Acoustic monitoring of nocturnal migration in Europe

Presenter/Co-author: Magnus Robb / Arnoud B. van den Berg, and Mark Constantine (The Sound Approach, United Kingdom)

Since ancient times, Europeans have heard migrant birds calling overhead at night. However, it is only very recently that acoustic monitoring of nocturnal migration has become popular in Europe, despite the example shown by Evans, Graber, O’Brien and others in North America. For nearly a decade, The Sound Approach has been making all-night recordings in Europe, searching for NFCs by scrolling through sonagrams afterwards. Our primary motivation has been to record poorly known nocturnal flight calls (NFCs), and we have collected examples from c160 species. We identify calls both aurally and visually, seeking matches with calls of known provenance. Although most NFCs are identical to diurnal flight calls, eg in shorebirds, wagtails and buntings, other species use their primary flight calls only at night. For example, grebes and rails rarely fly any distance during the day, and chats and Old World flycatchers do not appear to use flight calls during the day, but all these birds have NFCs. Most old world warblers appear to migrate without calling at all. Their silence means that a much smaller proportion of European passerines can be detected at night compared to North America. While at least one European bird observatory has been recording nocturnal migration for decades, others are now taking their first steps. As American pioneers showed long ago, acoustic monitoring will produce many  new insights into migration activity overhead. Although some species will not be detected, others are unlikely to be detected by any other means.