"The Aquatic Warbler is the rarest songbird in mainland Europe. Its numbers declined by 95 percent during the 20th century. But this is not for lack of effort on the warbler's part. Over the last fifteen years, researchers investigating the sex life of this small, retiring brown bird have uncovered a pattern of promiscuous behaviour, with male birds "continuously ready to mate and testing every female for her willingness to copulate". Almost two-thirds of all broods of young Aquatic Warblers have more than one father."
Link: Full story from BirdLife International