HOST RELATIONS OF PARASITIC COWBIRDS 31 



than as an adaptation to make the parasites more readily acceptable 

 to theu' intended victims. Selander and La Rue themselves admit 

 that they had no observations of cowbu'ds actually using this preen- 

 ing mvitational display in an attempt to appease hostile hosts near 

 their nests. They do cite Chapman's report (1928) of seemg a 

 female giant cowbird, Psomocolax oryzivorus, tmce bow her head 

 and present the fluffed feathers of the nape to a female Wagler'soropen- 

 dola, Zarhynchus wagleri, at the latter's nest, but even here there was 

 no evidence of immediate intended parasitism. 



One fiu-ther thought should be expressed. Selander and La Rue 

 write that the invitation to preen is "all the more unusual in that 

 cowbirds only infrequently dh'ect the invitational display to members 

 of then* own species and do not themselves indulge in social hetero- 

 preening." This suggests that we have here something akin to what, 

 in human beings, might be looked upon as a "childhood memory," 

 even though in each case the species invited to preen might be other 

 than the actual fosterer of the inviting bird. The lack of identity 

 of the invited preener with the origmal fostering species thus might 

 be looked upon as an mdication of a general absence of specific host 

 imprinting on the parasite while a nestling. This would be not at all 

 inconsistent with the general picture we have of cowbird-host relations. 



Foster Parent -Offspring Relationship 



In the discussion of the cowbird's invitational display for preenmg 

 by other birds, it was suggested that such behavior might reflect, 

 although faintly, some earlier experience with the alien species or, 

 in broader terms, with "birds other than cowbirds." This suggestion 

 naturally leads to the speculation as to whether or not nestling cow- 

 bu'ds become imprinted on their specific, individual fosterers; and 

 this speculation, in turn, makes it advisable to review the entire ques- 

 tion of imprinting, with special reference to parasitic birds. 



So far as the behavior of the foster parent is affected, the hatchmg 

 of a cowbird egg in its nest is no different from the hatching of one 

 of its own, and from then on, until the j^oung parasite is ready to fly 

 and become independent, the adult host reacts to the chick just as it 

 would to its own young. The fosterer acts, and seems, completely 

 unaware of the substitution, even in cases wherein the young parasite 

 grows to a size beyond that of the host's own typical offspring. This 

 apparent unawareness or indifference is probably one of the conditions 

 which permits brood parasitism to succeed. 



On the other hand, the young cowbird differs from the 3'oung of 

 its victims and of many other birds in that it reveals little or no evi- 

 dence of becoming imprinted on its foster parents as a particular 

 species. On the contrary, once it has fledged and has left the care 

 of its foster parents, the young cowbird joins others of its own kind 



