202 BULLETIN 196, UNITED STATES NATIONAL MUSEUM 



cycle through an entire season on Mount Mansfield. The following 

 nesting data are based largely on the 13 occupied nests studied that 

 summer, supplemented by less detailed observations in 1933 and 1936. 



Nest-building on Mount Mansfield in 1935 started during the first 

 days of June, in less than a week after the arrival of the birds. On 

 June 4, and again on June 6, birds were found laying materials for 

 the foundations of their nests. On June 5 a bird was found working 

 on a nest that was already probably several days old. Later observa- 

 tions on egg-laying, incubation, and hatching pointed to early June 

 as the period of most active nest-building, with later nests probably 

 due to some interruption in the nesting cycle. 



Bicknell's thrushes build their nests by laying twigs and mosses 

 loosely across one to several horizontal branches, usually close to the 

 point where the branches diverge from the trunk. After a consider- 

 able mass of material is assembled, they start building up the side 

 walls with moss and supporting twigs, and shape the cavity by sitting 

 in the nest and working with rotary motions of the body until the 

 cavity conforms to the size of the bird. In consequence, a sitting 

 bird, thus perfectly adjusted to its nest, forms an efficient watershed 

 for the eggs and young during rainy weather. Sometimes there is a 

 delay of a day or two before the final lining is added. Nest-building 

 activities were spread over the first half of June, though no individual 

 structure is known to have required that long for construction. At 

 two nests where an attempt was made to determine the nest-building 

 period more accurately, the birds deserted. 



Fresh green moss and small twigs comprise the bulk of the typical 

 Bicknell's nest. Usually moss predominates with just enough twigs 

 to give strength and stiffness to the structure. Long green strands 

 of an abundant pleurocarpous moss (Calliergon schreberi) are most 

 frequently used, often mixed with, or more rarely, entirely replaced 

 by, the erect lighter green tufts of sphagnum. Supporting twigs are 

 mainly spruce and balsam, usually with the addition of some minor 

 items such as nonconiferous twigs, flower stalks, the pinnae or stems 

 of ferns, dry leaves, shreds of bark, rotten wood, or hair. In one case 

 the exterior of a nest was beautifully ornamented with a network of 

 gray-green lichens. Moss, so readily available on the mountain for 

 building purposes, is freely used by both ground- and tree-nesting 

 birds, but none use it so freely as Bicknell's thrushes. Nests of the 

 olive-backed thrush in the same region characteristically lack the 

 mossy character of the Bicknell's nest. 



Moss and twigs constitute only the body of the nest. The interior 

 is filled with partially decomposed organic debris, apparently dug up 

 beneath evergreen trees and no doubt replacing the inorganic mud 

 used by robins and wood thrushes. Into the well-rounded cavity is 



