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In my world, instead of mammals, birds have become the dominant order. Similar to how Quadrabirds replaced the grazing mammals; Aquabirds will take up the niche cetaceans occupy.

My goal is that the Aquabird would be similar to animals like the orca or the narwhal, but I am stuck on the evolutionary process of such an animal. Considering that there are aquatic mammals, could aquatic birds exist? If so, what would they look like and how would they evolve?

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  • $\begingroup$ Birds are the dominant order of warm-blooded vertebrates in our world, too. There are more species of bird than of mammal, found in a wider range of habitats (there are no mammals in Antarctica, for example, and there were very few in New Zealand until a few centuries ago). $\endgroup$
    – Mike Scott
    Aug 14 '16 at 18:49
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    $\begingroup$ @MikeScott both points are wrong. Cetaceans occupy Antarctica and biologists all over the world agree that we live in the age of mammals. $\endgroup$
    – TrEs-2b
    Aug 14 '16 at 18:51
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    $\begingroup$ There are cetaceans in the Antarctic, but not in Antarctica. Because it's dry land with no significant rivers or lakes, and cetaceans only live in water. $\endgroup$
    – Mike Scott
    Aug 14 '16 at 18:53
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    $\begingroup$ Mammals aren't a species. They are a class. In scientific taxonomy, species is very specific. As to dominance, this is always categorical. Microbes didn't enter into Mike's point because they have neither a spine, nor are they warm-blooded. What dominance is defined as--that's another question. Scientifically this is answered by biomass, or control of energy in an area, or intelligence, or really, whatever the scientist writing the paper says it is. Think you both have good points, but the word dominance as I have seen it in academic papers has been used too many ways. $\endgroup$
    – Erin Thursby
    Aug 14 '16 at 19:55
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    $\begingroup$ I have seen diversity of species within a class used in the way Mike has used it, but it's pretty uncommon for most biologists to use the word "dominance" this way. It is, however, not totally invalid. TrEs-2b is right that most biologists don't define it in this way. livescience.com/46866-planet-apes-next-dominant-species.html $\endgroup$
    – Erin Thursby
    Aug 14 '16 at 19:59
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Well, this probably starts with diving birds that gave up flying. We have example species for that already, such as penguins. To stay in water for long periods, they're going to need to grow larger so that they can carry a thick layer of insulation, such as blubber: all the example aquatic mammals we have are fairly large for just this reason. Being small and un-insulated in the sea is a great way for a warm-blooded creature to die of cold.

That gets you to creatures that fill similar niches to seals and walruses, who come out of the sea to give birth, and your aquabirds will need to do that to lay and incubate their eggs. The eggs need to be incubated, or the embryos will die of cold if they have a bird's metabolism. Turtles run much cooler.

Having eggs that could float in the sea probably isn't viable, on the grounds of heat loss, and of inability to get enough oxygen through an eggshell. The shell has close to the minimum surface area for its volume, which is exactly what you don't want for a gill system or equivalent.

If you want whale-sized aquabirds, it gets harder. They're too big to come out of the water to lay eggs. I can see two ways to do it:

  1. Give birth to live young, which need to be fairly sizable to carry insulation. But that's boring, and un-avian.

  2. Have major sexual dimorphism. The males grow to the size of whales. The females are smaller, at least for the breeding phase of their lives. They come on land to lay and incubate eggs. They're fed by the males, somewhat like the way real birds feed chicks: the male gathers and pre-digests food, and the female doesn't have to hunt, just go out to sea a little and collect food from him.

It's a bit of a stretch, but it looks sort of right.

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    $\begingroup$ Dougal Dixon's book 'After Man: A Zoology of the Future' has penguins which retain the egg inside them until the moment of hatching. They have taken the niche of toothed whales and baleen whales in the Southern Ocean. $\endgroup$
    – DrBob
    Aug 14 '16 at 20:21
  • $\begingroup$ It may be un-avian, but evolution from oviparity to ovovivarity to viviparity has happened before. $\endgroup$
    – rek
    Aug 15 '16 at 3:51
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    $\begingroup$ i know this is old but 'un-avian' isn't really a valid argument, half of sharks are 'unlike fish' with their live birth, as well as ichtyosaurs being 'un-reptilian' for the same reason. Also, it goes both ways - monotremes (platypus and echidnas) lay eggs, and they are mammals. Using the 'its too different' defense doesn't make sense regarding evolution. A bird could very well develop a womb, or even go for the shark approach and hatch the egg inside. $\endgroup$
    – XenoDwarf
    Sep 8 '16 at 4:30
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The problem with aquatic birds is the eggs. Either they come to land to lay and incubate their eggs, in which case they're basically penguins, or they give birth to live young, in which case they're somewhat unavian.

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    $\begingroup$ turtles spend their entire lives in the oceans, only coming to land for laying eggs. they don't wait around for them to hatch like penguins either. the little turtlelets have to make their own way to mom and dad in the big blue... $\endgroup$
    – EveryBitHelps
    Aug 14 '16 at 18:59
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    $\begingroup$ But birds are warm blooded, which is why their eggs need incubation. $\endgroup$
    – James K
    Aug 14 '16 at 21:47

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