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On our own world, following the fall of the dinosaurs, mammals became the dominant class. This was caused by a cluster of factors, such as the rapidly changing temperatures and their ability to adapt. No matter how they became the dominant life forms, they are; this makes me wonder could birds become the dominant class?

I mean obviously it is possible, but surely there are ways that I can increase the odds of birds becoming dominant, or even guarantee it. What evolutionary environments would help guarantee birds become the dominant class?

A list of all Planet of the Aves questions can be found here

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    $\begingroup$ Please define "dominant". There are two definitions given by Wikipedia: "Ecological dominance is the degree to which a taxon is more numerous than its competitors in an ecological community, or makes up more of the biomass." (My boldface.) The answer might very well be different for the two. $\endgroup$ – a CVn Aug 12 '16 at 17:33
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    $\begingroup$ "Ok, so what do you need? Besides a miracle." "Bird seed... Lots of bird seed." $\endgroup$ – Xplodotron Aug 13 '16 at 17:41
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First off - who says mammals are dominant? There are about 10,000 species of birds, and 5,500 species of mammals. Mammals only get the 'dominant' label because (a) we are mammals and therefore biased, (b) the herds of megafauna which we like to watch in wildlife films - wildebeest, kangaroos, deer - are mammals.

Some of the birds had a bloody good go at being dominant. For instance, the terror birds were successful carnivores in South America for about 60 million years. Their mammalian competitors were marsupial carnivores, not placental carnivores for most of this. (The herbivores they were preying on were placentals). There were similar but unrelated giant killer birds in Europe and Asia - the Gastornithidae.

Where birds are going to lose out to mammals is being big. To be big they have to give up flying, because you can't be the weight of a sheep or a wolf and still get airborne. Basically to become enormous you have to become flightless: ostrich-like or terror-bird like.

Any niche which the birds don't occupy (because they need to stay small to fly) will be exploited by a mammal or reptile.

Mammals have teeth. That makes them really good at processing tough plant foods such as grass, or crunching up bones. A bird can do anything a mammal can in terms of making its intestines longer, or its stomach more efficient, but it can't chew its food. You could give your birds teeth again, but you will have turned them back into feathered dinosaurs.

Eggs can't run away. Mother mammal is lugging her babies around inside her womb, at the point that Mother Bird & Father Bird are sitting on their babies in eggs. So mammals can do things like travel while pregnant, but birds are stuck in one place until the eggs hatch. (Once hatching/birth has happened, things even up a bit). You could have some of your birds evolve live birth, and thus give them a life style more like a mammal's. There are fish, reptiles and amphibians which give birth to live young, so it's not completely outrageous to suggest a bird can too.

Eggs are size limited. The elephant bird's egg seems to be the maximum size which can be achieved. Even giant sauropod dinosaurs had eggs not much bigger than an ostrich egg. Birds therefore can't do the mammal thing of producing one enormous baby per year - they can't have a foal or a calf or a fawn. An adult ostrich and an adult deer might weigh the same amount, but the deer's newborn fawn is far ahead of the ostrich's newly hatched chick in the race to reach adult size. Again, live birth sorts this out.

Mammals have milk. It's a brilliant way of turning mum's fat and protein reserves into baby food. The milk can be specially tailored to baby's needs: high fat for baby polar bears, for instance. Birds (mostly) just have to collect what nature offers for their babies. However, pigeons and flamingoes produce crop milk which has the same function AND can be produced by both mum and dad. Make crop milk a more widespread feature of your birds, and they can feed their babies more like a mammal.

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    $\begingroup$ You missed what I would call the key point. Mammals have, or at least can evolve, hands. Not just monkeys. Raccoons and squirrels have considerable dexterity. Cats aren't far behind. Birds are in an evolutionary "trap". To dominate on land they'd have to de-evolve wings before they could gain hands. Can't happen with mammalian competitors in place. Now if any of the dinosaurs with forelimbs had survived, the story could have been very different indeed! $\endgroup$ – nigel222 Aug 13 '16 at 17:40
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    $\begingroup$ Not sure why you think hands are necessary to dominate. Plenty mammals have hooves, which are useless for dexterity. (Agility now, that's another thing entirely). My answer assumes that 'dominate' means 'vast herds of large ones in every ecosystem' plus 'being top carnivore'. $\endgroup$ – DrBob Aug 13 '16 at 18:06
  • $\begingroup$ I interpreted "dominate" to mean ultimately evolving to the point of acquiring technology. Hence the need for hands. They are also pretty handy (sorry) for lesser mammals. $\endgroup$ – nigel222 Aug 14 '16 at 8:29
  • $\begingroup$ You could have a bird that got really big and still flew, like the Pterosaurs. Granted, pterosaurs were not dinosaurs or birds, but I'm positing some independent evolutionary path that made birds get big and still fly. $\endgroup$ – Xplodotron Aug 15 '16 at 6:38
  • $\begingroup$ @Xplodotron - it's not size, it's weight (mass) which is the problem. The prehistoric bird Argentavis magnificens is estimated to be 70kg (ish) and is probably the heaviest bird to ever fly. It was so big that some scientists believe it couldn't get airborne unless there was a stiff breeze to help it generate lift. Flapping alone would not do the job. en.wikipedia.org/wiki/Argentavis Think man + hang-glider - they need to climb a hill and throw themselves into the wind to take off. $\endgroup$ – DrBob Aug 15 '16 at 14:54
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The meteor missed. The end-cretaceous mass extinction event didn't happen. Velociraptors and their friends lived on. They continued to evolve, to be large, and to eat whatever they wanted.

We now understand that late "dinosaurs" were very close to birds. They had feathers, hopped on two legs, and maybe even made sounds closer to chirps and eagle-cries than to Hollywood roars. In fact, even though we call both T-Rex and Stegosaurus "dinosaurs," A T-Rex has more in common with a modern sparrow, and lived closer in time to a sparrow, than to a stegosaurus. (Yes, this is an xkcd reference).

Clearly, at least a few things survived the end-cretaceous[citation needed] and became what we call birds. Had more things survived, and carried on as they were, they would have remained dominant. Mammals would have been kept in check and never got past being tiny rodent-like things scurrying around in burrows. The alt-world avians of today might not look exactly like real-world birds, but would be very close. And they would totally rule!

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    $\begingroup$ +1 Am I the only one who find this excellent answer comical because it's been delivered by a blue duck? $\endgroup$ – Henry Taylor Aug 12 '16 at 19:39
  • $\begingroup$ No, you're not....I do too, but it's nevertheless a great answer. To the point, thorough and worth a re-reading. $\endgroup$ – Joe Aug 13 '16 at 1:43
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I'm afraid 'evolution' and 'guarantee' don't play well together, however, you could improve the odds by giving a few species of avian life some of the advantages that mammals have. Once there were more than three types of mammals, not all made the evolutionary cut. Monotremes , Marsupials and Placentals all share milk for the young , I think that a parent being able to feed the young from stored resources (unlike Avians) is a profound advantage. Secondary, although hominids are upright, most mammals are not. To my knowledge, all Avians are upright. Without forelimbs or other means to grasp and do tasks like making tools, evolution is limited. Reselect a small to mid-sized Avian population, and give them the ability to feed the young on a short term basis, even in the absence of food, and the ability to grasp and shape the world around them, and you might have a winner... just my opinion.

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As an alternative to making the world more suited to birds, you could go with a few events to make the land less feasible to mammals and other creatures leaving only those that can go airborn to survive. And I think it's possible with a few geological and salt changes on our Earth.

There are several large scale extinction events in our not so distant history, most notably the Quaternary extinction events within North America that devastated the large mammal species. As a glacial period ends, you are left with giant melting ice sheets. These ice sheets melt at the surface, leaving large lakes on the top of them that grow and grow as the ice melts. Eventually these lakes breach the glacial ice wall holding them in and you have the entire contents of the lake then wash across what could be an entire continent. Lake Agassiz (larger than the great lakes combined) and it's associated drainage event into the Hudson Bay is attributed to an over 2.5 meter increase in global sea levels in some areas. Birds survive this event of course as they ride it out while air-born. Mammals and other land dwellers aren't so lucky.

Continuing these thoughts...there is a good reason why land-bound dinosaurs did not survive when the meteor impact hit while their air-born relatives survived. Being able to fly and the mobility this gives greatly increases a species ability to survive worldwide disasters.

As such my answer to your question is an increase in global disasters/extinction events, such as the toba extinction event that almost snuffed humanity as is, would inhibit all those that weren't yet able to fly. Leaving the birds as the only class to survive the repeated onslaughts of meteors, floods, volcanoes, and the nuclear winters associated with some of these events.

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