"that" in nonrestrictive attributive clause?

“Duck,and like them the pigeons,are endowed with steel-like muscles,【that】 are a good part of the weight of the bird,and there will ply the short wings with such irresistible power that they can bore for long distances through an opposing gale before exhaustion follows.”
i don’t know why there is “that” in nonrestrictive attributive clause.


There are other problems in that sentence but the main problem is using a comma before ‘that’ when it’s used as a relative pronoun. ‘That’ as a relative pronoun has to introduce a defining relative clause.


what is the diffrence between “that” and “which” in the same position of that sentence.