NO. 4 AVIAN GENUS CLAMATOR — FRIEDMANN 75 



may possibly be some unknown and, as yet unsuspected ecological 

 factor in the area of polymorphism of each of these species that has 

 made possible a local decrease in selective pressure and thereby 

 enabled two morphs of each to develop in a state of passive neutrality. 

 This may be something akin to what Stresemann vaguely suggested, 

 although transposing the factor from the "allele producing mecha- 

 nism" to the environment. There is no evidence even tangential to 

 this concept that may be cited, and the thought is merely inserted for 

 its suggestive value. Considering that both species are highly migra- 

 tory, at least in their southern sections (exactly the section where 

 jacobinus is polymorphic) , it is difficult to account for this apparent ge- 

 netic isolation in the light of present knowledge. It seems that in coastal 

 Kenya, Percival's statement (cit. supra) notwithstanding, levaillantii, 

 with its local black phase, is relatively nonmigratory, but no such as- 

 sumption can be maintained for either in southeastern Africa. The 

 absence of migrant, or of "wintering" examples of black-phase C. 

 jacobinus serratus from equatorial Africa during the southern winter, 

 when it is known that both phases are absent from their relatively 

 well-observed austral breeding range and have presumably gone north, 

 is a real puzzle. This is discussed more fully in our account of 

 migratory behavior (see pp. 84-85). In the present connection it may 

 be hypothecated that, wherever they may "winter," all the individuals 

 of southeastern serratus return to their home area for breeding, and 

 thus remain unmixed with their adjacent conspecific populations. 



Aside from the well-developed melanistic morph in the adults of 

 jacobinus and levaillantii, the latter species also has a rufescent 

 Juvenal morph, reminiscent of the hepatic phase of the young in 

 Cuculus canorus. I know of only two examples of this rufescent 

 phase, both from the northeastern portion of the Repubhc of the 

 Congo (former Belgian Congo). One such bird, a young male, taken 

 at Poko, in the Uelle district on July 31, now in the British Museum, 

 is bright cinnamon rufous above and below, only the remiges and 

 retrices being darker, less reddish, as in the "normal" juvenal. The 

 other one came from near Beni, in the Ituri district. Another, possibly 

 partial, rufescent bird may be one mentioned by Granvik (1934, p. 

 24) as having the undertail coverts pale rufous, although the rest 

 of the plumage was probably "normal" as it evoked no comment from 

 the describer, 



MIGRATORY BEHAVIOR 



The evolutionary picture of migratory behavior in the genus pre- 

 sents some peculiar features. All four of the included species are 



