Like a loon's, a tern's feet have this feature, yet terns are not notable swimmers
The UM Exhibits museum 
has it evolved from thecodont reptiles, the .... No keel on sternum, large cnemial 
crest on tarsometatarsus for swimming muscles like loon & grebe but different 
origin,... [DOC] [TLE] Swimming [PAR] With seemingly no effort, they rocket around using their wings to "fly" through the 
ocean.