Copy constructor calls when creating a new thread

I'm reading the book C++ Concurrency in Action to learn more about threading and the C++ memory module. I'm curious about the number of times the copy constructor is called in the following code:

struct func
{
    func() = default;
    func(const func& _f) {}

    void operator()() {}
};

int main()
{
    func f;
    std::thread t{ f };
    t.join();

    return 0;
}

When I walk through this code in the Visual Studio 2013 debugger, I see the copy-constructor called four separate times. It's called three times from the main thread and then once from the new one. I was expecting one, as it made a copy of the object for the new thread. Why are three extra copies created?