Tracking seasonally changing resources is regarded as a widespread proximate mechanism underpinning animal migration. Migrating herbivores, for example, are hypothesized to track seasonal foliage dynamics over large spatial scales. Previous investigations of this green wave hypothesis involved few species and limited geographical extent, and used conventional correlation that cannot disentangle alternative correlated effects. Here, we introduce stochastic simulations to test this hypothesis using 222 individual spring migration episodes of 14 populations of ten species of geese, swans and dabbling ducks throughout Europe, East Asia, and North America. We find that the green wave cannot be considered a ubiquitous driver of herbivorous waterfowl spring migration, as it explains observed migration patterns of only a few grazing populations in specific regions. We suggest that ecological barriers and particularly human disturbance likely constrain the capacity of herbivorous waterfowl to track the green wave in some regions, highlighting key challenges in conserving migratory birds.
Bibliographical noteFunding Information:
We thank Lei Fang, Qin Zhu, Hui Yu, Yuzhan Yang and Zhujun Wang for fieldwork; Xianghuang Li, Hongbin Li and Shujuan Fan for collecting remote-sensing datasets; Stuart Pimm, Devin Johnson and Aaron Olsen for constructive comments. We appreciate assistance of the staff of the Poyang Lake National Nature Reserve, Nanjishan National Nature Reserve, East Dongting Lake National Nature Reserve, Shengjin Lake National Nature Reserve, U.S. Geological Survey Western Ecological Research Center, Wildlife Science and Conservation Center of Mongolia and Korea Institute of Environmental Ecology, during the field study. We thank Alyn Walsh, John Skilling, Ed Burrell, Mitch Weegman, Angus Maciver, Arthur Thirlwell and Maurice Cassidy for help with the catching and tagging of the geese in Scotland and Ireland. We thank K.-M. Exo and K. Oosterbeek for supporting obtaining the tracking data of barnacle geese (three populations). We thank David Cooper, Peter Ertl, José Frade, Stanislav Harvančík, Benjamin Keen, Marcel Langthim, Yann Muzika and Manish Panchal for kindly providing images. Bird capture and logger deployment in this study in the UK was carried out under license from the BTO, and elsewhere in accordance with the guidance and permission (no. rcees-ddll-001) of Research for Eco-Environmental Sciences, Chinese Academy of Sciences. The study was supported by the National Key Research and Development Program of China (grant no. 2016YFC0500406), the National Natural Science Foundation of China (grants no. 31661143027, 31670424, 31500315), China Biodiversity Observation Networks (Sino BON), Scottish Natural Heritage, FlySafe (http://iap.esa.int/flysafe)—a project of the European Space Agency Integrated Applications Promotion (IAP) programme (http://iap.esa.int/), the Israel Science Foundation (grant no. 2525/16), the Minerva Center for Movement Ecology, the Adelina and Massimo Della Pergola Chair of Life Sciences and China Scholarship Council—Hebrew University of Jerusalem Scholarship Program.
© 2019, The Author(s).