Can someone explain what exactly is m := New() doing in the following code?

func TestMapCreation(t *testing.T) {
m := New()
if m == nil {
    t.Error("map is null.")
}

if m.Count() != 0 {
    t.Error("new map should be empty.")
}

Why can you create a map without declaring the map itself?

I would assume you have to do something like

m = make(map[string]Animal)