BICKNELL'S THRUSH 215 



foundland and the gray-phase bird more typical of continental regions. 

 Wallace (1939) has given a full description of these phases and their 

 distribution. 



From this it would appear that an observer reasonably familiar 

 with the Hylocichlae can, in most cases, readily separate members of 

 the gray-cheekedfspecies from olive-backed thrushes but that only 

 the extremes of the two size races can be safely determined in the 

 field. Color differences are in some cases discernible in the field but 

 are of no help in determining the size races, since both color types 

 occur in both size groups. 



Enemies. — Though the restricted distribution and relative rarity of 

 BicknelFs thrush may be in large part due to their highly specialized 

 habitat requirements, it may also in some way be correlated with their 

 low breeding rate and the associated mortality factors that prevent 

 further spreading. Tufts (Macoun, 1909) remarked that the Seal 

 Island birds had several destructive enemies to overcome during the 

 nesting season — the abundant crows and ravens, which constantly 

 circled over the island, and the feral cats, which had been liberated 

 and were breeding wild in considerable numbers. On Mount Mans- 

 field there were no crows or ravens, and the thrushes encountered 

 only natural predators, yet their nesting success during the summer of 

 1935 was low. Nine nests out of the 13 that were carefully watched 

 came to complete grief, and, of the surviving four, two were only 

 partially successful. Nesting success on this basis was only 25 

 percent; or, if calculated from the total number of eggs (45), reproduc- 

 tive efficiency was 24.4 percent, and the total loss of eggs and young 

 75.6 percent. In other words, 13 pairs of birds raised 11 young 

 (0.85 per pair) to the nest-leaving age, a rate at which the adults 

 would not be replacing themselves in two seasons. Considering 

 postnest juvenile mortalit} 7- , adult losses, and unmated birds, it seems 

 doubtful if these figures can represent more than a stable, if not 

 indeed, a decreasing population, though of course the results of a 

 single summer cannot be considered typical of all years. 



The main loss in 1935 was the unexplained disappearance of the 

 eggs and young of 6 nests out of 13 (19 eggs or young out of 45 eggs, 

 or 42.2 percent) . This was believed to be largely if not entirely due to 

 red squirrels, which were abundant and active arboreal hunters in 

 the nesting groves. Weasels were present in the area, but they are 

 mainly terrestrial hunters, like the more numerous chipmunks. 

 Bobcats were seen only once, though subsequent winter-trapping 

 returns, turned in to the State for bounties, indicated their presence 

 had been generally overlooked. 



Thus, more than half the known loss (42.2 percent out of 75.6 



