12 PROCEEDINGS OF THE NATIONAL MUSEUM vol. ism 



instances the variable characters are in agreement with obvious 

 environmental conditions on the breeding grounds and in accordance 

 with the classical Gloger's, Bergmann's, and Allen's rules. 



The numerous exceptions probably are accounted for by adaptation 

 to critical environmental conditions encountered by these populations 

 while in migration or on their wintering grounds. Such exceptions 

 have been documented frequently for intensively studied species 

 such as the juncos (Miller, 1941b), vireos (Hamilton, 1958), and 

 white-crowned sparrows (Banks, 1964). 



Characteristics and Nomenclature of Populations 



The generally accepted name for the hermit thrush has been 

 Hylocichla guttata (Pallas) (type-locality: Kodiak Island, Alaska) 

 since the American Ornithologists' Union (1902) decided that the 

 previously accepted name, Hylocichla aonalaschkae (Gmelin), was 

 based on an unidentifiable description. 



Inclusion of the hermit thrush in the genus Catharus is based 

 on evidence presented by Ripley (1952) and Dilger (1956). There 

 are rather marked morphological differences between most of the 

 Neotropical members of the group originally separated in the genus 

 Catharus and the North American species that previously have been 

 included in the genus Hylocichla. These differences include relatively 

 long leg bones, short rounded wing with relatively long tenth primary, 

 small sternum, brightly colored e}^e ring and bill in the tropical 

 group. These differences, however, are not as pronounced in some of 

 the more northern members of the tropical group. As Dilger (1956) 

 has pointed out, all members of the genus Hylocichla, except H. 

 mustelina, are more like Catharus, while the wood thrush is more like 

 Turdus. The relationships of H. mustelina to Turdus noted by Dilger 

 were in aggressive display, nest construction, and serological re- 

 semblance. In addition, I have noted that certain skull characters, 

 particularly a deep groove above the auditory bulla for the attach- 

 ment of the aductor mandibulae muscle, are present in typical mem- 

 bers of genus Turdus and to a lesser extent in H. mustelina but which 

 are quite indistinct in other species of Hylocichla and Catharus. 

 Everything considered, there appear to be sufficient resemblances 

 and overlap of characters to warrant placing all species that have 

 been grouped in the genus Hylocichla, except H. mustelina, in the 

 genus Catharus, and equally good reasons for not including H. mus- 

 telina in this genus. Since the wood thrush is the type-species of 

 Hylocichla, this probably should be maintained as a monotypic genus, 

 as Dilger (1956) has suggested, until such time as its relationships 

 are better understood. 



