34 U.S. NATIONAL MUSEUM BULLETIN 233 



shiny cowbird and its various hosts. The reactions of the hosts to 

 the cowbu'ds is, then, not usually to the adult parasites but to the 

 actual interference in the hosts' nests. 



On several occasions Nice (1943, p. 158) noted song sparrows using 

 a threat-posture against cowbu'ds and even striking them. A "threat 

 note" was given against cowbu'ds, but not against them exclusively, 

 as it was given also when the intruders were juncos, cardinals, and 

 once even a young rabbit. Nice (p. 262) found that, if a hen cowbird 

 came close to a nest, the pair of sparrows frequently attacked her. 

 The antagonistic reactions of the sparrows to the cowbird seemed to 

 depend on some conditioning in their past experience, either in early 

 life or later; recognition of the cowbird was something learned, not 

 innate, in the sparrows. Thorpe (1956, p. 121) considered that 

 recognition of the parasite as an enemy might be based on an instinc- 

 tive mechanism in some host species, but, in others, was probably 

 somethmg handed down from generation to generation, "not so much 

 by the experience of the dire results of attack but by the alarm dis- 

 played by the parent birds or by members of other species when they 

 see one of these enemies." 



Birds react in several ways to the intrusion of cowbird eggs into 

 their nests: they may accept them as if unaware of the eggs being 

 different from their own (whether they are aware or not, we usually 

 do not Icnow); they may desert the nest and build a new one; they 

 may build new nest linings over the strange eggs, thus effectively 

 flooring over or burying them; or they may throw out the parasitic 

 eggs. The same species may show more than one of these four 

 reactions. In many species the reaction depends on whether or 

 not the bh'ds have already laid some eggs of their own prior to the 

 moment of parasitism and also whether or not the cowbird removed 

 one or more eggs or damaged the nest. 



Many of the records, especially the older ones, have little or no 

 supporting data; they are merely statements of sets of eggs collected, 

 with places and dates. The following summaries of the three non- 

 accepting reactions of the victims are based only on such cases as 

 were reported with the pertinent observational data. 



Nest desertion. — The reaction observers call actual desertion 

 of the nest is frequently an inferred rather than an ascertained fact. 

 In the following summary I have included only those species wherein 

 the evidence points to a cause-and-effect relationship between para- 

 sitism and desertion. While accurate and detailed observations 

 are still needed for proof, apparently nest desertion is more apt to 

 occur when the parasite lays before the host has laid its first egg 

 or at least when the host is still early in its own laying schedule. 



The birds listed below have been observed at least once to desert 



