204 BULLETIN 19 6, UNITED STATES NATIONAL MUSEUM 



the nesting season, others, owing to their solid structure and protected 

 situation, may survive through one or more winters, their mossy walls 

 still green, still structurally perfect except for the debris-filled interior. 



Eggs. — Three or four bluish-green, lightly spotted eggs are laid. 

 Seven nests out of 13 had three eggs, and six contained four eggs, 

 averaging 3.46 eggs per nest for the 1935 season on Mount Mansfield. 

 Possibly this average is too low, since nests with three young were 

 assumed to have had three eggs; but in two cases one and two eggs 

 were known to disappear from a nest without affecting the remaining 

 eggs. The brown spotting on the eggs is a variable feature ; the eggs 

 of the grayer-backed birds (there are two color phase types of Bicknell's 

 thrush on Mount Mansfield) are less spotted, sometimes nearly immac- 

 ulate, the eggs of the browner-backed birds more heavily spotted. 

 However, there was often variation among the eggs in the same nest, 

 with two or three eggs nearly immaculate and the other more heavily 

 spotted. 



In dimensions the eggs of eight sets (29 eggs) varied from 21.0 to 

 23.0 millimeters in length, averaging 21.9, and were from 16.0 to 

 17.5 millimeters in width, averaging 16.6. The eggs of the olive- 

 backed thrush in the same area are typically larger and more heavily 

 spotted. 



Egg-laying in 1935 began on June 9 and continued through most of 

 June, with one clutch of July eggs; but egg deposition after mid-June 

 was believed to be due to the failure of earlier nesting attempts. 

 Apparently one egg a day is laid until the set is complete. In the 

 one instance observed the egg was laid at noon. Judged from hatching 

 dates and from one observation, incubation starts with the deposition 

 of the third egg, regardless of whether there are three eggs or four in 

 the nests. 



Young. — In the few instances that served as a reliable check the 

 incubation period proved to be 13 to 14 days, but unfortunately 

 three of the six nests for which egg-laying dates were known were 

 broken up before hatching, and in another only one egg hatched, 

 which could have been the fourth (last) egg hatching on the thirteenth 

 day or an earlier egg hatching on the fourteenth day. In one study 

 nest three eggs hatched after 13 full days of incubation, and the 

 fourth young emerged half a day later. The three eggs in another 

 nest hatched during the thirteenth day of incubation. 



Considerable variability in hatching procedures was noted. At 

 one nest the female apparently did practically all the work of liberat- 

 ing a feeble and underdeveloped chick, while at another nest a chick 

 nearly freed himself by kicking open a slightly cracked shell during the 

 absence of the female. 



The newly hatched chick is blind, relatively helpless, and entirely 



