470 PROCEEDINGS OF THE ACADEMY OF 



STUDIES OF THE AMERICAN FALCONIDiE MONOGRAPH OF THE 



GENUS MICRASTUR. 



BY ROBERT RIDGWAY. 



Introduction. 



No group of American hawks has presented more numerous 

 (lilliculties in the way of elucidating and strongly characterizing 

 its species; and so complex are the obstacles presented in this 

 case, that the conclusions of authors, who have made the genus 

 Micrastur a subject of special study, are very conflicting. The 

 brevity and otherwise unsatisfactory nature of the original de- 

 scriptions of the older species is the main difficulty, but the per- 

 plexing stages of plumage, which in this genus are very remark- 

 able, and so far as we know entirely peculiar among the Falconidse, 

 add still more to the confusion into which their synonymy has 

 fallen. In the latter we have reference to the fact, perhaps not 

 yet generally known, that two distinct phases of color, a gray and 

 a rufescent, occur in some of the species at all stages of growth, 

 which circumstance has but very recently been taken into con- 

 sideration in defining the specific characters. 



The undertaking of this monograph would have been impossible 

 to the author but for the kind assistance of Mr. Osbert Salvin, 

 the eminent English ornithologist, who transmitted for examina- 

 tion his entire series of specimens, representing every known 

 species of the genus. 

 National Museum, Washington, D. C, July 30, 1875. 



Literature. 



The first species of this genus was described by Vieillot in 

 1817, and since that time numerous other names have been pro- 

 posed, which are as follows, in chronological order: 1 



H) Sparvtus ruficolln, Vieillot, Nouv. Diet. Hist. Nat. X, 1817, p. 822. 

 (= M. ruficollis, adult, rufous phase.) 



(2) 8parviu8 gifoicollis, Vieill., 1. c. p. 323. (=M. ruficollis, gray phase, 



adult?) 



(3) iSjiiirn'/is melanoleueus, Vieill., 1. c. p. 327. (=M. melanoleucus, ad.) 



1 The names following in parentheses are those adopted in this work. 



