36 U-S. NATIONAL MUSEUM BULLETIN 223 



pattern. In these respects cuckoos are vastly greater specialists than 

 cowbirds, but they probably have been practicing parasitism for a 

 much longer time." To this statement I add that the weavers are as 

 new if not newer than the cowbirds as brood parasites. 



That it is not possible to point with any certainty to discrete, 

 mimediate causes need not, in itself, be disturbing. Such informa- 

 tion would indeed be most welcome, but the improbability of obtaining 

 it is also significant. Brood parasitism is not necessarily an advanta- 

 geous mode of reproduction toward which, as an ultim.ate goal, we 

 might expect to find traces of antecedent trends in varying stages 

 of development. 



To make this point clear, I must digi'ess briefly. Wlien v/e examine 

 the life history of a parasitic bu'd, we find that it seems to flourish at 

 the expense of its hosts. In other words, it appears to have a rela- 

 tively "easier" or more advantageous way of life. These advantages 

 we as humans read into nature and are not actually present in it. 

 No evidence supports the view that self breeding makes life more 

 difficult for other birds and that any tendency to "get away" from 

 these portions of their behavior would necessarily become an evolu- 

 tionary asset. Brood parasitism is merely another way of breeding. 

 It has been established in a number of groups of birds but this fact 

 does not imply that it is a better or more efficient way. 



In their recent book Rothschild and Clay (1957, p. 38) discussed 

 the insecurity attendant on a parasitic existence, aud pointed out 

 that egg or larv^al mortality is vastly higher in most parasites than in 

 their hosts. While this fact is true especially of arthropod and 

 nemathelminth parasites, these authors extended their generalization 

 to include even avian brood parasites like the European cuckoo, which 

 they claimed lays four or five times as many eggs as do its hosts. 

 This generalization is only partially applicable to avian brood para- 

 sites, e.g., the great egg mortality in one of the South American 

 parasitic cowbirds, Molothrus honarlensis}'^ 



In parasitic birds generally, such "advantages" as have been read 

 into their lives by their describers are not, in themselves, biological 

 goals toward which we may expect directive evolution, more success- 

 fully attained in one group or species and less completely in another. 

 These "advantages" are merely the particular features of a habit that 

 have given it sm-vival value. Their status as "advantageous" ele- 

 ments is something determined by the operation of natural selection 

 after the advent, and not something toward which the particular 

 species involved was "striving." Thus, if "striving" were present, 

 it would be expected to leave observable traces, and we might well be 



<» Tanagra bonaricnsia Gmelln, Caroli a Llnni . . . systema naturae, ed. 13, vol. 2, 17S9, p. 898 (Buenos 

 Aires). 



