216 BULLETIN 196, UNITED STATES NATIONAL MUSEUM 



percent) was attributable to red squirrels or unassigned predators 

 (one loss — not included in the above percentages — was fairly definitely 

 assignable to a blue jay that chanced to pay one of its rare visits to 

 the mountain from lower elevations), and in addition two infertile 

 eggs and two deserted eggs suffered the same fate. Moreover, of the 

 five empty nests of the year that were found in July, at least three 

 gave evidence of having been plundered. 



One nest came to grief through some disease, which may possibly 

 have been Protocalliphora parasitism. When half grown the young 

 became too weak and sickly to eat and slowly waned to the point of 

 death in spite of the assiduous attention of the female parent. On 

 the basis of 45 eggs this gives a 7 percent loss due to this disease, but 

 if based on the number of young raised to the susceptible age (14) 

 the loss would be 21.4 percent. Moreover, a nest observed in 1933 

 apparently suffered the same fate, the undeserted young dying in the 

 nest when about half grown, and still another nest was found with 

 the dried skeletons of half -grown young, a tragedy that may possibly 

 have stemmed from this undetermined disease. In several of the 

 study nests in 1935 the young, *on about the sixth day, showed 

 symptoms of sickness].but recovered. 



Other losses were due to infertile eggs or defective embryos (about 

 11 percent) and to deserted eggs (about 9 percent). Neither of these 

 percentages is very accurate, however, as the former is based on the 

 total egg count, 15 of which disappeared before hatching, and the 

 adults of the deserted eggs renested, producing a second set of eggs, 

 which was then lost to predators. 



These heavy nesting losses appear to be typical of other local 

 birds, in spite of which the blackpoll warblers, whitethroats, and 

 j uncos managed to maintain dense population, and the BicknelPs 

 and olive-backed thrushes are certainly as abundant in their respective 

 habitats as are their generic relatives, the hermits and veeries, at 

 lower levels. It is entirely possible that the Bicknell's thrushes on 

 Mount Mansfield are close to their optimum numbers and that the 

 various limiting factors here noted merely tend to keep them at this 

 favorable level. This introduces the age-old and probably unanswer- 

 able problem of whether their rarity and limited distribution is due 

 to low productivity or whether slow reproduction is an adjustment 

 to their enforced occupation of a restricted ecological niche. 



Fall and winter. — The characteristic late-summer seclusion referred 

 to above comes to a termination early in September with the manifesta- 

 tion of premigratory movements. The birds again become active 

 in the woods, feeding among the groves, calling loudly and frequently, 

 and sometimes bursting into short snatches of song. On September 

 10, my last day on Mount Mansfield in 1935, the thrushes were still 



