1. F. A. Johnson (https://orcid.org/0000-0002-5854-3695) ✉ (fred.johnson@bios.au.dk), Aarhus Univ., Dept of Bioscience – Kalø, Rønde, Denmark.
2. K. Koffijberg (https://orcid.org/0000-0003-0146-2269), Savon, Dutch Centre for Field Ornithology, Nijmegen, the Netherlands.