Part II.] NATURAL ENEMIES OF BIRDS. 251 



Even the water snake (Natrix sipedon) is said at times to 

 rob the nests of marsh wrens and to eat the eggs and young 

 of rails, but it is not believed to feed commonly on birds. 

 Probably all field ornithologists of large experience have wit- 

 nessed the robbing of birds' nests by snakes. The parent 

 birds occasionally are killed in defense of their homes, and it 

 is probable that now and then an adult bird, while feeding, is 

 surprised and caught by a snake, although I have never known 

 this to happen. Snakes probably do not require a great quan- 

 tity of food. Individuals have been known to live without 

 food for more than a year, and investigators, capturing snakes, 

 find a large proportion of stomachs empty. The digestion of 

 snakes is slow, and probably they do not consume nearly as 

 much food in proportion to their size as would a bird or even 

 a mammal. Hence they probably are not individually as de- 

 structive to birds as are warm-blooded rapacious creatures, but 

 where the larger snakes become too numerous they may exert 

 a serious depressive influence on the numbers of birds. 



Frogs. 



There are many tales of frogs swallowing ducklings but no 

 such case has come under my observation. Mr. Robert B. 

 Lawrence reported that a frog in his brother's duck pond was 

 killed after it had devoured a young pin-tail duck, and that, as 

 many young wood ducks had disappeared, it was believed that 

 frogs had eaten them.^ This observation is corroborated by 

 others. Only very large frogs are able to catch and swallow 

 birds and such frogs should not be allowed in ponds with young 

 ducklings. 



Fish. 



Trout, salmon and other large fish capture young birds which 

 fall or alight upon the water. Pike and pickerel are so destruc- 

 tive to the young waterfowl, it is said, that young ducks can- 

 not live in water where they abound. 



Insects. 



The insect world is potentially the greatest of all dangers 

 to bird life. Wherever birds and other natural enemies of 



» Huntington, Dwight W.: Our Wild Fowl and Waders, 1910, p. 88. 



