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All your birds are belong to us

July 23, 2010

New I and the Bird #130 at Count your chicken! We’re taking over!


Prior to the connection of North and South America by the Panamanian landbridge some 3 million years ago, the two continents were on wildly different evolutionary trajectories.  Pleistocene North America has a distinctly Eurasian flair, as the two continents have a long biogeographic history and many of the major animal groups that existed there are able to trace their origin either to the vast super-continent of Laurasia or from an arrival across the Bering Strait over the last several million years.  This included well-known families like Bears, Deer, Rabbits, Dogs, Cats and others who share close relatives in the Eastern Hemisphere.

South America was far more bizarre, having been part of the massive Gondwanaland Southern super-continent and sharing close biological ties with, of all places, Australia.  The rest of the Gondwanaland, Africa and India, continued on a collision course with Asia where their Pleistocene fates became intertwined, but South America and Australia stayed islands throughout most of the Cenozoic era, which allowed for the evolution of strange and bizarre creatures like those well known in Australia.  What is less well known is that South America had it’s own assemblage of oddities, and the fossil record shows monotremes like the Platypus and giant flightless predatory birds, not unlike Cassowaries and Emus of today in structure if not voracious appetite.  Some still exist in some form even today; in sloths and anteaters and the Rhea, but most of the biggest and baddest of South America’s fauna winked out by about 1 million years ago.

Why?  Well, you can lay the blame on Panama.  Once the two continents were connected it allowed terrestrial mammals that had been isolated for millions of years to interact in the geologic blink of an eye.  This hugely important paleozoographic event was called the (deep echoing voice here) Great American Biotic Interchange, and while the initial interchange was largely even-steven with both continent’s fauna making equal inroads, over time the Northern species won out by virtue of a few quirks of geography in their favor (a larger land mass offering greater selective pressure mainly).  In any case, this is why there are Bears and Dogs and Cats and Camelids in South America whereas the only mammals of South American origin to stick it out up here are the noble North American Porcupine, mostly cause no one wants to screw with it, and the Virginia Opossum, though it must be said that the Nine-banded Armadillo continues to make a valiant effort too. Perhaps there’s something to be said for elaborate defensive armory.

Ok, so why is this important?  This is, after all, a bird blog.  It turns out the prehistory of birds is a little less cut and dried.  Bird bones are fragile and don’t preserve well so how birds handled the (deep voice) Great American Biotic Interchange has largely been a mystery for a long time, especially with regard to neotropic-nearctic migrants like Warblers and Grosbeaks and Thrushes and such.  Did these birds evolve in North America to take advantage of South American wintering grounds, or did they evolve in South America to take advantage of the North American seasonal abundance?  In essence, does migration evolve towards off-season survival or towards new breeding success?

For a long time the second theory, called the Southern Home theory, has held sway, but modern bird science has some serious tricks at its disposal and recent findings using molecular data and phylogenetic evidence from over 100 genera of birds give more weight to the idea that those birds evolved here in North America rather than vice versa.

The results reveal that while ancient birds could fly most species did not cross the water between the two isolated continents, so were subject to the same constraints as their land based mammalian counterparts. The land bridge was therefore crucial in facilitating cross continental migration.

“This inter-continental migration was far from even. While within the tropics around the equator exchange was equal in both directions, between the temperate zones of North and South America it was not,” said [Brian Tilston] Smith [from the University of Nevada]. “Avian lineages from the northern Nearctic regions have repeatedly invaded the tropics and radiated throughout South America. In contrast species with South American tropical origins remain largely restricted to the confines of the tropical regions.”

And even more, the distribution of these birds of northern origin nearly exactly mirrors the distribution of mammals following the Interchange.

When considering the perching birds oscine and suboscine the team found that despite having northern ancestral origins, 55% of New World oscine species now breed in South America, many of them in tropical habitats. In contrast, only 2.4% of suboscines have secondarily adapted to North American temperate zone habitats.

For reference, oscine refers to the vocal organs that birds use to sing.  Most passerine families are oscine and commonly referred to as “songbirds” because of their elaborate songs.  Suboscines don’t have elaborate vocal organs, and thus typically don’t sing. They’re represented mostly by the Tyrant Flycatchers, who notable to this study reach the peak of their diversity in South America, but also by some other distinctly neotropical families like Antbirds and Contingas.  Our flycatchers though, are those 2.4% of birds that have adapted to North America against heavy odds, very much like the Porcupine and the Armadillo.

So it seems our migrants, despite spending most of the year in the neotropics and offering colors that seem definitively tropical, are in fact North American to the core.  Lucky us.

  1. July 23, 2010 9:38 am

    Haha. Love the post title. I’ve never played the game myself, but it seems to be one of those references that’s seeped away from its source into a broader culture.

    That’s really neat about the evolutionary origin of our summer breeders. I’d always just assumed they were southern birds who pushed north to take advantage of abundant summer food.

  2. July 23, 2010 10:25 am

    Like Seabrooke, I love the title and was educated by this post. I wonder how many South American bird species were lost in the (deep echoing voice) Great American Biotic Interchange.

    By the way, when I start a band it is totally going to be called the Great American Biotic Interchange.

  3. Nate permalink*
    July 23, 2010 10:54 am

    @Seabrooke- I always thought the same, that neotropic migrants are fundamentally South American, but it’s of of those things that makes sense when you think about it. Bird generally have strong fidelity to their nesting grounds but can range widely the rest of the year. You could see how something like that could evolve into long-distance migration over as little as 3 million years.

    @Corey- I expect it’s impossible to know exactly how many since songbird fossils are so rare. We know about the giant “terror birds” but there would have to be smaller species to. And I’ll bet there were some pretty spectacular ones if the diversity of a family like the Cotingas is any indication.

    I also want to be in that band.

  4. July 23, 2010 10:04 pm

    Those South American birds were sure set up the bomb.

    Sorry, couldn’t resist 🙂

  5. Nate permalink*
    July 23, 2010 10:22 pm

    @Grant- They had no chance to survive make your time.

  6. July 24, 2010 12:12 am

    Hoatzin: For great justice!

    (Is it just me, or does the Hoatzin just seem like an apt choice for the commander of South American birds?)

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