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Background

This is a Terror Bird:

Specifically, this is Titanis one of the largest of the carnivorous Phorusrhacidae; it was perhaps 2.5 meters tall and 150 kg.

I would like a terror bird that is at least 1000 kg, but it would be even better if it were more like 5000 kg; the size of a Tyrannosaurus. Fundamentally, Tyrannosaurus is more closely related to bird than it is to extant reptiles like crocodiles. Of all the birds that we know of, the terror birds come closest to matching the physical anatomy of Tyrannosaurus; that main difference being that the birds lack the heavy tail of Tyrannosaurus.

Question

Two overlapping questions:

  • Can a terror bird weighing 1000 kg survive in any modern ecosystem?

  • Can a terror bird weighing 5000 kg survive in any modern ecosystem?

Considerations

  • Modern ecosystem means anything that exists today, though assume that human hunting pressure doesn't exist so herds are plentiful.
  • A modern physical environment means current temperatures and oxygen concentrations, among other things. There is some speculation that higher oxygen levels could be related to dinosaur size. I want to be sure that a terror bird shape can bio-mechanically be extended to large enough size.
  • The terror bird must be able to survive at all sizes as it grows (at whatever rate it can) into a full sized adult.
  • Given that terror birds are probably extinct due to competition with modern modern predators, assume that the terror bird can successfully compete with modern carnivores (lions, etc).
  • Do not assume that a terror bird can catch modern herbivores (can it catch a horse?) This an important part of the question.
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    $\begingroup$ I think such a monster is a perfect fit for modern Australia. $\endgroup$ – Renan Sep 12 '18 at 20:21
  • $\begingroup$ @Renan Australia doesn't have a lot of large meat items these days. I would have thought 1000kg would be overkill. $\endgroup$ – kingledion Sep 12 '18 at 20:22
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    $\begingroup$ In terms of predator competition, I think the correct evolutionary question isn't "Can this bird compete with modern predators?" as it is "What evolutionary pressure would make this bird grow to the size of a T Rex?" Size gives an advantage in competing for carrion, but that only provides an evolutionary pressure to be the largest carrion bird around - any more growth than that wouldn't make sense as an evolutionary adaptation. And there certainly is no existing prey species requiring your bird to be that size. $\endgroup$ – tbrookside Sep 12 '18 at 21:08
  • $\begingroup$ Do you require a Terror Bird like the one in your picture, scaled up? Or will you accept an avian of the requisite size, proportioned so as to produce much Terror? $\endgroup$ – Willk Sep 13 '18 at 22:40
  • $\begingroup$ @Willk It must be avian, carnivorous, and large. $\endgroup$ – kingledion Sep 13 '18 at 22:44
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Modern and extinct flightless birds adjust for weight by swinging the whole body farther back, making the pelvis stick back very far, and making the femur more vertical (see image). this moves the feet further forward. This does mean the forces on the femur will be huge at the largest size so it will need to be far thicker than seen in T-rex. At the largest sizes it may not be functionally sound.

The other problem is finding something to eat, but that is a bigger problem for the larger size so I address each individually.

5000kg The only issue is what prey they attack that requires them to be that big, they will be to slow to catch smaller animals so they had better be hunting things close to their size. there just are not many animals available. elephants, hippo, and rhino are about it so you are probably restricted to Africa or parts of Asia in the modern world. During the Pleistocene you have many more options: Africa, Eurasia, and North America basically the places that had mammoth.

1000kg This much easier, depending on the time period most of the world will work. But you still want and abundance of ~1000kg prey animals, which limits your to a few places on earth.

enter image description here

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  • $\begingroup$ I don't think the first objection is really much of a problem, as the terror birds could always re-evolve a longer, heavier tail. Off the top of my head, the largest existing land predators are tigers at about 200 kg, and lions a bit less. (Kodiak bears go to ~700 kg, but they're omnivores.) In the current world, it seems unlikely that there'd be sufficient prey to sustain larger carnivores. $\endgroup$ – jamesqf Sep 13 '18 at 3:34
  • $\begingroup$ the tail may be possible but despite many flightless birds none have, which makes me wary of saying they can. I am going to edit to make the last part a bit clearer. keep in mind there are pleistocene predators in that size range. $\endgroup$ – John Sep 13 '18 at 13:56
  • $\begingroup$ I don't think there were predators in the 1000 kg range, let alone 5000 kg. The only contender that I can find in a quick search seems to be the South American giant short-faced bear, with one weighing in at an estimated 1600 kg: news.nationalgeographic.com/news/2011/02/… (IDK if it was a pure predator, or an omnivore like modern bears, though.) Others like the American lion seem to run about 50% heavier than their modern counterparts. $\endgroup$ – jamesqf Sep 14 '18 at 17:00
  • $\begingroup$ the 1000kg range obviously not the 5000kg range, note there are living terrestrial predators (saltwater crocodile) that weight over 1000kg. Also the short faced bear is a predator, it is adapted for better running ability than modern bears. There is no such thing as a pure predator, the closest you can get are hyper-carnivores and there are not many of those. $\endgroup$ – John Sep 14 '18 at 17:21
  • $\begingroup$ re: large game animals outside of Africa: wouldn't the North American bison qualify, before we hunted them to near-extinction? $\endgroup$ – papidave Sep 15 '18 at 13:36
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Large predators such as crocodiles use ambush. Being living dinosaurs themselves, that means it is a proven method.

Now the terror birds face the challenge of Hot Blooded species; our rate of calories expenditure is way higher than a reptile. Cold blooded ambushers can snack once and sleep all week.

For a hot blooded hunter, this places a larger pressure on your ecosystem. Now your Apex predator is feeding more than once per week.

So your deciding factor is; what changes are needed in your prey to support your predator?

For once, increased birth rate. Maybe they are faster, but migration trips to and fro foraging zones, expose them to the predators. Maybe it is an exclusive prey that lacks any other predators. Architeuthis dux, AKA papa squid falls in this category. Only sperm & pilot whales munch on them. Other reason to favor a large size is cold weather, in turn requires a large predator to take them down. Polar bears already rock 700kg, so it isn't impossible to reach 1000kg. (Add more feathers for the cold!)

Over 5,000 Kg, you method of transportation depends on the soil. You will go ankle deep in mud, totally sink on snow to say nothing of making you too vulnerable to overheating.

The dealbreaker is calories.

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    $\begingroup$ House sparrows are indeed dinosaurs, but crocodiles aren't. (But both dinosaurs, including sparrows, and crocodiles are archosaurs.) The most recent common ancestor of dinosaurs and crocodiles lived in the early Triassic, about 250 million years ago. $\endgroup$ – AlexP Sep 13 '18 at 0:07
  • $\begingroup$ I recognize my ignorance on the subject. Used the crocodiles as a baseline of large predator, which has survived the evolutive race for millions of years. $\endgroup$ – Gustavo Sep 13 '18 at 20:06
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Make the terror birds aggressive herbivores living in a herd:

All large land mammals are all aggressive :

  • Elephant
  • Hippopotamus
  • Rhinoceros
  • Buffalo
  • and last but not least: Giraffe :-)

Except for the rhinoceros, they live in herds, are fiercely territorial, (yup cute dumbo and funny hippo will rip you and your car to shreds), and look in the mirror ;-) and you'll be reminded giraffes have horns! and one kick of a giraffe's hind legs can split a lion's skull in 2.

Once they have these characteristics, why can't they become as heavy as:

  • A Black Rhinoceros (680–1,810 Kg)
  • A White Rhinoceros (1,400–2,300 Kg)
  • An Asian Elephant (3,600–4,500 Kg)
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Big flightless birds extinct and extant have a similar body habitus - long neck, bulky body, no tail.

flightless birds http://safariostrich.co.za/2016/05/the-ostrich-dinosaur/

Probably it is the lack of tail for balance combined with a need for speed that limits the maximum size of these large flightless birds.

https://www.newscientist.com/article/dn24625-losing-their-dino-tail-limited-size-of-flightless-birds/

Theropod dinosaurs, the ancestor of birds, walked with their legs vertical using their tail for balance. As their descendents evolved to fly, their heavy tails became a liability and shrank to a stub. To keep their balance, the evolving birds began to crouch, with their femurs tilted back to keep their knees under their centre of gravity. They kept this position even after some had ceased to fly, and the extra strain made their femurs stronger... The dino femurs grew longer and thicker as body size increased, Chan says. But in birds, there was a “distinct plateau” between 300 and 500 kilograms, with femur length constrained by the need to keep the knee near the centre of gravity. That makes it harder to support a large body, and so it limits their size, argues Chan.

I wonder also if there is a limit to how strong a hollow femur bone can be. These flightless birds have strong bones, but they are all still hollow. Also the light skeleton reduces the mass of these birds - a giraffe of 5 meters might weigh 1000 kg (weight to height ratio = 2) but a big 3 meter ostrich weights only 150 kg (weight to height = 0.5). Good if you want to be a fast bird, but not if you are trying to achieve a weight target requested on world building stack.

But there is a different type of body a flightless bird can have. https://www.pinterest.com/pin/537476536770332251/ penguin skeleton

Penguins are unusually upright, with their legs set far back on their bodies. Uniquely among birds their bones are not hollow. Penguin tails, or pygostyles are very robust.

https://fossilpenguins.wordpress.com/2011/10/28/tour-of-the-penguin-skeleton-iii-the-pygostyle/

Penguins have a pygostyle, but it is quite different in shape from the standard avian pygostyle. In penguins, the element is more elongated and less flattened. Rather than being plate-like, it is almost triangular in cross-section with a flattened base. Penguins also have a very different set of tail feathers. Rather than forming a fan, penguin tail feathers are very stiff and quill-like, and stick out somewhat like the bristles of a broom. This is especially true of penguins from the genus Pygoscelis – the Linnean name Pygoscelis actually translates to “stiff tail”. These penguins are prone to be caught slouching around, partially propped up on their tail feathers. It seems that without the necessity of maintaining a “fan” of tail feathers, penguins have gone ahead and modified their pygostyle to a shape more suited to supporting themselves on land than steering in flight.

This has the makings of a large land animal: weight stacked vertically over the stable tripod of stocky legs and tail. This is a different route to bipedalism than either an ostrich or a tyrannosaur. This is a thing that can get very big - like the ground sloth Megatherium which could measure 6m and weigh 4000kg.

megatherium

The giant terror penguin will, however, be a predator in the pattern of a tyrannosaur. Why did the Tyrannosaur need to be so big and fierce? Was it to take down and devour huge formidable prey? No - it was to intimidate smaller, fierce carnivores. A carnivore never wants a fight it might not win - any injury might prove fatal because of consequent inability to hunt and starvation. Wild dogs are tremendously efficient hunters but half of their kills are stolen from them by larger predators.

Like T.Rex (may have been!) the massive terror penguin is a scavenger and kill stealer. No other predators have a chance against it and they leave when it arrives, carrying off what they can. Its bulky body offers no vulnerable spots within reach, and the bulk serves to anchor a long snakelike neck and sharp slashing beak. Because of its diet of carrion, its head will be bald, like a marabou stork or vulture. The long neck also allows it to eat without actually getting down prone on the ground, and the size of the bird means it can eat the entire kill itself.

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Due to your demand for specifics, I'll have to split my answer into two:

1,000 kg - This is at least semi-plausible. This thing would chew through even the largest predators in its habitat, being 4x the weight of an adult male African lion. It would be mostly confined to Africa and south Asia, subsisting mostly on elephants, rhinos, and hippos, while occasionally snacking on smaller creatures such as zebras, wildebeest, antelope, and sika.

5,000 kg - No matter where in the world you put this thing (unless it had gills and could live in the ocean and hunt whales) it would starve.

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