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.

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

Summer Tanager Photo

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.

wire owl enclosure for purple martin gourd rack

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.

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