Mean Clutch Size Clutch size varies with the timing of the season; it is 

 lower in late seasons. In a study of 354 nests from 1963-71 on the Yukon Delta, 

 Alaska, Lensink (1973 Ln Bellrose 1976) found an average clutch size of 4.26 

 eggs. Average clutch size varied from 3.3 in a late season to about 5 in an 

 early season. 



Incubation Period The most accurate figure for the incubation period of 

 the Whistling Swan in the wild is based on a single instance in which the last 

 egg of a clutch hatched after a 31-day interval. In another instance, incuba- 

 tion took about 32 days (Lensink ijn Bellrose 1976). 



Hatching Success We found no precise figures for hatching success expres- 

 sed as the proportion of eggs laid that hatched. Lensink (In Bellrose 1976) 

 thought that nesting success — considered as the proportion of nests in which at 

 least one egg hatches — was very high on the Yukon Delta; he believed that at 

 least some eggs hatched in over 90% of the nests. Bellrose (1976) suggested 

 that perhaps one egg of the average clutch fails to hatch, basing his remarks 

 on average clutch size and the size of broods seen in June and July. 



Fledging Success No definite data are available. Bellrose (1976) pre- 

 sented data indicating that production of young is low. On the other hand, 

 adults accompanied by young tend to have two or more (Bellrose 1976), suggest- 

 ing a high success for some clutches and the complete loss of others. 



Age at Fledging Bellrose (1976) suggested that most cygnets probably fly 

 at 60 to 70 days, but noted that some might need 75 days. 



Age at First Breeding Lensink (Ln Bellrose 1976) suggested that few birds 

 breed before their third summer, and that most probably first breed when older. 



Mortality of Eggs and Young Palmer (1976a) considered egg-gathering by 

 Eskimos and Indians a significant mortality factor in some areas. Lensink ( in 

 Bellrose 1976) noted nest destruction by gulls and foxes. 



Early freezing of water in fall accounted for some 3-5% of pre-fledging 

 mortality in young from the Mackenzie-Anderson River Delta area. In other areas, 

 freezing was reported as an occasional source of major mortality (Bellrose 1976, 

 Palmer 1976a). 



Renesting Swans occasionally lay repeat clutches if nests are lost early 

 in the nesting cycle, but the chances of northern swans doing this successfully 

 in the wild are considered poor because of the short nesting season (Kear Ln 

 Scott 1972). 



Maximum Natural Longevity A bird banded on the Anderson River Delta, 

 Northwest Territories, attained an age of 16 years and 2 months (Clapp et al. 

 in press) . 



Weight Mean weight of 42 adult males was about 16 lb (7,260 g) and 63 

 adult females averaged 13.9 lb (6,300 g). The mean weight of adults winter- 

 ing in Utah was 17.3 lb (7,850 g) and that of immatures was 13.3 lb (6,030 g) 

 (Bellrose 1976). 



