===> SPOILERS AHEAD! <===
Playthrough and story
Having seen the two endings I think I get the meaning of the game. Or maybe I am reading too much into it, but I had fun thinking this out so why not write it down? Despite having tried to get a more "neutral" one in the second run, it turns out there being only two endings is justified. I will elaborate on the game, it's ideas, and what I think about it below:
The game starts by telling the player that "the kingdom of birds has fallen and will not rise again." While that is a reference to the phoenix bird, the way I see it, the bird "god" is actually a roc, which is more fitting for a malevolent being as the roc is actually said to be as big (or bigger than) mountains in mythology and eat elephants. I could think of no better villain for such a story.
Anyway, the protagonist emerges from it's shell and finds the roc who commands the little birb burd to bring him food. The first task is easy, but immediately after giving the little bird the order to search for more the little bird runs into vulnerable and orphaned chicks. Here the "god" shows a disdain for weaker birds as he tells the protagonist to ignore the chicks and feed him instead, and right before that he even says that if the player wants to use checkpoints/teleporters ("return to me sooner") he would have to kill fellow birds yet to hatch.
From that point on, player choices determine the fate of the chicks, the god, the protagonist, and the world... Several rather frustrating deaths aside, specially if the player is trying to go for the best ending. Eventually the player finds another bird who resembles a cross between a robin and a hoopoe - the birds which, according to persian mythology, resisted the crow's influence and helped lead the flock of birds away from the mist and towards the path to meeting the "king of birds", or Simurgh.
At any rate, If the player remains loyal to the roc, the chicks starve and presumably the unborn are slain, their nests to be used as teleporters. This ending is the easier one to get and also rather unsatisfying - the hoopoe-robin chastises you for having been "led astray by power" and for believing blindly in heaven, whereas the world they lived could have been made into such a thing. I actually found the implication that the little bird was killed and sent to hell as punishment for the evil caused to be a poetic end.
If you choose to go for the good ending instead and feed the chicks (and also do your best to avoid murdering the unborn birds), the "god" becomes progressively angrier until he actively starts using his power to attack you, admitting his intention to destroy what remains of the world. While the game is at it's hardest, the ending you can get after slaying the tyrant-god is quite worth it: The hoopoe-robin praises you for your choices, saying that the world they live in can be made into heaven now that the kingdom of birds is no more. He ends on a hopeful note: "If nothing matters, why not let there be love?"
What I think about it
Again, assuming I am not seeing things were there are none, the game depicts a sick society at it's last throes where a weakened, cornered tyrant is desperately trying to finish off what remains of a deeply wounded world. At first I thought this was some sort of critique of modern day society, as it is implied there were other gods that formed the fallen kingdom but are now no more. But the whole "god commands bird to murder fellow birds" brings to mind biblical scenes and blind adherence to dogma more than modern decay instead.
The game is ultimately about doing what you think is right, but you will be judged for it. Do you really think some vague idea is worth more than those who are alive, or not yet born? Do you really think defying authority is worth it, even if it ends what is familiar to all? Which version of heaven is better to you?
Personally, I do not agree with nihilism as a philosophy. I never saw the appeal of it - to me things are not so much meaningless as they are subjective. There is a saying: "a man's trash is another man's treasure", so to me what a fellow person finds happiness and meaning in can be completely meaningless to me - and vice versa.
I also have to add that the game is quite inconsistent in it's approach to it. The bird who is opposed to the god is clearly espousing a morality that is not nihilistic - indeed, why let there be love? Nothing matters, remember? And when nothing matters, evil and good have no difference: Death is the same as life, joy is the same as misery, good is the same as evil. None of them hold any weight, zero, and infinity multiplied by zero is still zero.
While the good ending is more, well, meaningful to me as-is, I think perhaps it would have been better if the kingdom of the birds had been a good thing which the little bird attempts to serve and the other bird condemned the little bird's actions not so much based on morality but on futility - the kingdom has fallen and will not rise again, so all the protagonist has done is either delay the inevitable or hasten it, not really changing it.
Another approach would have been to ditch the nihilism entirely. This world clearly has good and evil defined, so why not roll with it from the get go? The hoope-robin asks the protagonist to consider the choices taken, but why not appear and judge earlier? Have him actively work against the evil god, condemn the player for not feeding the chicks, try to make him question Aevali's existence from the beginning, warn him that the "god" is actually a hateful entity. And when the die is cast and all choices are taken, rather than a bleak or hopeful outro you simply get what you worked for: The destruction of the world, or it's salvation.
Final words: It was quite an enjoyable experience, gameplay-wise though there are a lot of traps which could have been done without considering the game is more about moral choices than it is about platforming. At least that was how I experienced it. I could also talk more about the presented ideas but I think I have done enough about that already.
Also, yes, I was really bored. ¯\_(ツ)_/¯