Why Weren't Birds Top Predators After 66Mya

[Mediospirifer]

Ramo, I think the "living fossil" phenomenon that sharks typify is due to robust physiology coupled with behavioral adaptability. Yes, a lot of marine life did go extinct in the K-T catastrophe, and possibly a number of shark species were among them. The ones I would expect to have the best shot at surviving a catastrophe would be small, cold-water (or highly migratory) species that ate a wide variety of food species. They'd be able to get by with less available prey (being small), could survive the colder water partly by being adapted to it and partly by swimming to warmer climes, and eating a variety of prey would allow them to survive the extinction of some food sources. That's my hypothesis, anyway. Being cold-blooded would help: they wouldn't need to eat as much as, say, a sea otter would.

There's also the observation that the shark body plan is excellent for fast swimming, although not as good for maneuverability as most of their prey.

[siteseer]

Among sharks the only two large genera (those that apparently ranged over 10 feet long) that survived the K/T extinctions were Sphenodus and Notidanodon - forms known from deepwater environments). The small sharks and rays that survived were generalist feeders (Galeorhinus, Squatina, etc.) and/or those that frequented deeper water.

Ramo, I think the "living fossil" phenomenon that sharks typify is due to robust physiology coupled with behavioral adaptability. Yes, a lot of marine life did go extinct in the K-T catastrophe, and possibly a number of shark species were among them. The ones I would expect to have the best shot at surviving a catastrophe would be small, cold-water (or highly migratory) species that ate a wide variety of food species. They'd be able to get by with less available prey (being small), could survive the colder water partly by being adapted to it and partly by swimming to warmer climes, and eating a variety of prey would allow them to survive the extinction of some food sources. That's my hypothesis, anyway. Being cold-blooded would help: they wouldn't need to eat as much as, say, a sea otter would.

There's also the observation that the shark body plan is excellent for fast swimming, although not as good for maneuverability as most of their prey.

[Mike from North Queensland]

Without getting to specialised into the science of evolution. One point seems to have been overlooked. Most mammals live in packs or extended families including ourselves so this could have been the major contributing factor for repopulation plus many were omnivores. Omnivores do have one main advantage that in the ability to vary there food source so possibly as a group able to mutate to a different species easier due to utilising available food source. Birds tend to be more specialised either herbivore or carnivore since the loss of teeth and the larger predatory birds for the most part only live in pairs, hunt mammals and there young leave within a year. The birds would have had an advantage where they can easily travel and spread great distances but this is reliant on a suitable food source that is mostly specialised. As a simplified example look at the common rat, prey for predatory birds when foraging for food but nests and breeds in large numbers in a hole where it is safe and the handful the birds kill does not affect the population level. The bird is safe from the rat while in the air during the day but a night a rat can take out a breeding nest in one night and do this several times in a year and no young birds for that year and in a few years the bird population is gone.

Mike D

[adthefossil]

@Mike:

I think some raptors hunted in packs (they could even talk according to a movie I once saw ;-0 ) and I think a bird could easily be omnivorous. They'd recently been eating meat and some had presumably adapted to seeds. A parrot (which is almost a falcon) eats seeds but it can easily take your finger off, and modern crows and pigeons (also almost a falcon) eat absolutely anything. Eating seeds and berries can be a challenge because many are toxic, but the genes for digesting meat were certainly in the birds waiting to be reused anytime. Not many birds turn their noses up at insects, worms, etc.

Having said that, the thread already agreed that a bird would have to give up flight if it planned to wrestle with bears, but at the time of interest there were more mice than bears so predatory birds wouldn't have to be big just yet. What confuses me is that they apparently couldn't continue the size race by giving up flight and pecking the bears to pieces. By that time their lungs and bones were better than ever and the strategy should have been viable, so how come every terror bird failed?

As for the vulnerability of nests, dinosaurs had always had that problem but flight went a long way to relieve it.

@Mediospirifer:

Let's assume hibernation is a really difficult trick that the mammals learned while staying out of trouble, and let's assume it's sometimes necessary but no birds know how to do it. Why would this be worse for ground based predatory birds than for flying, seed eating ones? You'd have to assume that seeds were available all year round while more or less all the likely prey was hibernating in secure burrows, and you'd need that to be true more or less all over the world. Any place that had more prey than seeds in winter would be a place we'd expect predatory birds to beat seed eaters. Can that assumption hold?

OTOH if we assume that hibernation is an easy trick to learn, then we'd see that hibernating in a tree is probably a lot more dangerous than in a ground-based burrow, and we'd see that feathers are better insulation than fur, especially in the wet. So under the easy trick assumption the problem is getting worse.

Also, you rightly say that small animals are more likely to hibernate because cold is not a big problem for a big animal. There are plenty of small predatory birds. What I'm looking for are the big flightless ones, who wouldn't be particularly interested in hibernation.

[Rockwood]

I don't mean to belittle any of the interesting thoughts here, but one more came to me when I realized that some of what I was considering random was actually just unknown.

At what point does the limit imposed by the data available get overtaken by the complexity of the possible interactions causing the whole situation to become effectively random ?

[adthefossil]

@Rockwood: I think you could argue that all randomness is just lack of data, except perhaps quantum randomness, but that probably doesn't affect paleontology. OTOH, even if you had all the data in the world and intended to run a simulation to find out what was going to happen next, would the simulation itself be part of your data set?

[adthefossil]

I was discussing this with some SVP experts who mentioned that birds have huge eggs compared with dinos. I figured that must be because of the feathers and ended up reading about how to feed chickens. Apparently they don't like high protein (compared with energy) but do need loads of methionine and cysteine in order to make feathers. It also appears that fish and certain seeds and nuts are the best way to get that.

So if you're a pure predator, you don't want feathers. You could be naked, but what you really need is fur, which birds don't know how to make.

But this only makes things worse because we do have eagles, and they need their feathers. It's the ground based predatory birds who could be naked that are missing.

[Mediospirifer]

@Mike:

I think some raptors hunted in packs (they could even talk according to a movie I once saw ;-0 ) and I think a bird could easily be omnivorous. They'd recently been eating meat and some had presumably adapted to seeds. A parrot (which is almost a falcon) eats seeds but it can easily take your finger off, and modern crows and pigeons (also almost a falcon) eat absolutely anything. Eating seeds and berries can be a challenge because many are toxic, but the genes for digesting meat were certainly in the birds waiting to be reused anytime. Not many birds turn their noses up at insects, worms, etc.

Having said that, the thread already agreed that a bird would have to give up flight if it planned to wrestle with bears, but at the time of interest there were more mice than bears so predatory birds wouldn't have to be big just yet. What confuses me is that they apparently couldn't continue the size race by giving up flight and pecking the bears to pieces. By that time their lungs and bones were better than ever and the strategy should have been viable, so how come every terror bird failed?

As for the vulnerability of nests, dinosaurs had always had that problem but flight went a long way to relieve it.

@Mediospirifer:

Let's assume hibernation is a really difficult trick that the mammals learned while staying out of trouble, and let's assume it's sometimes necessary but no birds know how to do it. Why would this be worse for ground based predatory birds than for flying, seed eating ones? You'd have to assume that seeds were available all year round while more or less all the likely prey was hibernating in secure burrows, and you'd need that to be true more or less all over the world. Any place that had more prey than seeds in winter would be a place we'd expect predatory birds to beat seed eaters. Can that assumption hold?

OTOH if we assume that hibernation is an easy trick to learn, then we'd see that hibernating in a tree is probably a lot more dangerous than in a ground-based burrow, and we'd see that feathers are better insulation than fur, especially in the wet. So under the easy trick assumption the problem is getting worse.

Also, you rightly say that small animals are more likely to hibernate because cold is not a big problem for a big animal. There are plenty of small predatory birds. What I'm looking for are the big flightless ones, who wouldn't be particularly interested in hibernation.

Many birds are opportunistic. Ring-necked pheasants are generally considered seed-eaters, but I was told by one of the workers at our local gamefarm where they're raised that if a bird dies in the pen, the other birds will readily peck at the carcass and eat it. Many birds also live in flocks, while cats and weasels are both fairly solitary predators. I'm not sure that living in a group is relevant to a large predator.

Around here (New York state), many of our small songbirds are considered seed-eaters. Chickadees are a fairly typical non-migratory example. They frequent birdfeeders all year round. During nesting season they also hunt insects and grubs. During winter, they eat seeds and insect eggs. If the weather turns dangerously cold (even with their feathered coats), they seek out sheltered hideaways to get out of the wind. If extreme cold persists, or the available food runs out, then they fly south until they like the local conditions. I've never seen all of the black-capped chickadees disappear in winter, but I've heard reports of brown-capped chickadees (a more northern species) being seen in this area during extreme winter cold.

By contrast, robins are specialist predators: they feed exclusively (or nearly so) on earthworms. They simply can't feed during a New York winter, so they migrate to warmer climes for winter. Many predatory birds follow this pattern.

Redtailed hawks are a generalist predator. They'll eat any small animal they can catch: rodents, rabbits, the occasional bird, snakes, whatever. They typically don't migrate long distances, although they will travel short distances and congregate in winter in a particular location that has a high population of prey animals. I've seen as many as 23 perched around the gamefarm I mentioned above during cold weather. The person that I talked to there told me that they're attracted to the rats that are in turn attracted to the pheasants' feed, although the hawks will occasionally take a pheasant.

The point of this is that omnivory is fairly common among birds. A bird with a pointed beak (like a chicken), can tear out a bit of flesh and swallow it, and can probably digest it without problems. Trying to live exclusively on meat is a different proposition for an animal adapted to plants: they usually need the high fiber content of plant foods, and often require vitamins that aren't found in meat. The vitamin problem runs the other way, too: cats can't survive on a purely vegetarian diet. They need taurine to stay healthy.

I also want to illustrate the typical bird response to seasonal food shortages: migration. When the food runs short, my local bird populations fly south. A landbound bird can't do that! That's why I think terror birds would have a problem with cold winters. A terror bird would need a good coat of feathers to keep its neck and legs warm, and enough available prey to maintain the high avian metabolism. A wolf or cat can go several days or even a week between large meals, but I don't know of any bird that can do that. It's that high metabolism that I think is the biggest barrier to hibernation for a bird.

Terror birds may be able to migrate on foot, as caribou do, but why should they? Flying is the most energy-costly way to travel, but it is also the fastest. A journey that takes a robin a few days would take a terror bird weeks. And lets look at the reasons for migrating. Caribou don't travel nearly as far as any migrating bird, nor do all caribou migrate. They're following the food, and they eat a lot. Birds fly north in summer (in the northern hemisphere!) to their breeding ground. The advantage of this seems to be that longer daylight hours give them a longer forging time to feed their young. They can raise larger broods in a northern summer that they could in a subtropical one.

I don't think your chicken feed research can be broadly applied to all birds. Wild chickens are (as far as I know) mainly seed eaters, with a significant proportion of insect foods in their diet. Eagles and hawks are exclusively carnivorous. The only plant foods in their diet would be from the prey's gut contents. Certainly they don't have any trouble building feathers!

[Mediospirifer]

One quick observation: According to the wiki pages for methionine and cysteine, meats are excellent sources of both. So your typical predatory bird will have no trouble with that. Neither will opportunists that eat eggs when they can get them.

[Scylla]

So all the mammals had was a placenta? Well, that is your answer then! I read once that all the modern humans had to do to totally displace the Neanderthals was to have a successful birthrate 1% higher. Over thousands of years they would out compete and displace the Neanderthals. Small survival differences really compound over time, like compound interest. I'm all for more efficient lungs and lighter bones, but evolution is based on fitness, ie # of offspring that reach breeding age. So if some terror bird could out fight and out hunt some saber tooth chipmunk, but got its eggs eaten just a few times out of a hundred, then it gets to be in a museum as a "Oh WOW!" exhibit, but the saber tooth chipmunk ends up as common as racoons.

[Mediospirifer]

So all the mammals had was a placenta? Well, that is your answer then! I read once that all the modern humans had to do to totally displace the Neanderthals was to have a successful birthrate 1% higher. Over thousands of years they would out compete and displace the Neanderthals. Small survival differences really compound over time, like compound interest. I'm all for more efficient lungs and lighter bones, but evolution is based on fitness, ie # of offspring that reach breeding age. So if some terror bird could out fight and out hunt some saber tooth chipmunk, but got its eggs eaten just a few times out of a hundred, then it gets to be in a museum as a "Oh WOW!" exhibit, but the saber tooth chipmunk ends up as common as racoons.

So placentals can breed faster than birds or marsupials? Interesting. I wasn't aware of that.

That would potentially explain what happened in the Americas, at least. As I understand it, terror birds thrived in South America where the only mammals they shared the continent with were marsupials. After the Panamanian land bridge formed, terror birds moved north, while wolves, coyotes, pumas, and jaguars found the way south. Only a few millennia after that, terror birds were extinct.

Do marsupials reproduce at a similar rate to birds?

[adthefossil]

@Placenta: It's more efficient than laying a thousand eggs in a hole in the hope that one doesn't get eaten by birds, but we need a factor that specifically punishes things like terror birds more than sparrows or eagles.

@Migration: But why do birds have such fast metabolism? I'd have guessed it's because they fly. If there's no other reason, I'd just counter that terror birds could have a slower metabolism.

@Methionine: But that farmer's article says chickens don't like high protein. I'm not sure if the same can be said of terror birds - probably depends whether you want to roast or broil them.

But I got a very interesting answer from Dr. Jeb Bevers via the SVP: Teeth!

A hooked beak is great for ripping flesh off a carcass but try bringing down a struggling bull with one. Keratin is flexible and a beak can only be stiff in the specific direction it's designed to be stiff in. Modern birds of prey use their claws to kill things, and because they're usually flying at the time, they don't have to stand on their feet at the same time as trying to kill stuff with them. Once the bird loses flight, it has to do two things at the same time with its feet. The image of a 9ft dodo dancing around like J.C. van Damme on a hot tin plate is an unlikely one.

This theory requires that all extant birds had lost their teeth by 66mya, but AFAIK that is indeed the case.

Any objections?

[Mediospirifer]

Why do we need a specific factor that punishes terror birds? They were quite successful until they encountered placental mammals. The question really is "what changed when the land bridge formed?"

It may have been simply competition from unfamiliar beasts able to adapt their behavior more quickly. Or there may have been a bird flu virus in North America--say, carried by prairie chickens--that the terror birds had no resistance to. Such a virus would have severely weakened the population, perhaps to the point that other carnivores could have wiped them out. Granted, viruses travel through a dense population more quickly than a scattered one, so solitary-lifestyle birds would have been less susceptible than flocking birds.

My point about methionine is that if we're talking about predatory birds, they'll naturally have more available in their diet than chickens do.

The point about teeth from Dr. Bevers is a very interesting one. It looks like he's saying that if a terror bird attempted the wolf tactic of biting down and hanging onto large, struggling prey, it would likely break its beak. I can accept that. The idea of a 9-foot flightless bird being unable to use its claws as weapons, however, I have to disagree with. Ostriches and cassowaries are fairly dangerous animals, because they have large claws and can kick very effectively. If you stand your ground facing down a cassowary, it can disembowel you with one kick! And ostriches are the fastest bipeds around; I can picture terror birds having similar agility.

This does bring up another question: If terror birds couldn't use their beaks as weapons, how did they hunt? Maybe they were small-prey specialists.

[adthefossil]

Terror birds weren't successful like the dinosaurs were. They didn't occur at all until 60mya (as far as anybody knows) and only popped up sporadically for short periods. So they often tried but never caught on. There does seem to be some evidence that one lot only survived until the dogs arrived. This is in stark contrast to the continuous success of therapods before the extinction, so there has to be some reason why a raptor-like strategy was either unavailable or unviable for birds since the extinction, and that reason may not punish sparrows, crows, eagles, herons, or any other form we can see to be successful.

The chicken feed idea was indeed a bit daft of me.

Dr. Bevers just mentioned the birds' lack of teeth without going into too much detail. There's also this excellent blog article:

https://qilong.wordpress.com/2014/10/27/toothed-birds

As for flightless birds being dangerous, I've read about that too and was therefore quite scared of the emus that a friend of mine farms. They used to like him until they figured out that he was cooking their babies. One day he got attacked in just the way you describe. The result was that he was left standing with his trousers around his ankles.

Anyway, to get attacked like that you have to basically stand there like an extra in a kung fu movie and wait for it. It doesn't work so well on something that's running away. A lion can grab the prey's back with its front claws and gradually drag it down. If a bird tried that it would just be going for a ride. I believe cassowaries can also deliver quite an impressive Glasgow Kiss, but that's not useful when applied to the prey's butt.

How did terror birds hunt? I guess they hunted vulnerable stuff to extinction and then starved. That would explain why they kept popping up for short periods. There seem to be more and more doubts that they hunted at all. I read about one with a 40 degree blind spot in front, but I can't remember which. A big scary beak can also be useful for opening coconuts, and perhaps the giantism was to scare away lions. I think humans have an emotional desire to glamourise strange animals especially by beefing them up. For national symbols we always seem to choose something macho like a bear, lion or eagle rather than something useful and sociable like a bee (although Portugal chose the chicken for some reason.)

[Mediospirifer]

The thought occurred to me (after I walked away from the computer) that terror birds may have been scavengers. Vultures have meat-shredding beaks, too, and they are quite successful. Terror birds would have had them for competition, and may only have been viable when they could scare predators away from a kill. A clawfoot kick may be useless for bringing down running prey, but could be quite effective for scaring off a predator that wants to stand its ground. I expect they would have defended their nests the same way.

I hope your friend with the emus wasn't seriously hurt! That sounds quite scary.

[siteseer]

Ramo,

Sharks and rays did suffer heavy losses during the K/T extinction. The latest estimate is that 56% of Late Cretaceous genera did not continue into the Early Paleocene (Kriwet and Benton, 2004). Henri Cappetta was the first to focus on shark/ray extinctions since the Jurassic. His 1987 study estimated 45% of genera disappearing at the K/T boundary but this article was overlooked to some extent because his Handbook on Mesozoic and Cenozoic chondrichthyans was published the same year. The numbers change because new taxa got described on both sides of the boundary during the intervening years and others might be synonymized or found to have survived in some Early Paleocene site.

http://www.academia.edu/4843200/2004kriwet

The larger genera frequenting shallow seas (Scapanorhynchus, Squalicorax) died out. The only large sharks that survived were deepwater forms (Sphenodus, Notidanodon). Many of the sharks we see today (makos, carcharhinids, Hemipristis) would not become common until the Late Eocene-Oligocene.

The rays were hit hard at the end of the Cretaceous. Most families were wiped out - all the Cretaceous sawfishes (Ischyrhiza, Schizorhiza, Dalpiazia, Sclerorhynchus, etc.). Forms that were rather rare (Dasyatis, early myliobatiforms) would become common and diverse in the Paleocene-Eocene. A new family of sawfishes, unrelated to the Cretaceous groups, appeared or at least became common during the Eocene.

Jess

Cappetta, H. 1987.

Extinctions et renouvellements fauniques chez les Selaciens post-jurassiques. Mem. Soc. geol. France. N.S. 150: 113-131.

Very interesting reading. I appreciate all who have added to this discussion. The discussion here seems to be heavy on what lived on the land, and in the air. What about the water? The sharks survived the KT unscathed, and for the most part, unchanged. Did they reach the "end" of their evolutionary ladder millions of years before us mammals even started climbing?

[jpc]

Another problem is that they no longer think Gastornis/Dyatyrma was carnivorous because of some calcium isotopes. I don't know how convincing that calcium test is.

Do you have a source for this? I am doing a talk on fossil birds soon, and this topic is part of the talk....

@ mediospirifer... Robins spend the winter here in Wyoming, where the season is comparable to yours. They survive on berries... I have seen them on juniper trees and Russian olives fattening up.

I never imagined the large carnivorous flightless birds as chasing down beasts their own size, but more like a heron, eating lots of small guys. So, I think the argument Dr Bevers makes is moot.

Fun discussion, kids. For the record, I have spent a lot of time in the Eocene of Wyoming and have yet to find even a shred of Diatryma.

Edited January 27, 2015 by jpc

[siteseer]

In placentals the embryo develops within the mother before it is born which is a big difference from birds since the embryo is not as well-protected - vulnerable to predators and the elements. In marsupials the embryo partly develops by the time it is born but it must climb up into the pouch to complete its development and it doesn't always make it. You'd have to look for references that provide gestation periods, litter ranges, and average survival rates of young to get an idea of efficiency.

I think the ancestors of birds must have already had a fast metabolism to be able to become active fliers. There must have been a lot of jumping and/or climbing before getting airborne. I would think that in order for terror birds to have competed for so long with mammals, they must have kept the fast metabolism of their ancestors.

From my time spent in Florida I can say that terror bird fossils (Titanis) are very rare so they were probably not common. Fossil collectors prize them more than sabercat bones because they are much rarer. They made it into North America during the Pliocene leaving remains in Texas and Florida so far but no remains have been found in Pleistocene deposits.

@Placenta: It's more efficient than laying a thousand eggs in a hole in the hope that one doesn't get eaten by birds, but we need a factor that specifically punishes things like terror birds more than sparrows or eagles.

@Migration: But why do birds have such fast metabolism? I'd have guessed it's because they fly. If there's no other reason, I'd just counter that terror birds could have a slower metabolism.

@Methionine: But that farmer's article says chickens don't like high protein. I'm not sure if the same can be said of terror birds - probably depends whether you want to roast or broil them.

But I got a very interesting answer from Dr. Jeb Bevers via the SVP: Teeth!

A hooked beak is great for ripping flesh off a carcass but try bringing down a struggling bull with one. Keratin is flexible and a beak can only be stiff in the specific direction it's designed to be stiff in. Modern birds of prey use their claws to kill things, and because they're usually flying at the time, they don't have to stand on their feet at the same time as trying to kill stuff with them. Once the bird loses flight, it has to do two things at the same time with its feet. The image of a 9ft dodo dancing around like J.C. van Damme on a hot tin plate is an unlikely one.

This theory requires that all extant birds had lost their teeth by 66mya, but AFAIK that is indeed the case.

Any objections?

Edited January 27, 2015 by siteseer

[Mediospirifer]

Lots of interesting comments in this discussion!

jpc, I haven't observed robins in winter around here. Early spring, when the ground is just starting to thaw, yes--sometimes large flocks! I find it interesting that they can overwinter eating berries.

Smaller prey for a terror bird is certainly plausible. I can picture one trying to chase down a jackrabbit... I did once see an amusing video of a young golden eagle trying (repeatedly) to catch a jackrabbit. The camera was following the jackrabbit. The bird would stoop on the motionless hare, the hare would remain motionless until the last possible moment then hop to the side causing the bird to miss. Eagle plows into the ground and looks at the hare looking back and wiggling its ears. After six or eight repeated tries, the eagle was clearly becoming frustrated! The last part of the video that I saw was of the eagle chasing the hare on foot, wings raised and beak open, with the hare loping along staying just out of reach. I don't know how intelligent jackrabbits are, but at that point I thought the beast was seriously just teasing the poor, hungry bird. I think the hare would have had to work a lot harder to avoid getting eaten by a terror bird!

The hooked beak of a terror bird would have allowed it to consume larger prey (compared to itself) than a heron's straight beak. Herons swallow their prey whole, therefore have an upper limit to how large a fish is worth pursuing. The hooked beak of a raptor is a meat-shredding tool. Given a dead buffalo, the bird would happily fill its belly.

siteseer, I agree with you about the vulnerability of eggs vs. placental development. I don't know much about marsupials: I did know that red kangaroos will gestate their embryo internally for a short while, then the newborn had to climb into the pouch. I have no idea how the failure rate of that journey might compare with the placental miscarriage rate. Both mammalian systems have the advantage (over eggs) in that the young are carried around by their mobile mother, not subject to being left behind if a predator finds her nest until birth. And in large herbivorous mammals, for whom the main predator avoidance strategy can best be described as "run for your life!", the newborns are up an running with the herd very quickly, while the predatory cubs are nearly helpless and hidden in the den. Among birds, the precocial/altricial divide seems to be along ground nesting vs. tree or cavity (or other inaccessible site) nesting.

[adthefossil]

So it seems like everybody agrees that terror birds didn't hunt large animals but either hunted small stuff, scavenged, or were veggie. It makes a lot of sense that they would have defended themselves in the same kicking style as pretty much all modern ratites, and that their size was driven by the need to fend off other big scavengers. But in the case of Gastornis there's still that pesky calcium isotope test.

[adthefossil]

But wait! Who says you can't bring down a bull with a beak? (Don't answer that - I know - it was me.)

Velociraptor used its long row of teeth to just hang on to the prey while it slashed away with its toenails. It probably used the remains of its front legs to position itself facing the prey given that it would have been flung around a bit as the prey twisted and turned. For an ostrich to do that, all it needs for a beak is a big hook that it can drive through the skin. Skin is fairly tough, and the beak could be designed to avoid cutting the skin like a scalpel. Such birds would probably hunt in packs.

So I'm stumped again as to why we don't have elephant-eating ostriches.

[Stocksdale]

Since there are twice as many species of birds as mammals today, birds have been more successful than mammals in many niches.

But it is interesting that they never really filled the ground-dwelling niche.

Edited January 28, 2015 by Stocksdale

[Stocksdale]

So I'm stumped again as to why we don't have elephant-eating ostriches.

I'm certainly not an expert in evolution so would be glad to be corrected by those who are...... but I often thing of evolution (and history in general) as a one-way street. It is the 'arrow of time' that only goes one direction. So once a change in body form takes place, that change can't easily be reversed.

One particular thing that comes to mind are gills. Once the sea-to-land transition occurred and tetrapods lost gills, they were never able to regain those. We've had numerous times when reptiles, dinosaurs, mammals, crocodiles have returned to become fully marine creatures. But in not one of those cases have they been able to breath under water again. They had to continue with the lungs designed for land breathing.

I imagine that many of the adaptations that happened in the small subset of avian dinosaurs are not easily reversed. The wings can't easily return to being grasping appendages. Once teeth are foregone to make flight easier, they can't be returned once the birds become flightless.

Just my thoughts....

Edited January 28, 2015 by Stocksdale

[Mediospirifer]

Velociraptor also had four clawed limbs, not two. That does make hanging on somewhat easier!

Also, keep in mind the point about keratin flexibility and the possibility of breaking a beak. A toothed predator can still hunt if it loses a tooth in combat (and in the case of reptiles will regrow it soon enough), but a broken beak is deadly to a bird, more analogous to a broken jaw for a reptile.

A better strategy for a bird would be to use strong feet to cling to a bull's back, flapping wings for balance, and slash with the beak. The terror bird's long neck might even allow it to go for the throat and sever the jugular or carotids, if it could crouch. The question there would be whether its claws would allow grabbing and holding like a perching bird. Looking up cassowary claws, I see that they don't have a back toe, just the three forward toes and the slashing claw. The wiki article on terror birds (Phorusrhacidae--I can't get a link to work from this computer!) doesn't describe the feet in detail, but the feet in the photographed skeleton don't look like perching feet even though they do have a rear toe.

The article suggests that the largest terror birds would have been capable of striking downwards with the beak hard enough to break bones, but would not have been able to withstand side-to-side forces. Holding on to a struggling large animal would have been dangerous to the beak. The author suggests that the birds kicked the prey to injure it, then finished it off with blows from that beak, and possibly could have taken large prey.

I'm wondering if they hunted large prey cooperatively, like wolves. Three or four terror birds together might have been able to surround their prey and keep it at bay while they kick and peck it to death.

It's interesting to speculate!

[siteseer]

In 90's geochemists started looking for carbon isotopes with the idea that isotopes of what animals ate would become present in the bodily fluids and would later be detectable in the bones. They found that modern East African herbivores such as antelope did have the particular carbon marker for the grasses they ate in the enamel of their teeth. Trees, shrubs, and grasses of a cool-temperate climate have a different carbon marker than grasses of a warmer climate where it rains in the summer. They took the idea even further by sampling fossil teeth and got consistent results relative to the known plant material in the area at the time.

These days, they also take samples of fossil teeth of mammals to see what oxygen isotopes are present. Scientists have an idea when the early whales were becoming more fully-adapted to the ocean because they can tell when one Eocene group was no longer drinking freshwater. Back in the 90's, they were already thinking that water from a river would give a different result than water from a lake because one isotope would be lighter than another and evaporate away so the heavier one would be present in higher concentration.

I assume that different calcium isotopes must be what works better with birds for some reason and that the isotope present in known Diatryma bones must be a clear marker for a herbivorous diet.

There was an article in Terra, the now-defunct magazine published by the Los Angeles County Museum of Natural History, published in the late 90's that explained it for the layman:

Harris, J.M. 1998.

Mammal Enamel: Diet leaves its mark in the Fossil Record. Terra. 35 (4). July/August.

I have seen only a limited run of this magazine but have found some paleo-related articles. Before the USGS library is completely depleted, I am going to try to get high-resolution copies of what I have. It was a great magazine on natural history and other subjects - too good to last long, it appears.

Jess

@jpc: It's an interesting idea that perhaps they did exist directly after 66. But we'd still need to know why they eventually failed, and that mysterious reason could probably also explain why they never got started.

Another problem is that they no longer think Gastornis/Dyatyrma was carnivorous because of some calcium isotopes. I don't know how convincing that calcium test is.

Edited January 30, 2015 by siteseer