Brown-headed Cowbird 



Molothrus ater (Boddaert) 

 Discussion 



There are several reasons at the present time for presenting a 

 new and comprehensive host catalog for this species. The amount 

 of information brought together here is much gi'eater than what 

 was available at the time of my earlier (1929) account: thousands 

 of additional instances of cowbird parasitism have been noted and 

 correlated with the thousands of cases previously reported. More- 

 over, the additional information pertinent to, or even tangential to, 

 the problem, as well as the actual discrete bits of new data on many 

 of the previously included species, permits much more satisfactory 

 analysis and mterpretation than formerly were possible. 



The rise in the number of known hosts is due to several factors. 

 For one, the brown-headed cowbird has increased its range and its num- 

 bers in some areas, such as parts of the Gulf and southern Atlantic 

 states, California, and parts of Canada. In Alabama, for example, 

 Imhof (in htt., 1960) informed me that it now breeds throughout the 

 state, whereas only 10 years earlier its range was restricted largely 

 to the coastal belt. Monroe (1957) produced e\idence that the 

 bird was breeding in Florida. Webb and Witherbee (1960) have 

 collected and summarized data establishing the extension of its 

 breeding range across the western half of Georgia, as well as into 

 northwestern and northern Florida. Grinnell and Miller (1944, p. 

 437) noted that the bird had "increased phenomenally in southwest- 

 ern California since about 1915, in the San Francisco Bay region 

 since about 1922, and in the Sacramento Valley since 1927, if not 

 earlier . . . ." In Ontario, Snyder (1957, p. 35) concluded that 

 "unquestionably the species has moved northward 200 miles or more 

 during the present century." Such geographic expansions have 

 brought the parasite into contact with additional potential victims. 

 Furthermore, since the number of observers has increased greatly 

 in the older ranges of the bird, many additional data have been placed 

 on record — tliis in spite of the decline of egg collecting, which was 

 once the primary source of information. Still another factor in the 

 increase of the host hst is the great change in our understanding of 

 the specific and subspecific status of many of the birds involved. 



Although I have brought out a number of supplements since my 



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