384 PROCEEDINGS OF THE NATIONAL MUSEUM vol. lu 



Nongeographic Variation 



Moults and plumages: I believe that Dwight (1900) is essentially 

 correct in his diagnosis of the moults of Swainson's thrush. There 

 is no prenuptial moult but some birds go through a "brightening" 

 stage in which the upperparts acquire a rich, bright hue. At first 

 glance, it might appear that the bird had just completed a prenuptial 

 moult, but observation under a low-powered binocular-microscope 

 reveals that the feathers are considerably worn. Miller (1941, p. 181) 

 observed that some races of j uncos undergo a similar brightening 

 stage. There is a sequence of abrasion to the feathers of the upper- 

 parts that is helpful in segregating specimens in breeding plumage. 

 Females arrive on the breeding ground later than males (sometimes 

 as much as two weeks) and do not begin to show signs of wear until 

 another week or ten days has passed, where as males begin to show 

 signs of wear (sometimes going through a " brightening" stage) almost 

 as soon as they arrive on territory. Specimens of subadult birds 

 taken in spring migration are invaribly more worn than adult birds 

 taken during the same season. There is evidence that both age groups 

 go tlu-ough a prenuptial moult of the tail feathers (239929 and 208991), 

 sometimes extending well into the breeding season (187190 and 

 187176). 



Age of museum sldns: Many of the specimens in the U.S. National 

 Museum are very old. Consequently, the problem of color alteration 

 due to age was given special attention. A series of skins known to 

 be badly foxed were used as a control group. Foxing produces a 

 peculiar artificially dyed appearance to the plumage that is at once 

 apparent after a little experience is gained in recognizing this type 

 of pigment alteration. As a secondary result of having the control 

 series available, it was determined that several birds collected on the 

 east coast and labelled "ustulata" were actually foxed specimens of 

 swainsoni. These were 236009, female, Sept. 18, 1881, Highland 

 Falls, N.Y., E. A. Mearns; and 263403, sex unknown, Sept. 21, 1897, 

 Ai'iel, Amite Co., Miss., collected by A. AlHson. 



Ecological Comments 



With the possible exception of the race H. u. oedica, the nest-site 

 requirements of Swainson's thrush are almost invariably associated 

 with the northern and mountain coniferous forest climaxes and their 

 subclimaxes. In general, H. u. ustulata shows a decided preference 

 for the deep forests of the Humid Transition Zone west of the Cascades 

 in Oregon and Washington and the Coast Ranges of British Columbia. 

 Here the heavy rainfall produces a rain forest of almost impenetrable 

 undergrowth that is dominated by a climax forest of Sitka spruce. 



