8 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 124 



the eastern parts of its range is a darkening of the pigmentation of 

 the upper parts in the northeast. 



In the south, transition from the very small birds of the California 

 coast to the large type of the Great Basin and southern Rockies is 

 quite abrupt through intermediate populations of the Sierra Nevada 

 and Cascades. Color change is a relatively slight darkening above 

 and an increase in both number and darkness of ventral spots pro- 

 gressing from west to east in this area. 



Morphological Variation and Environment 



References to apparent correlation of color intensity of birds with 

 environmental moisture and of size with temperature are too nu- 

 merous to mention although experimental evidence of the exact 

 environmental factors responsible for the variations is rare. 



Bowers (1960), in a significant pioneering study, found a relation- 

 ship between environmental moisture and background color of 

 wrentit (Chamaea fasciata) habitat that, in turn, was correlated with 

 the color of the birds' plumage. The fact that wrentit plumage color 

 and habitat background color varied together, in a gradient passing 

 from moist to drier environments away from the coast in California, 

 indicated a definite relationship. Natural selection for protective 

 coloration by predation, as shown experimentally for deer mice by 

 Dice (1947) and for moths by Kettlewell (1955, 1956), seems to be 

 the likely explanation of this phenomenon. It is probable that the 

 evidently indirect effect of environmental moisture on color is brought 

 about by natural selection through predation in many species of 

 animals, including the hermit thrush to some degree. It is assumed 

 that cryptic coloration, if it exists in hermit thrushes, should be 

 judged on the basis of general resemblance to the forest floor or 

 very low vegetation since this species nests and spends most of its 

 time on or near the ground (Bent, 1949). Thus, general tone of leaf 

 and twig litter on the forest floor would be significant. Moisture 

 would affect the shade of the litter and humus, and density of the 

 understory would control the amount and quality of light reaching 

 the ground; therefore, as Bowers (1960) suggests, the general back- 

 ground tone in vegetated environments is darker in moist climates 

 and paler in arid regions. Other important observations on the cor- 

 relation of bird plumage coloration with habitat backgrounds include: 

 Moreau (1930), Behle (1942), Davis (1951), Meinertzhagen (1950), 

 and Selander (1954). 



McCabe and McCabe (1932) and Munro and Cowan (1947) have 

 commented on the general correlation of variation in hermit thrushes 

 with environment in the Northwest. Certainly both the morphology 



