heard of at least a dozen more, all of the same 

 species, that were seen in fresh ponds or 

 rivers, dipping their heads under water re- 

 peatedly. In either case two interesting ques- 

 tions suggest themselves: first, How did a 

 bird whose whole life from birth to death is 

 spent on the sea first learn that certain mus- 

 sels will drown in fresh water? and, second, 

 How do the other birds know it now, when 

 the need arises unexpectedly ? 



