I'm working on a Spec Evo project and I'm wondering if there's a possibility for domesticated chickens, the closest living relative to the T-Rex, to regain those massive sizes.

How did these chickens get so big? Firstly, I imagine these giant chickens while directly related to domestic chickens have a intermediary species where a few chickens who've taken to a purely carnivorous diet evolved into a more ratite/raptorial build similar to cassowaries and other ratites, due to their inability to fly because of there oversized flight muscles.

Then similar evolutionary pressures to the Jurassic and Cretaceous eras, where a large Oxygen boom happens due to the lack of human deforestation. The oxygen levels in this current era are around 50% of the highest they've ever been in Earth's history and a gigantism wave starts, their prey becomes bigger and so too, do these "Terror Chickens" grow bigger to better take them down (I imagine that their prey is a new form of insects I call "saurian-like insects" basically insects with adaptations and biology similar to large dinosaurs allowing them to grow much bigger)

So going back to the top, how big could the domestic chicken evolve? I'm okay with cheating a little like for example, the chickens coat of feathers making them look bigger than they actually are or the chicken's height being mostly a long neck like ratites.

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    $\begingroup$ I am pretty sure that ostriches are at least as closely related to T. rex as chickens are. And even if by a cruel twist of fate the Tyrannosauridae prove to be more closely related to the Galloanserae than to the Struthioniformes, which is unlikely, it still remains clear that ducks and geese share the same degree of relatedness as chicken. $\endgroup$
    – AlexP
    Commented Jun 10 at 23:44
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    $\begingroup$ Considerably larger if they were aquatic chickens. $\endgroup$
    – AJFaraday
    Commented Jun 11 at 9:02
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    $\begingroup$ Considering how little we really know about evolution this is impossible to definitively answer, but based on what we do know, given enough time anything is evolutionarily possible. So the real question is, do we have enough time? $\endgroup$
    – JBH
    Commented Jun 12 at 2:02
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    $\begingroup$ Real world examples to check would include the Australian Cassowary, and the New Zealand Moa (extinct) $\endgroup$
    – Criggie
    Commented Jun 13 at 3:05
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    $\begingroup$ Keep in mind potential bottlenecks like upscaling respiration, joints, and digestive processes, which are now heavily selected for for millions of years to allow small size. But 65 million years turned a rat-like ancestor into a blue whale, so anything's possible. frontiersin.org/articles/10.3389/fevo.2019.00242/full $\endgroup$
    – DWKraus
    Commented Jun 14 at 13:12

5 Answers 5


Evolutionary pressure alone is probably sufficient to allow for the development of T-rex-sized mega-chickens - but only if there are certain drivers and a lack of competition (especially at the early stage in the process).

Firstly, a carnivorous chicken is unlikely to compete successfully with an existing carnivorous mammal - cat, ferret, fox etc., so maybe you need an initial cataclysmic event (mammal-specific disease maybe?) that removes existing carnivores from a large island or sub-continent.

Secondly, I doubt that even in a high-oxygen environment, prey insects could grow to a sufficient size to require a carnivore to upscale to T-rex size. So maybe the chickens grow as insects grow to guinea-pig (or maybe even rabbit) dimensions, but beyond that they probably need to swap their prey to reptiles, amphibians, mammals, or other ground-dwelling birds.

Thirdly, the process would require many recognizably different intermediate chicken-ancestored species, rather than "[an] intermediary species" as you suggest.

And finally - the final carnivore will almost certainly still have a beak - no big T-rex teeth.

  • $\begingroup$ Voted to approve the edit, but "mammal" was given a specific other alternate spelling so consistently, I'm worried that's perhaps how its spelled somewhere outside the US. $\endgroup$
    – T.E.D.
    Commented Jun 11 at 13:47
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    $\begingroup$ Not their primary prey, but ordinary chickens already eat small reptiles, amphibians, and mammals when circumstances allow. I'd think that apart from the problem of finding enough prey, the early intermediate macrochickens wouldn't need to change their lifestyle much. $\endgroup$ Commented Jun 12 at 15:15
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    $\begingroup$ @RussellBorogove chickens eat damn well anything, including other birds when they can catch them. $\endgroup$ Commented Jun 13 at 7:19
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    $\begingroup$ Also second the "chickens eat anything" comments. I've kept bantam-sized chickens at a stage, and have personally observed them catching and devouring a mouse and a frog that strayed into their coop. If they were, say, ostrich-sized, I'm pretty sure the neighbor's cat/dog would not be safe. $\endgroup$
    – frIT
    Commented Jun 13 at 16:16
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    $\begingroup$ No big teeth? Might not be that far away in evolutionary terms cell.com/current-biology/fulltext/S0960-9822(06)00064-9 $\endgroup$
    – Yorik
    Commented Jun 13 at 18:57

The biggest terror-bird ever, grew up to about 3 meters tall and 300 kg.

The way I see it, your Giant Chicken could not grow any bigger than this, without sacrificing its chickenity.

As creatures grow bigger, their body plan needs to change to handle the weight. The legs of a bird must grow proportionally thicker. The feet proportionally larger. the point of balance needs to be lower and closer to the hips. The beak must be bigger to support more rapacious diet. The neck must be thicker and stronger, but thus, less flexible. The wings must be reduced to lower the weight.

Basically, if you grow a chicken to be the size of a terror bird, you must effectively end up with a terrorbird-shaped creature, just with a chicken's crest. That would barely look like a chicken at all. Anything bigger than that is not eve a bird anymore, basically a t-rex in a chicken costume.

A ballpark estimate: keep your Terror Chicken under 2 meters tall and under 200kg in weight, to make it look chickeny still.

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    $\begingroup$ "Chickenity" might be one of my favorite words, now. $\endgroup$ Commented Jun 11 at 15:27
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    $\begingroup$ Please check your spelling. I'm sure it's "Chickenality". Not to be confused with "Chicken Ala King". $\endgroup$
    – gns100
    Commented Jun 11 at 19:22
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    $\begingroup$ @gns100, I always assumed "chickenity" is about fundamental traits of a chicken, while "chickenality" more about the symbolic appearance of being one. Like say, a man in a chicken costume who pecks around is very chickenalicious, but has no actual chickenity. $\endgroup$ Commented Jun 12 at 7:23
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    $\begingroup$ @Criggie pretty sure chicknasity refers to particularly nasty chicks $\endgroup$ Commented Jun 13 at 6:39
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    $\begingroup$ You're batting 1000 on the verbiage. Suggested edit: "under 200kg in weight to preserve its chickenality." $\endgroup$
    – Tom
    Commented Jun 14 at 0:45

I could imagine this chicken living on an island such as New Zealand. Animals throughout history have been known to change size on islands (Foster's rule). Garganornis (a giant goose) lived on the islands of Miocene Italy and grew to 1.5 metres tall. (The goose’s habitat is marked on the map with a black asterisk.) enter image description here

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    $\begingroup$ A giant goose sounds more terrifying than a T-rex sized chicken to me. Maybe OP should change the plan. $\endgroup$
    – bracco23
    Commented Jun 11 at 14:46

The Aepyornis lived in Madagascar up until about 1000 AD. The remains suggest they were 3m tall and could weigh up to 1000 Kg.


Is 3.6m big enough?


Some New Zealand Moa species were that big and only became extinct after humans arrived about 700 years ago.

Elephant Bird

On the other side of the world, in Madagascar, Elephant Birds roamed. These birds, while not as tall as Moa, were much more massive, sometimes weighing over a ton. Their extinction is even more recent, lasting into the 1700s.

You do need to get rid of the mammals who are generally better at filling the ground-borne ecological niches than almost everything else: it's no coincidence that these large birds were on isolated islands. Also, get rid of us because we are the ultimate apex predator; lions and tigers and bears only exist because we let them exist, oh my!

As descendants of flying birds, you will need evolutionary changes because chickens lack the skeletal structure to support such body mass. However, don't let that bother you—mammals evolved on land, yet whales and seals returned to an aquatic lifestyle.


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