Bird, Christopher D.; Emery, Nathan J. (2009). "Rooks Use Stones to Raise the Water Level to Reach a Floating Worm". Current Biology. 19 (16): 1410–1414. doi:10.1016/j.cub.2009.07.033. PMID19664926.
Bird, Christopher D.; Emery, Nathan J. (2009). "Rooks Use Stones to Raise the Water Level to Reach a Floating Worm". Current Biology. 19 (16): 1410–1414. doi:10.1016/j.cub.2009.07.033. PMID19664926.