If a human shapeshifter turned into a flying bird they would be the largest one alive, assuming mass is conserved.

No, they would turn into a normal bird. The extra mass is shunted into z-space where afterwards it can be called back to return the shifter to their previous form. Just don't stay a bird for more than two hours.

That is a weird way of saying "Humans are bigger than birds," even then though, I think on average the ostrich will win.

Edit: I missed the flying part, but still.

By quite a lot too. The kori bustard is the heaviest known flying animal at around 18kg or 40lbs.

Marahute!

CAW bitchhhh

You ever seen an ostrich?

Bro they are bigger than humans.

not as large as big bird

Just because mass is conserved doesn't mean size/shape is conserved. If you're always human-sized, you're a pretty shitty shapeshifter. No offense.

When I was a kid I had a book called The Science of the X-Men (which that site lists for $11 but eBay lists for anywhere from $100-1500) that attempted to use real world physics to explain how the powers of the X-Men might work.

Mostly the explanations came down to "I dunno, maybe black holes?" For example, it speculated that Jean Grey might have a microscopic black hole in her brain and had subconsciously learned to use it to open the other end in other brains and somehow vibrate the brain matter in a way to communicate telepathically.

Anyway, I don't recall whether it covered any shapeshifters, but if so, it probably handled it the same way Animorphs did (as others have mentioned in this thread), but with black holes instead of z-space.

imagine a human-sized flying bird, that would be terrifying

Preserving a mass while maintaining the ability to fly would require you to significantly increase in size, which comes with all sorts of drawbacks.

Humans can't fly precisely because we're too dense. Birds and other flying creatures have plenty of adaptations meant to reduce mass (or, rather, density) by all means possible.

if a fan was stuck in my ass I'd be a windmill.

Are you also preserving the ratios of materials that the body is made of? We'd have very heavy bones for a bird (or a lot of bones at bird density) and probably not enough muscle to lift 2m long wings. Also all the keratin in our hair and nails wouldn't make many feathers. We'd be a mostly plucked bird

you would need to turn into something like a Pterosaur given the hollow bones and everything. Those could be anywhere half the mass of a person to like twice the mass or something.

And if my mom had wheels she’d be a bicycle.

So if they turned into an elephant they'd be the cutest widdle elephant ever??

Biggest by mass, but a bird with the mass of a human cant fly (biomechanically impossible), so there needs to be magic fuckery involved.

Meaning that a shapeshifter that turns in a flying bird with the same mass as a human would likely not be as large as you think. Humans are very dense compared to birds. Assuming we keep that density, we could be smaller than some of the larger eagles or carrion birds.