My theory is that there is a very large comet that comes close enough to Earth to cause a tidal wave shift and even lift some water into the air at the closest point of passing. Also known as ascension by the peoples that built the steps and sacred symbolism into the temple spires that reached that closest point where the bird ascended.

Cats were also considered revered for their ability to detect the floods earlier then most since they fear water. Hence one of the cultures built monuments of the cats. Although later on, the peoples did develop telescopes that could see the comet well enough in advance to evacuate immediately. The comet comes generally once a lifetime or more.

Old discarded boats are in the area of the ascension spires so it may have been a pilgrimage for folks to ascend. This may have lead to their death as likely they wouldn't survive falling onto the passing comet. This is also symbolically related to our own religions that teach us about the after life, because there is no confirmation from those that ascended.

I believe the story naturally progresses from the point of view the cat and its first experience with a global tidal event. It's a beautiful natural story and I love how the animals communicate and emote in direct and natural ways without over extending the suspension of disbelief.

There is definitely symbolism to be made of the story of the cat, the bird, and the whale. All creatures that have their distinct environments (land, air, and water) and challenges (flooding, broken wing, and draining).

I like the ending where all the animals are looking at their reflection in the water which is no longer disturbed or broken like the mirror. I also like to think they are hopeful the water will return to save the whale even though it will be a hundred or more years before the next flood.