Like a loon's, a tern's feet have this feature, yet terns are not notable swimmers
... Loon-like birds have 
been around for a long time as shown by fossil evidence dating back 40 to ... ful 
swimmers, able to dive to great depths and maneuver exceptionally well, 
propelling... [DOC] [TLE] The Evolutionary History of Birds | U-M LSA Museum of Zoology [PAR] Look at fossil birds, and other fossils that had bird-like features.