Tracing A Recursive Method

What is printed to the console when the following program is executed?

public class BitCount {
    public static int countOnes(int n) {
        if(n == 0) return 0;
        return (n % 2) + countOnes(n/2);
    }
    public static void main(String[] args) {
        System.out.println(countOnes(13));
    }
}