Engzell, J. “In the Grashopper Warbler Locustella Naevia, Males With Better Nutritional Condition at the Winter Ground Arrive Earlier Than Other Males at the Breeding Ground”. Ornis Svecica, vol. 27, no. 2–4, Oct. 2017, pp. 139–143, doi:10.34080/os.v27.19567.