Here's my theory.
In my opinion, the Zora couldn't find enough fish to feed themselves on the Great sea. This was probably becuase the fish of Hyrule are fresh water. Therefore they're gills and fins which encouraged them to swim, which had no benefit in obataining food, actually led them into the sea, which was filled with dangerous monsters. Therefore those who tried to find food on the ocean were killed by monsters. Those who lacked gills or fins didn't try to make a living off fish, travelled to Dragon Roost Island. Their they discovered Valoo a dragon who's scales gave them the power of flight, which was a beneficial adaption as you can avoid most of the monsters on the Great Sea by flying. Over time the Rito adapted to haave a more humaniod body in order to climb to the top of the Volcano (scales are slippery and a fall from that hieght could be fatal). I don't know about the beaks becuase the Rito's diet isn't gone into in WW. This is just speculation, but I think it cover's most of the Rito's adaptations.
It states several times in the game that there are no fish on the Great Sea. Also there are clearly and abundance of monsters. This is what happens when you try to apply logic to Zelda.

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Formerly Ritogirl

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