Six million years ago, the skies of Argentina were home to fearsome predator - Argentavis magnificens, the largest bird to ever take to the air. It weighed in at 70kg and had a wingspan of 7m, about the same size as a Cessna 152 light aircraft.
Argentavis was a member of an extinct group of predatory birds understandably called the teratorns - 'monster birds'. They are related to storks and New World vultures such as turkey vultures and condors. But Argentavis completely dwarfed even the massive Andean condor, weighing six times more and with a wingspan over twice as long (in the picture below, its silhouette is placed next to a bald eagle for scale).
There is no question that Argentavis flew. It has all the characteristics of modern flyers including light, hollow bones and strong, sturdy wings. It's how it flew that palaeontologists have puzzled over, given its massive size in relation to modern birds. For a start, how did it get its large bulk off the ground in the first place? The heaviest living flier, the Great Kori Bustard, is over three times lighter than Argentavis, and even it can only take off after arduously 'taxiing' like a airplane.
Sankar Chatterjee from the Museum of Texas Tech University decided to model the giant's flying style by running simulations with known fossils. He found that Argentavis simply couldn't have generated enough lift from a running-take-off. It needed height to get airborne, but it could manage with surprisingly little. Even a gentle down-slope of 10° and a light headwind would have given it enough extra power to avoid an embarrassing crash. Albatrosses and hang-glider pilots use the same technique today.
Once in the air, the flapping flight that small birds use was out of the question for the giant predator. By studying its skeleton, Chatterjee estimated the maximum amount of power that its flight muscles could have generated. And while substantial, it was still 3.5 times less than the minimum amount of power needed to fly.
Instead, Chatterjee believes that Argentavis was a master glider. It was capable of soaring for great distances at a shallow angle of 3°, continually re-shaping its wings to control its glide. Unlike flapping, the efficiency of gliding doesn't change very much with size, if a bird sticks to the standard body plan. So despite its enormity, Argentavis sailed through the air with as much grace as much smaller species like the buzzard or white stork.
Like modern soarers, Chatterjee believes that Argentavis used two techniques. By flying along the Andean ridges, it stayed aloft using upwards air currents produced by wind deflected up the cliffs. The several fossils found at the Andean foothills support his idea.
Because of its efficient gliding, it could stay aloft using relatively slow drafts of wind. Chatterjee calculated its top speed at about 70 km/h, allowing it scan vast tracts of land for prey. It's a very energy-efficient style and today, eagles and vultures use it to great effect, sometimes covering hundreds of miles without a single wing flap. When the bird switched from the mountains to the wide, open spaces of the pampas, it switched to a different method - thermal soaring, where rising columns of hot air provided it with lift.
Popcorn-like cumulus clouds betray the location of thermals, and by circling around one, Argentavis could have risen through the air, giving itself enough height to soar to the next thermal. Despite its large size, Chatterjee calculated that Argentavis was manoeuvrable enough to manage the tight circular turns needed to stay within a thermal column.
Even with this reliance of thermals, Argentavis was pushing the limits of even gliding flight. Any heavier and it would have exceeded the maximum weight for safe gliding. So why are there no equally sized giants today? Chatterjee thinks that the late Miocene's climate provided the answer. Six million years ago, Argentina was much hotter and drier than it is today - just the weather needed for generating the powerful thermals needed to lift such a large bird.
Argentavis was beautifully adapted to take advantage of this large, open habitat, where it could travel across large distances in search of prey. And unlike modern condors, it was no mere scavengers. Its skull was as long as my forearm and ended in a formidable hooked beak - it was an active hunter, possibly taking prey on the wing. .
Reference: Chatterjee, Templin & Campbell. The aerodynamics of Argentavis, the world's largest flying bird from the Miocene of Argentina. PNAS doi.10.1073/pnas.0702040104.
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That's so interesting. I wonder whether with global warming that size of glider will evolve again.
So Argentavis spots a scurrying mammal on the pampas below and swoops to nail the little bugger. Does it land and cover it (like many hawks) and waddle over to a take-off slope? Or, does it capture it (like a sea eagle) and, using the excess airspeed generated in its dive, soar to glidable heights while noshing on its prey in the air?
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MartinDH
So Argentavis spots a scurrying mammal...
Actually, as a close relative of condors and turkey vultures, wouldn't Argentavis have been more likely to seek out the giant, mouldering corpse of a ground sloth or a glyptodon? In my experience with turkey vultures, while they can be magnificent to watch, they're pretty leery around animals that look like they might scurry.
I get a lot of opportunities to observe turkey vultures, and they're not above eating their fill of roadkill and then just standing around for a while. Of course, they're capable of short bursts of flapping flight to take off from ground level, but on the other hand, I don't live in the Andes, where slopes aren't exactly in short supply.
Aha! I do hope Darren Naish and David MarjanoviÄ are reading this. They have pooh-poohed me several times for making this argument (admittedly, without actual data).
Hmm, all I will say is: oh look :)
How do these considerations apply to similarly sized (and even bigger) pterosaurs? Do they apply at all, given the differing anatomy?
Chatterjee's calculations are wrong.
Look at the supplementary material. He gets the "available power" by taking the basal metabolic rate, multiplying it by the usual 20% for metabolic -> mechanical, and concludes that was the power available, 170W.
How, exactly, is the *basal* rate the power limit? Basal rate is when the animal is *resting*. Many animals have active rates about 10x the basal.
For context: the maximal power muscles can generate, depending on species, is typically between 350-450 W per kg. There are *frogs* that generate more than 170W of absolute power in a jump.
A bird of that size, if even 5% of it's body mass was flight muscles (a VERY low number), could generate a minimum of 1400 W of power.
That is, hands down, one of the most embarrassing mistakes I've ever seen in a paper.
HP:
"In my experience with turkey vultures, while they can be magnificent to watch, they're pretty leery around animals that look like they might scurry."
american black vultures are of the same family, and are aggressive enough to chase turkey vultures off of carcasses, and predatory enough to take small scurrying critters and newborn livestock. nasty buggers, heh. myself, i can see a hunting cathartid.
To leave this sensible discussion briefly, please note the diagram of the birds getting a free ride up the thermal coilspring escalator, gliding down, and climbing again. Do cumulus clouds float on a coilspring mattress? If so, then at night do they lower?
As a hang glider pilot this doesn't entirely make sense to me. Hot weather isn't necessarily good for lift. Post cold-frontal conditions are best for thermals, when the lapse rate is high. When it's hot, you get temperature inversions that slow climb rates, and may completely stop thermals.
Having said that, modern birds can stay up in thermal lift as long as they like. The bigger the bird, the better they do it. Almost every bird larger than a small songbird will save energy by turning in thermals - seeing birds turn is how I find thermals once in the air. Vultures always land back on a cliff once the thermals start to decay, and wait for the next one to come through.
Albatrosses use dynamic lift (wind shear layers) to maintain height by diving from a high-energy layer to a lower energy layer, then back again, often at wave height. This technique is not available to a land bird.
But you always end up with the landing problem, and the subsequent take off. Shallow slopes don't work well; if you take off from a shallow hill, you'll very likely end up at the bottom of said hill, even if the wind is conveniently coming up the slope. What do you do if you have a katabatic wind? Prey is unlikely to be conveniently organizing itself on a steep slope. I fetch the car to get back on top of the hill - these birds are going to have to flap their wings...
It has long been speculated that the constant strong winds of that area were even stronger in the Miocene, and that Argentavis put them to good use, perhaps being incapable of taking off without them and accordingly dying out with them. Good to see that someone tries to throw some numbers at this question.
I have yet to read the paper or the supp. inf., but comment 7 is not encouraging. Confusing basal and active metabolic rate is a major headdesk moment.
A paper that confuses basal and active metabolic rates represents a failure of journal review, too. The reviewers should be as embarrassed as the author. This is the first time I've seen Ed taken in by a howler.
So, the conclusion is that A. magnificens could produce three times the power needed to take off from level ground, and could therefore climb rapidly too. No doubt it spent most of its time soaring anyhow. Wouldn't you? (In fact, why don't you? Lessons are surprisingly inexpensive.)
"could not generate enough lift?" that's a load of bs. Just use common sense. If the creature cannot generate enough lift, how the hell did it survive? not mention growing to such a size?
Typical scenario:
Bird hungry; bird sees prey and wants to eat... but the landing area is no steeper than 10 degrees. If bird lands, bird cannot get up. Therefore bird dies from starvation.
If such a size is a disadvantage such that it cannot take off at will, then natural selection would favor those that can! Therefore, the stupid scientist's argument is totally wrong. His argument is as ridiculous as those who claimed that T-Rex was a scavenger.