baeolophus bicolor), mockingbird (Mimus polyglottos), brown thrasher (Toxostoma rufum), robin (Turdus migratorius), cardinal, and summer tanager.  Bird remains dating to the Pleistocene are most commonly found in caves but have also been excavated from river deposits.  Neither is reliable as a complete inventory of former avifauna diversity.  Caves harbor remains of species that prefer cave habitats or were vulnerable to birds of prey that roosted in caves. Remains of  bird species adept at avoiding predators may never be found in caves. And birds that don’t often fly over water won’t be found in river deposits.  Nevertheless, aside from genetic studies, the fossil record is the only evidence of ancient avifauna abundance and diversity.

I searched the paleodatabase and the Florida Museum of Natural History database for the Pleistocene occurrence of the 7 species of birds nesting near my property.  Mockingbirds, an extremely common species, are known from just a single Pleistocene-aged site in Florida.  Brown thrashers are known from 4 sites dating as far back as the mid-Pleistocene including 2 in Florida and 2 in Virginia.  Robins are known from 8 sites located all over the continent.  The tufted titmouse has been found at just 1 site in Virginia.  Cardinals have been identified from 4 sites, all in Florida.  Tanagers are known from 3 sites–1 in Florida, 1 in Alabama, and 1 in Virginia.  Carolina wrens are absent from the fossil record.

Obviously, these species did live during the Pleistocene and were more common than the fossil record suggests.  But it is likely that some were less common then and much prefer human-altered habitat.  William Bartram, the 18th century naturalist, noted while traveling through wilderness that the forest was silent, but he knew when he was approaching civilization because he would begin hearing bird songs near human settlements.  Humans create varied habitat that is more attractive to a greater number of bird species than unbroken wilderness.  Instead of virgin old growth forest, anthropogenic habitats include agricultural fields, overgrown orchards, land left fallow, and abandoned land reverting to young forests.  Some Pleistocene landscapes may have mimicked this mix of habitats, but these were the result of megafaunal interactions with the environment along with the rapid cyclical climate changes of the Ice Age.

I suspect cardinals, mockingbirds, and Carolina wrens were less common during the late Pleistocene than they are today.  Cardinals have greatly expanded their range north, thanks to bird feeders provided by people.  But 10,000 years ago, cardinals were likely a bird restricted to the southeast.  Carolina wrens like to nest on human made substrates and would’ve been hard pressed to find quality nesting locations.  The other 4 species nesting near my yard may have been as common during the Pleistocene as they are today in some habitats.  Robins probably found heavily-grazed locations to their liking.  The high tree canopy of virgin forests would have made tanagers happy.  Today, chimney swifts almost exclusively nest in chimneys, but large hollow trees in old growth forests served as nesting colonies for chimney swifts just a few centuries ago.  And the leaf litter of untouched woods is the perfect foraging ground for brown thrashers.  In addition birds of deep wilderness not seen today in my neighborhood probably nested near the location of my property then.  I think hairy woodpeckers, ruffed grouse, turkeys, ravens, and magpies may have nested here 36,000 years ago.

Some people are really into keeping purple martins. Note the installed net used to protect the birds from snakes. Before native Americans practiced agriculture, purple martins were likely an uncommon bird. They are absent from the fossil record.  Now, they are common and entirely reliant on anthropogenic nesting structures.

The purple martin (Progne sobis) is an example of a bird that has become entirely dependent upon humans.  Until about 8,000 years ago, this species was likely an uncommon bird that used abandoned woodpecker holes and natural hollows for nesting.  But Indians began cultivating a gourd-like squash, and purple martins nested inside the hollowed out squash.  Indians discovered this habit and starting hanging multiple containers made of dried squash on tree saplings to attract the birds.  Purple martins eat noxious insects and chase crows away from cornfields.  What may have begun as an entertaining curiosity for Indians became a beneficial practice.  Purple martin populations increased because the proximity of the nests spurred orgiastic behavior, greatly improving rates of reproduction.  Some of the bird species nesting in my yard today may be less extreme examples of dependence on human-altered habitat, but they seem to like this location.

Lynch’s Crater

Long ago, a volcano collapsed, creating an 80 yard deep crater.  Lynch’s Crater, located in northeastern Australia, has since become half-filled with 230,000 years worth of lake and marsh sediment. This sedimentary deposit has preserved pollen evidence spanning 2 complete glacial/interglacial cycles.  From this evidence scientists know a wet rain forest prevailed in this region until man arrived here about 40,000 years ago.  The actions of Australian aborigines converted the rain forest to a dry woods dominated by fire-adapted trees such as eucalyptus.  This environmental change was not associated with any shift in glacial cycle.  Instead, man overhunted the megafauna into extinction and began setting frequent fires.  The large biomass of megaherbivores was no longer consuming vast amounts of vegetation, leaving lots of flammable material for men to burn.  Grazers and browsers were no longer suppressing plant growth, facilitating seed dispersal, and recycling nutrients in their dung.  Species of fungi, dependent upon megaherbivore dung for reproduction, declined in abundance.  Scientists use the measurable quantity of dung fungus spores in dated cores as a proxy for the former biomass of large herbivores.  Dung fungus is actually a  more reliable indicator of former megaherbivore presence than the fossil remains of these beasts because bones are rarely preserved.  Scientists have used this clue to study ancient megafaunal populations in North America, Europe, Madagascar, and Australia.  However, some researchers have noted some problems with using dung fungus spores as a proxy for megafaunal populations.  Chris Johnson, an Australian zoologist, along with other scientists, have addressed these concerns by implementing solutions in a study of data collected from Lynch’s Crater.

Location of Lynch’s Crater.  Sediment within the crater provide a 230,000 pollen record, illustrating how plant and animal communities changed over time.

 

Photo of Lynch’s Crater.  For over 100,000 years it was a lake but over the past 50,000 years it has been a marsh.

Some researchers have noted that dung fungus spores disperse over short distances, and their abundance can be effected by drought.  This can cause a variability in spore abundance unrelated to the abundance of megeherbivores.  Another problem is the variation in the amount of pollen produced by plants.  Because dung fungus is numerically expressed as a value relative to pollen counts, it can be difficult to compare fungus proxy values between studies.  Dr. Johnson and his colleagues executed 3 solutions to these problems.

1. They took core samples from different locations within the study area to minimize local effects.

2. They expressed dung fungus abundance independently from pollen counts.  They found interpretations of spore counts when expressed as a percent of pollen were not influenced by changes in vegetation type.

3. They compared trends in the quantity of dung fungus spores with spores from fungi that don’t rely on megaherbivore dung for reproduction.

 

 

 

 

Sordaria humana.  This species of dung fungus prefers human and dog shit.  While other species of dung fungus declined in abundance following the extinction of the megafauna, the abundance of this species remained strong and even increased after humans colonized Australia.

This study counted the volume of spores from 5 genera of fungi extracted from dated cores.  Sporormiella and podospora depend upon megaherbivore dung for reproduction.  Sordaria, coniochaeta, and cerophora spores occasionally land on megaherbivore dung, but these are generalist genera not as dependent upon megaherbivore dung for reproduction. There was a significant difference in decline between fungi dependent upon megaherbivore dung and generalist fungi.  Sordaria humana is a species of fungus that reproduces readily on human and dog feces.  Sordaria spores remained steady in abundance after 40,000 BP when sporormiella and podospora declined.

This study found that the volume of dung fungus spores in Australia prior to 40,000 BP was similar to numbers from studies conducted of Pleistocene North America, Pleistocene Europe, late Holocene Madagascar, and modern livestock producing regions.  This suggests the biomass of megaherbivores in pristine environments was close to what modern pastures can support.  Data from this study also show the extinction of Australia’s megafauna is closely associated with the initial presence of man.  It appears as if man hunted these animals into extinction within a 1000 year time span.  The transformation of wet tropical forest to dry fire-adapted woods occurred after the megafauna became extinct, precluding the possibility that climate change was a factor in the extinction of Australia’s megafauna.

Reference:

Johnson, Chris; et. al.

“Using Dung Fungi to Interpret Decline and Extinction of Megaherbivores: problems and solutions”

Quaternary Science Reviews Feb 2015

 

 

 

 

Surprising Discoveries of Large Carnivore Dietary Preferences on the Pleistocene Mammoth Steppe

The mammoth steppe was a vast continuous environment that stretched from western Europe to Alaska during the coldest phase of the most recent Ice Age.  Glacial ice locked up so much of earth’s atmospheric moisture that sea level fell, creating land bridges connecting the British Isles and Alaska with Eurasia.  The mammoth steppe consisted of desert grassland, cold and windy but without much snow cover.  This environment supported a wealth of megaherbivores including woolly mammoth, bison, yak, musk-ox, woolly rhino, horse, megaloceros (a giant deer), caribou, camel, and saiga antelope.  Such a wide prey selection attracted several species of large predators.  Scientists long speculated about the relationships between predator and prey on the mammoth steppe, but now it’s possible to determine which prey species each individual species of predator favored.   In an ingenious study, Herve Bocheren, a German professor, used stable isotope tracking in combination with mathematical models to learn about the diet of late Pleistocene carnivores on the mammoth steppe.  Some of his findings are quite surprising.

Map of the vast mammoth steppe ecosystem that existed between ~28,000 BP-~15,000 BP.

Various species of plants have distinct ratios of carbon and nitrogen isotopes, and therefore herbivores that eat these plants have similar ratios in their bone chemistry.  Carnivores that then eat these herbivores also attain these distinct ratios of carbon and nitrogen isotopes.  By analyzing the chemistry of ancient bones found in caves, the diets of these animals can be pieced together.

Stable isotope tracking suggests the spotted hyena (Crocuta crocuta) was the dominant predator in Europe until about 25,000 BP–before climatic conditions caused the expansion of the mammoth steppe grassland.  Between 60,000 BP-28,000 BP forests and open woodlands still grew amidst the grasslands, and climate remained temperate though there were rapid fluctuations.  The spotted hyena, the same species found in Africa today, thrived in temperate climates, and they outcompeted wolves, lions, and even Neanderthals here during this time period.  Isotopic evidence shows hyenas ate a wide range of prey including mammoth, horse, and rhino; relegating wolves to prey such as elk, giant deer, and chamois.  But hyenas were unable to survive in Eurasia during the following colder climate phase, and they became extirpated from the mammoth steppe.  Hyenas must have a limiting minimum temperature limit that they can endure.

The  most surprising result of Dr. Bocheren’s study was the discovery that cave lions (Panthera spelaea) relied on caribou for at least 25% of their diet.  The lion of the mammoth steppe was not the same species as the African lion (Panthera leo).  It was 10% larger but males had smaller manes.  The evidence from this study supports conjecture that it was a solitary predator, unlike its extant cousin.  Packs of hyenas and wolves were able to restrict access of this solitary predator from more desirable prey such as bison and horse, forcing cave lions to rely more on caribou.  There is also a great variation in each individual lion’s choice of prey.  One individual favored caribou and deer.  Another specialized in cave bear but also took mammoth, deer, and rhino.  A 3rd individual fed upon cave bear and deer.  And a 4th ate the same mix of desirable prey that hyenas ate.  Each individual learned to hunt certain prey animals, whereas a social predator would’ve likely taken a wider mix of prey.  An individual lion killing an huge cave bear must have been an impressive sight.  There is also fossil evidence of lion bite marks on bear bones.  Today, certain Siberian tigers are known to specialize in hunting brown bears (Ursus arctos).

Dr. Bocheren studied the bone chemistry of scimitar-toothed cats (Dinobastis serum or Homotherium serum depending on whose nomenclature one chooses) as well.  Unlike saber-tooths (Smilodon fatalis) this fanged cat was not an ambush predator but chased down its prey instead.  Complete skeletons of scimitar-toothed cats have been found in Friesenhan Cave, Texas associated with many bones of juvenile mammoths.  Because of this single site, scimitar-toothed cats were thought to be specialists in hunting juvenile mammoths.  This study casts doubt on that assumption.  Instead, the favorite prey of scimitar-toothed cats in Eurasia was the yak (Bos grunniens), along with bison and caribou.  Less commonly, they did eat musk-ox, mammoth, and horse.  They were a generalized predator, not a specialist.  Scimitar-toothed cats are rare in the fossil record compared to other large carnivores and probably were extirpated from the mammoth steppe along with hyenas and leopards when the climate deteriorated about 25,000 BP.

Painting of lions on a wall in Chauvet Cave, France.  Looks like the representation of a pride of lions.  I’m not convinced the extinct European cave lion was a solitary animal as suggested by this study.

The yak (Bos grunniens).  Isotopic tracking studies suggest this was the favored prey of the extinct scimitar-toothed cat.

Wolves (Canis lupus) replaced hyenas as the dominant predator in Eurasia after 25,000 BP.  There was a wide genetic and morphological diversity among Pleistocene wolves on the mammoth steppe.  The large extinct Pleistocene wolf ecomorph that lived in Alaska ate mostly horse, bison, and caribou but not mammoth.  This line of wolves became extinct.

Isotopic evidence shows Paleolithic humans living about 28,000 years ago ate mammoth but did not allow their primitive dogs to consume the mammoth meat.  Instead, humans fed their dogs caribou and musk-ox.  However scavenging predators such as wolf, brown bear, wolverine, and fox did take advantage of anthropogenic mammoth hunting.

Dr. Bocheren’s isotopic study confirms the cave bear (Ursus speleus) was almost entirely herbivorous.  Brown bear diet varied.  Brown bears were more carnivorous in regions where they overlapped with cave bears but were more herbivorous in regions where they overlapped with highly carnivorous giant short-faced bears (Arctodus simus).  Brown bears apparently avoided completion with larger bear species.  In Alaska giant short-faced bears ate caribou, musk-ox, and other predators but plant foods may have made up to 50% of their diet.  Surprisingly, they didn’t eat much horse or mammoth–2 common prey species here.  The diet of this North American species south of the ice sheet has not yet been studied.

Note 1: I think the common names of cave lion, cave bear, and cave hyena are misnomers.  99.9% of the individuals of these species that ever lived never stepped inside a cave.  Their bones were more likely to be preserved in caves, hence the cave appellation.  Nevertheless, it’s misleading to think of them as cave dwellers.  This is just a pet peeve of mine, but I wish they would be given different common names.

Note 2: I’m not entirely convinced that Panthera spelaea was a solitary species, nor am I convinced this species played second fiddle to wolves and hyenas.  I’ll think more on this and perhaps comment at a later date.

Reference:

Bocheren, Herve

“Isotopic Tracking of Large Carnivore Paleoecology in the Mammoth Steppe”

Quaternary Science Reviews March 2015