THE AQUATIC VERTEBRATES 1025 



well as robbers. The kingfisher is a poor swimmer, but the terns, 

 gulls, and pelicans rest gracefully on the surface. In the latitude 

 of the Ohio the kingfisher is found between early March and No- 

 vember, rarely even in December. The terns migrate to the 

 northern lakes in summer and the pelicans are but stray visitors. 

 The terns, gulls, and pelicans have certainly acquired the adapta- 

 tion to the water at the ocean. 



The third ecological group is formed by the grebes and loons. 

 They are pelagic birds, swimmers par excellence, both upon the 

 surface and in the water. The term diving ought not to be ap- 

 plied to the performance of both kingfisher and loon. 



The fourth ecological group is formed by the bottom-feeding 

 ducks, the mudhen, geese, and swans. They are littoral or abys- 

 mal forms securing their food in the mud at the bottom, largely 

 about the margins of ponds or lakes in water not too deep to pre- 

 vent them from reaching the bottom when "tipping." Many 

 of the ducks are good swimmers under water, and the bay and 

 sea ducks are said to reach the bottom at a depth of 100 to 150 

 feet. 



The fifth ecological group is formed by the herons, cranes, and 

 bitterns. These range in much the same zone as most of the ducks, 

 but their food, for the most part, is different. They stalk cau- 

 tiously, without jerk or sudden motion, or stand in water of a 

 depth not too great for their long legs. Their spearlike bill im- 

 pales fish or frog. 



The sixth and last of the ecological groups of aquatic forms con- 

 tains the rails and snipes. These are shore birds, wading in the 

 shallowest water or along the wet shores, frequently moving with 

 the advancing and retreating waves, picking the stranded animals 

 from the surface or probing for their prey in the soft beaches. 



All of the groups except the first, the swallows, nest as near the 

 water as possible. Less strictly aquatic are the swamp black- 

 bird and long- tailed marsh wren which build their nests in cat tails; 

 likewise the song and marsh sparrows, so abundant along margins 

 of stream or pond. From the nature of the case the waters of 

 northern and temperate zones are a closed book to all the birds in 

 winter. Hence, birds are not perennial elements of the aquatic 



