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Gustavsson, C.G. 2011. A white feathered area on the forehead is more common in hybrids of Greylag Goose Anser anser and Greater Canada Goose Branta canadensis than in the parent species. Ornis Svecica. 21, 1 (Jan. 2011), 37–44. DOI:https://doi.org/10.34080/os.v21.22617.