Vocal Signalling by Eurasian Spoonbills Platalea leucorodia in Flocks before Migratory Departure

We describe a vocal signal and associated postures shown by Eurasian Spoonbills Platalea leucorodia leucorodia just before the departure of (part of) the flock from a site, but also occurring in flight. Onomatopoetically called ‘ook’, it is a soft muffled sound that was noticed in Spoonbills at a stopover site during southward migration in southwestern France and at a post-breeding feeding site in The Netherlands. When making the ‘ook’-sounds, adults as well as juveniles contributed to what looked like dialogues, the sound of the juvenile having a higher pitch than that of the adult. On the basis of their specific occurrence in pre-departure contexts, we propose that the vocalizations function during collective decision-making, perhaps contributing to reach consensus before and during non-routine group movements from one place to another.