Part IL] NATURAL ENEMIES OF BIRDS. 217 



necessity and competition, it becomes plain that the creatures 

 which prey on such species and so keep their numbers within 

 normal bounds are essential to the welfare of all. 



NATURAL ENEMIES SUPPLEMENT AND CHECK ONE 



ANOTHER. 



When we examine broadly the relations of birds and their 

 enemies we find that some species appear far more destructive 

 to bird life than others. Certain swift hawks, for example, 

 seem to feed almost wholly on birds whenever they can obtain 

 them, while other slower hawks rarely take any except help- 

 less, sick or disabled birds, but feed largely on small mammals, 

 such as squirrels and field mice. Thus one species is seen to 

 take an excess of birds while the other takes an excess of 

 their enemies, the effect of the activities of one tending to 

 balance those of the other. Again, a certain animal may be a 

 destroyer of certain birds and a protector of others. The 

 skunk, for example, is known occasionally to destroy the eggs 

 and young of grouse and other birds which nest on the ground, 

 as well as those of domestic fowls. On the other hand, the 

 skunk is the appointed guardian and protector of young water 

 birds, which are unsafe except in shallow waters, where there 

 are no great fish to eat them. Therefore they frequent such 

 shallow waters, and there their greatest enemy is the snap- 

 ping turtle. Wherever these great turtles are numerous practi- 

 cally no young water birds can be raised, as these cold-blooded 

 monsters hide in the mud of the bottom or swim under water, 

 and pull down the young birds by their feet. 



During four summers I watched many of these turtles 

 depositing their eggs and burying them in the earth near the 

 shores of a river, and in every case within twenty-four hours 

 the skunk unearthed and ate every egg laid. Where skunks 

 are numerous it seems impossible for any snapping turtles' 

 eggs to hatch, but where skunks have been extirpated turtles 

 increase rapidly in numbers and in time prevent the multipli- 

 cation of wild ducks or geese. 



Although the turtle is an enemy of wild fowl, it may un- 

 wittingly befriend the ground-nesting birds locally by provid- 

 ing the skunk with a tempting supply of turtle eggs at a time 



