480 PROCEEDINGS OF THE ACADEMY OF 



analogy to the Owls is seen not only in the respects pointed out, 

 but also in the enlarged ear-aperture, of oval form and verti- 

 cal position (see Fig. 7), and the dense hair-like antrorse bristles 

 which cover and nearly conceal the cere. 



We have elsewhere demonstrated that this genus belongs to 

 the subfamily Falconinsef by reason of osteological affinites, not- 

 withstanding its exceedingly different appearance externally. 



From the examination of nearly one hundred and fifty specimens 

 of the genus, we have been able to make the following deductions 

 regarding the nature of the changes to which the plumage of these 

 hawks is subject: (1) There is no appreciable sexual difference, 

 either in the young or adult stages ; (2) there are two well-marked 

 growth stages, causing the young birds to differ very essentially 

 in plumage from the adult ; and (3) certain species are more or 

 less subject to erythrism, individuals being deeply rufescent and 

 others clear plumbeous, these phases being entirely independent of 

 age, sex, or season, and connected by specimens variously inter- 

 mediate between the two extremes. This dimorphism of plumage 

 is in every respect analogous to that so well known to charac- 

 terize certain owls, as the Syrnium ahtco of Europe, the common 

 Scops asio of the United States, and several species of Glaucidium, 

 in Tropical America. In the case of the Glaucidium ferrugineu m , 

 the rufous phase is far commoner in the Amazonian region than in 

 Middle America, it being the rule throughout the former district 

 and the exception in the latter ; in the same way, Micrastur rufi- 

 collis, of Brazil, etc., is rufous in perhaps seventy-five per cent, of 

 the individuals composing that species, and the development of 

 the erythrism is extreme ; but in M. guerilla, the northern form, 

 it occurs in perhaps less than fifty per cent., and then consists of 

 only an appreciable tendency toward this condition, seen in a rusty 

 wash overlying the plumbeous of the back, producing thereby a 

 warm-sepia tint. 



All the known species agree in the following 



Common Characters. Tail dusky tipped and irregularly barred 

 with white. (A 2 ) Adult : Above plain blackish or plumbeous, 



1 Outlines of the Natural Arrangement of the Falconidae. [Read before 

 the Philosophical Society of Washington, April, 1873.] <Bull. U. S. Geo- 

 logical and Geographical Survey of the Territories, No. 4, second series. 

 Washington: Government Printing Office, June 10, 1875. 



1 These letters and figures in parentheses refer to those of the following 

 table. 



