The function \( y = x^5 - 13 \) is nonlinear because it contains the term \( x^5 \), which is a polynomial of degree 5. Linear functions are of the form \( y = mx + b \), where \( m \) and \( b \) are constants and the highest power of \( x \) is 1.
Thus, the correct answer is:
2 (nonlinear)