The function \( y = x^5 - 13 \) is a nonlinear function because it contains the term \( x^5 \), which is a polynomial of degree 5.
Linear functions have the general form \( y = mx + b \), where \( m \) and \( b \) are constants and the highest power of \( x \) is 1.
Since \( y = x^5 - 13 \) does not meet this criteria, it is nonlinear.
Therefore, the answer is 2.