I just love to read Mark Twain
To answer your questions.... The story leading up to the sentence If a jay ain't human, he better take in his sign, that's all. Talks about all the human characteristics of the Blue Jay... the use of the word sign is a metaphor a list of the human characteristics of the Blue Jay. It is as if through the human characteristics displayed by the bird, the bird was holding a sign which read "I am Human".
To chance it simply means to accept the risk. If you purchase a lottery ticket then you are taking a chance (accepting the risk) that you will loose.
I hope this helps,
Student or Learner