NO. 2 BIRDS OF ISLA ESCUDO DE VERAGUAS — WETMORE IJ 



the flanks and under tail coverts are lighter brown. In all the changes 

 that have been described these wrens have remained uniformly chestnut 

 above, with black crowns. 



One possible explanation of this interesting gradient might be that 

 the plain, chestnut-breasted forms had become established fairly early 

 in the Central American area where they have continued with modifi- 

 cation toward the elimination of barring. In the South American area, 

 on the other hand, the barring became intensified. Through a subse- 

 quent spread of range in the latter population, the two groups have 

 been brought in contact, with resultant hybridization that has caused 

 the mixing that has been described. 



The races recognized as reditus and castaneus represent two stages 

 in this process. It would appear that the schottii group has been the 

 one in active expansion because of the extensive range that it now 

 occupies. It is interesting that the chestnut-breasted group is not 

 found farther north in Central America, though there would appear 

 to be no ecological barrier to prevent this. 



Hellmayr (1934, p. 180) includes another group, Thryothorus 

 scmibadius Salvin, found in tropical lowlands of the Pacific slope 

 from southwestern Costa Rica to western Chiriqui, also as a race of 

 nigricapillus, but this does not seem justified. The bird in question is 

 more finely barred, with 3 narrow dark bars on the individual feathers 

 of the breast, and the crown chestnut, concolor with the back ; also 

 it is smaller. In the schottii-nigricapillits group, which scmibadius 

 resembles superficially, the black bars are heavier, there are 2 bars 

 on the individual feathers of the breast, the crown and upper hindneck 

 are deep black, and the size is larger. There is no indication whatever 

 of hybridization between semibadius and the adjacent Thryothorus n. 

 costaricensis. While juveniles of the costaricensis-nigricapillus group 

 show spots or a slight wash of brown on the pileum and hindneck, the 

 crown cap remains plainly defined. Thryothorus semibadius would 

 appear to be an older offshoot of the ancestral stock that has produced 

 the forms with barred breast, and from its limited range one that may 

 be on its way to extinction. 



The following summary, based on extensive series throughout the 

 entire range of these birds, outlines findings as to their relationships 

 and distribution. It should be noted that museum series of skins almost 

 invariably include immature individuals that are not fully grown, 

 especially in the development of the wings. These are easily detected 

 and have been omitted in the measurements that are given under the 

 different forms. 



