Why does an empty vector call the value type's default constructor?

Using g++, I observe that creating a vector of size zero calls the vector's parameterized object type's constructor once. It then is deleted. Why does this happen?

#include <iostream>
#include <vector>
using namespace std;

class s
{
    public:
    s() { cout << endl << "default s constructor" << endl; }
    ~s() { cout << endl << "default s destructor" << endl; }

};

int main()
{
    vector<s> v(0);
}

Output:

default s constructor

default s destructor