So after being criticized for lack of nuance in your argument, you double down with hyperbolic anecdotes, and a "slippery slope" I'm going to assume that's a zero in the personal experience column.
Let's get rid of this slippery slope nonsense. Pointing anecdotal outliers as an example of why an entire group of people should stop petitioning for acceptance is a mad libs tactic you could use against literally any form of social progress, or any religion, political movement, or personal philosophy for that matter. A few terrible parents injure or kill their child in the name of Jesus every year, that shouldn't be used to discredit Christians, so don't hold trans people to a different standard.
I obviously agree that parents shouldn't try to funnel their child into a trans gender role, based on faint clues, like playing with dolls, nor should they try to raise their kid without gender, since that just guarantees you'll traumatize them regardless of their gender identity. But as I said, you're pointing to a hyperbolic anecdote.
I can't speak to the condition of bird people, but I wonder how many people spend their lives hiding the fact that they are "bird people" or if this is something that fades if they disconnect from their community. Bruce Jenner spent 30 years pretending to not be Trans, achieving ridiculous amount of success without feeling fulfilled. The bird-people thing sounds more like people who were bored and want to add something interesting to their otherwise lackluster identity, like the people who decide they were cowboys in a past life. I could be wrong though. If you have more insight into the long term mental condition, suicide rate, general disposition, and discrimination against bird people, I'd be happy to give it a read. Thankfully trans gender people have a good deal more information about them, and (despite your faulty assertions) are not categorized as delusional by the APA.
I mentioned this before, multiple times, that the mechanics of identity are more complex than simple biology. If someone's intersex, they're kind of expected to choose a gender and run with it. Because sex isn't gender, and gender isn't sex. My lab-mix doesn't have a gender. When he got his testicles removed it did not trigger an identity crisis, because he doesn't have a self perceived identity. He and his sister shit in the same place, because they don't have the capacity to perceive it as any kind of inappropriate.
Gender is a thing humans have, because we're the only things on earth capable of perceiving that as being important, and it's defined by an increasingly obsolete combinations of societal perceptions of masculinity and femininity. There's a reason we have terms like "girly girl" because there's a general recognition that a female can be more or less feminine, that it's not directly attributable to a binary (XX or XY) configuration. Now where masculinity and femininity separate from societal expectation and natural inclination is a bit of a gray area. Since identity a cultural biproduct, it's impossible to define one in the other's absence. I suspect feral humans would not have a very well honed sense of self.