110 U.S. NATIONAL MUSEUM BULLETIN 233 



headed cowbird upon it is such as to endanger its continued existence. 

 No other species is so heavily parasitized with such disastrous results 

 over its entire population. Unless something happens, or is done, to 

 reduce, if not to eliminate, the cowbird menace, the future of Kirt- 

 land's warbler is perilously insecure. 



Only a few years ago, however. Van Tyne (in Bent, 1953, p. 426) 

 wrote that, while the cowbird was a most important enemy of Kirt- 

 land's warbler, there was no reason to think that the parasite might 

 bring about its extinction, as Leopold had feared. Van Tyne thought 

 that it was more probable that the observed changes in the population 

 size of the warbler were the result of changes in the amount of suitable 

 habitat for it in its breeding grounds in Michigan or in its wintering 

 area in the Bahamas. Nevertheless, the alarming interpretation of 

 cowbird parasitism presented by Mayfield certainly suggests that the 

 losses caused thereby have brought Kirtland's warbler to a situation 

 perilous to its continuity. Here is a case wherein the parasite, as a 

 new enemy in the environment of the warbler, has "gotten out of 

 hand," and should be controlled, if not ehminated locally. 



Van Tyne has added some further observations on the relations 

 between this host and the parasite. He saw a female cowbird spend 

 hours apparently watching a female Kirtland building its nest. When 

 the warbler was not actually working on the nest, the structure was 

 left unguarded, and, "as soon as the main structure was finished — 

 even before the Hning was added — a cowbu'd (presumably the one 

 that had been watching the nest-building) came early in the morning 

 and laid in it. After watching many hours at recently completed, or 

 nearly completed, Eartland nests, I would judge that cowbirds laying 

 in a Ku'tland's nest during this early part of the cycle, which is the 

 period most favorable for the cowbirds' chances of producing young, 

 run very httle risk of detection and attack. But after the warbler has 

 begun incubation, the nest is rarely left unguarded, and the female 

 warbler will attack violently and diive away any cowbird she finds 

 in the vicinity." 



Although the cowbird is a major calamity to Kirtland's warbler, 

 the latter, in turn, must certainly be rated as a good or successful 

 host from the standpoint of the parasite. Based on his large amount 

 of data, Mayfield (p. 179) estimated that about 41 percent of all 

 cowbird eggs laid in nests of this host would survive to produce fledg- 

 lings. This is a very high rate, which in itself adds yet another 

 element to the danger facing Kirtland's warbler. While it can be 

 argued that the parasite's success may be of temporary duration since 

 it may decimate its host to the point of eliminating its own supply 

 of victimizable nests, such a result would involve the permanent 



