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Walking by the lake and watching the ducks the other day a thought struck me; there are lots of aquatic birds but all remain at least somewhat terrestrial, nesting on land.
Why is it that no birds have taken the evolutionary route of the dolphins and whales and returned to the sea?
Is there something in avian biology that means this is not possible?
Apart from one isolated group of birds in Australia (i.e., mound nesters found in Australia that use external heat for incubation - see link), all birds incubate their eggs (see link). All species that returned from land to sea have retained their reproductive cycle (marine mammals still give live birth and suckle their young) and still rely on air for oxygen intake. Extrapolating this mammalian evolution to birds would result in a creature that returns to sea, with all ties to land cut off, but that still has to incubate their eggs under water. As they would still rely on air, this is not very feasible as they would need to go up often to breath. This would leave the eggs vulnerable to predation. Even worse, it would leave the eggs at the mercy of a relatively cold ocean cooling them down quickly. Incubation means that the eggs are pretty much kept at body temperature which is difficult in the sea regardless of breathing parents. In all, while there may be certain benefits of underwater life for birds, it will require some drastic evolutionary changes in terms of the reproductive cycle. A more parsimonious way of colonizing the sea is to go back to land once and a while to lay eggs, incubate them, and raise the young until they are sea-worthy, like penguins do as @MattDMo mentioned.
This is a quite old question (more than 3 years now), and chances are OP will never read this. However, I'd like to give my contribution:
The issue here is not if birds incubate or not their eggs (see the other answer). Even if we suppose that no bird incubate its eggs, that is, that all birds lay their eggs and just walk away and that the eggs develop normally at cold temperatures, even so there could be no "marine bird" the way OP asks, like a dolphin. And the answer is in the egg itself.
Birds, which are a group of dinosaurs, together with the other reptiles and the mammals show an outstanding evolutionary innovation: the amniotic egg.
The amniotic egg allowed our Amniota ancestor to leave the water and to complete the whole reproductive cycle on land. It was a great evolutionary achievement, which allowed the tetrapod "conquest" of dry land.
An amniotic egg (tortoise)
However, there is a catch: that amniotic egg, which allowed our ancestors to be independent of water for reproduction, cannot be laid on water. Due to several reasons, mainly the (lack of) gas exchange through its shell, the embryo will not develop if the amniotic egg is laid on water.
That's why marine turtles have to leave the water to lay eggs, slowly crawling on the sand in a very exhausting task which will some time later put their very babies in danger. It would be preferable laying the eggs in the water, where they are already... but they can't.
So, how could some mammals take that evolutionary route? Because (most of) mammals don't lay amniotic eggs: they developed viviparity instead. And that viviparity allowed mammals such as whales and dolphins to spend their whole life in the water, never coming to the land.
A very interesting proof of this evolutionary explanation is the case of the ichthysaurs. This extinct group of marine reptiles spent all their reproductive cycle in the water, never returning to the land. How is that possible with an amniotic egg? The answer is that ichthyosaurs didn't lay eggs: just as mammals, they were viviparous. That viviparity allowed them to be truly aquatic.
Artistic representation of an ichthyosaur with its viviparous embryo
This now famous image (Motani et al., 2014) shows a very rare — and tragic, since they died — fossil, a pelvis of a Chaohusaurus mother with three embryos, one of them clearly visible:
So, we can only speculate that, the day some bird develop viviparity (or radical changes in the amniotic egg structure and physiology), then we can have a truly marine bird.
Source: Motani, R., Jiang, D., Tintori, A., Rieppel, O. and Chen, G. (2014). Terrestrial Origin of Viviparity in Mesozoic Marine Reptiles Indicated by Early Triassic Embryonic Fossils. PLoS ONE, 9(2), p.e88640.
x also belongs to that clade x, without exception. That's a basic phylogenetic rule. If a given bird evolves to an organism with fur and teeth, that organism is obligatorily a bird.
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