Is it even possible for someone who critter changes into a thunderbird to carry a character up a 10 story building...i think its unlikely they try it but if they do I have a test all ready...or should I just laugh and tell them it isnt possible.

-Josh

First of all Thunderbirds are cars and they don't have good enough aerodnyamics to take flight (even though rocket boosters might help, but then that is with a controlled ascent and an uncontrolled descent).

The critter who is large because of its windspan has to carry double the bodyweight normally carried. What will most likely happen is that they will get to the fourth floor and the thunderbird will let go of the person, because of lasting strength. It's the equivalent of running taking two forty pound bags holding them with arms wide at shoulder length and running up 10 flights of stairs. It can be done. It just demands immense training.

My First car was a Tbird...my parents got it for me for 50 bucks...for 50 dollars it wasn't a bad car...until winter hit...then it died forever lol.

I know its a longshot...but I just wanted to know because I could see them trying thanks for answer!

-Josh

Aside from the T-Bird thing ...

Isn't the Thunderbird an awakened creature? If so, the Shapechange (or Critter Form) spell does not allow the caster to take that form.

Exactly. (Critter) Form and Shapechange both specifically state you cannot use awakened animals.