What's the bird carrying Ark around?

As you can see, in English it was translated as a gull, which could mean any number of shore birds (source). If you think that's a bit vague, look at the Japanese version:

Here it is simply called wataridori, or migratory bird. Even the English translation hints to this in dialogues, showing how gull was just a short word to stand in for migratory bird which was impossible to use, given the cartridge limitations for the recurring names.

But there's more! On the one and only Japanese board 2ch someone pointed out that you'd need a really big migratory bird for this sort of task. Well, good thing there's already such a bird that is portrayed in both Western and Eastern mythology as carrying humans or transferring souls. Its name? Shirasagi, a term denoting all big migratory birds with long beaks and white plumage, such as the stork or the white heron.

And now you know what's that bird which carries Ark around in a bag, just like a baby (subject for infinite jokes among Japanese fans).

Even more, you can see how Ark's metamorphosis in the end all ties in neatly with this. After all, storks are symbols of the soul and its next voyage. The ancient shirasagi ceremony wonderfully portrays this role, with the dancers momentarily transformed themselves in the majestic white bird.

Let's just hope Ark turned back into a human after he reached Elle's house in Storkolm (lit., stork islet). It would be a tad hard to enter the door with such a long neck.