1. Department of Foreign Languages, College of Education, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
2. Department of Behavioural Neurobiology, Max Planck Institute for Ornithology, Eberhard-Gwinner Strasse, Seewiesen 82319, Germany
3. Max Planck Institute for Ornithology, Humboldt Research Group: Evolution of Communication, Eberhard-Gwinner Strasse, Seewiesen 82319, Germany
4. Institute of Cognitive Science, Comparative BioCognition, University of Osnabrück, Artilleriestrasse 34, Osnabrück 49076, Germany