COMPARING BILLS. 107 



on both sides, so that a picture of one side of them is precisely 

 like a picture of the other ; but this, the only bird in the world 

 that is colored unsymmetrically, is also the only bird that 

 moves always in the same direction while feeding, thus keeping 

 the same side exposed to its enemies. This is a wonderful 

 instance of what is called protective coloration, about which we 

 shall soon study. 



