struct bird {
  ~bird() { std::puts("in a bird"); }
};

bird in = bird(bird(bird(bird(bird()))));

The destructor is called once!

Thanks to C++’s return value optimisation. You can disable this behaviour by passing -fno-elide-constructors to the compiler. However, if you’re using a standard later than C++14 then it happens regardless. So does this mean we can’t let copy constructors cause side-effects, lest the compiler doesn’t bother calling them?

See copy elision and run the example in Compiler Explorer.