386 Relations between Species 



of feeding and caring for it even in the absence of the rightful young. 

 The spectacle of a small sparrow or warbler frantically trying to satisfy 

 the hunger of an adolescent cowbird perhaps four times its size is an 

 amazing sight. 



The "parasitic" activities of other kinds of birds are really in the 

 nature of "hold-ups." The reader may have witnessed the spectac- 

 ular sight of an eagle attacking an osprey high in the air, forcing it 

 to drop a fish from its talons, and then catching the fish before it 

 reaches the ground. Similarly, a skua or a jaeger will chase a gull or 

 a fulmar until the pursued bird drops its fish, or disgorges a meal al- 

 ready swallowed, and then the bully secures the food for itself. 

 Various other kinds of animals are professional highwaymen, includ- 

 ing some that compound the felony by robbing thieves. Such hi- 

 jacking is carried on by certain flies (Bengalia) that waylay ants re- 

 turning from a raid on a termite nest. The fly attacks an ant carry- 

 ing away a termite, causing it to drop its booty, whereupon the fly 

 quickly consumes the stolen goods. 



Parasitism. Strictly speaking, a parasite is an organism that 

 resides on or in the body of a larger living organism and derives 

 nourishment from its tissues. Accordingly, the foregoing instances 

 of exploitation are not regarded as parasitism in the more precise 

 meaning of the term, either because nourishment is not involved or 

 because one organism does not live in contact with the other. Thus 

 barnacles growing on the back of a whale, as well as other kinds of 

 epizoans and epiphytes, are not true parasites since they do not eat 

 the host's tissue. However, the limits of parasitism even in the strict 

 sense are not sharp. Just as gradations exist between parasitism, 

 commensalism, and symbiosis, so also many borderline situations ex- 

 ist between parasitism and predation. Some organisms derive only 

 a part of their nourishment from their hosts and some are in contact 

 with their hosts for only a part of their lives. The typical parasite 

 lives in its host without causing its death, and the typical predator 

 kills the prey upon which it feeds. Yet some parasites regularly kill 

 their hosts, and some organisms classified as predators eat only a part 

 of their prey— sometimes without causing significant harm. Generally 

 speaking in parasitism the weak benefit at the expense of the strong, 

 whereas in predation the relations are reversed and the strong exploit 

 the weak. 



Examples of partial parasitism are found among various kinds of 

 plants and animals. The mistletoes ( Loranthaceae ) grow like little 

 shrubs on the branches of trees. Their specialized roots penetrate 

 the vascular tissues of the host whence they obtain water with the 



