LONDON (Reuters) - Europe's rarest songbird is facing extinction, despite being the most promiscuous and energetic lover in the avian world, and concerned scientists are looking urgently for ways to save it. The male aquatic warbler is described as "continuously ready to mate" and able to indulge in record-breaking mating sessions, which in turn gives the females ample opportunity to sample and select the best mates. However, numbers have slumped to less than 20,000 in the past century -- a decline of 95 percent -- and its range has shrunk from continent-wide to isolated strongholds in eastern Europe as humans...