OVERLAPPING FORMS 151 



of the Rio Grande or the Gulf of Mexico. Between the two 

 lines thus roughly denned is a band of country about 1 ,200-1 ,300 

 miles long and 300-400 miles wide, which contains some normal 

 birds of each type, but chiefly birds exhibiting the characters of 

 both, mixed together in various and irregular ways. Even in 

 the areas occupied by the pure forms occasional birds are re- 

 corded with more or less indication of characteristics of the other 

 form, but within the area in which the two forms are conterminous, 

 the mixed birds are in the majority. The condition of these birds 

 of mixed character is described by Allen as follows: 



"As has been long known — indeed, as shown by Baird in 

 1858 — the 'intermediates' or 'hybrids' present ever-varying 

 combinations of the characters of the two birds, from individuals 

 of C. auratus presenting only the slightest traces of the char- 

 acters of C. cafer, or, conversely — individuals of C. cafer present- 

 ing only the slighest traces of the characters of C. auratus — 

 to birds in which the characters of the two are about equally 

 blended. Thus we may have C. auratus with merely a few red 

 feathers in the black malar stripe, or with the quills merely 

 slightly flushed with orange, or C. cafer with either merely a few 

 black feathers in the red malar stripe, or a few red feathers at 

 the sides of the nape, or an incipient, barely traceable scarlet 

 nuchal crescent. Where the blending of the characters is more 

 strongly marked, the quills may be orange-yellow or orange- red, 

 or of any shade between yellow and red, with the other features 

 of the two birds about equally blended. But such examples are 

 exceptional, an unsymmetrical blending being the rule, the two 

 sides of the same bird being often unlike. The quills of the tail, 

 for example, may be part red and part yellow, the number of 

 yellow or red feathers varying in different individuals, and very 

 often in the opposite sides of the tail in the same bird. The 

 same irregularity occurs also, but apparently less frequently, 

 in the quills of the wings. In such cases the quills may be mostly 

 yellow with a few red or orange quills intermixed, or red with a 

 similar mixture of yellow. A bird may have the general coloura- 

 tion of true cafer combined with a well-developed nuchal crescent, 

 or nearly pure auratus with the red malar stripes of a cafer. 



