pondweed , wild celery, and seeds of other aquatic plants. In a study of winter- 

 ing birds on Chesapeake Bay, Stewart (1962) found that the diet consisted large- 

 ly of crustaceans, molluscs, and small fish, with some plant material. Crusta- 

 cea (particularly crabs), insect larvae, amphipods, molluscs, and small fish 

 made up most of the diet in Britain, with plant material reported from only two 

 of four samples and in low percentages (Olney and Mills 1963). Pehrsson (1976) 

 found that Common Goldeneyes feed on smaller mussels (Mytilus edulis ) than those 

 eaten by other diving ducks in the same area. Thus, food habits seem to be the 

 same on both sides of the Atlantic. A study of the food of Common Goldeneye 

 ducklings in Sweden revealed that they fed almost entirely on aquatic insects, 

 both adults and larvae (Eriksson 1976). 



IMPORTANT BIOLOGICAL PARAMETERS 



Egg Laying Nesting may 'begin in mid-April and continue through much of 

 May with regional and seasonal variation (Bellrose 1976), probably depending 

 on local weather conditions. Eggs are laid every second day (Bellrose 1976). 

 The mean date of the beginning of egg laying at one locality in southwest Swe- 

 den, 1971-1977, varied from 15 April to 9 May (Eriksson 1979c). Other studies 

 in Europe found most eggs present from about mid-May through June (Cramp et al. 

 1977). 



Mean Clutch Size Clutches in 75 North American nests held from 5 to 15 

 eggs and averaged 9.21 eggs (Bellrose 1976). In Finland, the mean size of 63 

 clutches was 9.3, ranging from 5 to 17 (Linkola 1962 in Cramp et al. 1977). 

 Clutch size in Sweden varied from a mean of 8.5 to 10.2 over the period 1 971 — 

 1977 (Eriksson 1979c). The range of clutch sizes recorded for this area was 

 3-15 for the period 1974-1977 (Eriksson 1979a), but Eriksson defined clutch 

 size as the number of eggs in a nest in which incubation was known to have oc- 

 curred. Consequently, his figures would have included nests in which more 

 than one female laid eggs and those in which eggs were lost during egg laying. 

 Clutches laid late in the season in southwestern Sweden were significantly 

 smaller than those laid earlier (Eriksson 1979c). 



Incubation Period Incubation takes 28-32 days, with an average of 30 days 

 (Bellrose 1976). Cramp et al . (1977) indicated that most incubation periods 

 are 29 to 30 days and listed extremes of 27 and 32 days. 



Hatching Success An average clutch of 9.2 eggs produces an average of 6.3 

 ducklings but a mean of only about 4.1 survives to near-fledging age (Bellrose 

 1976). An average clutch of 9.3 eggs at hatching produced an average of 4.7 

 young reared in Finland (Linkola 1962 in Cramp et al. 1977). In Sweden, a mean 

 of 8.9 young hatched in nests in which at least one young hatched, but an aver- 

 age of only 27% of the clutches hatched any eggs (Eriksson 1979c). Studies re- 

 ported in Bellrose (1976), largely on North American Goldeneyes, indicate that 

 50-69% of the nests result in at least one duckling hatched. In a study con- 

 ducted in southwestern Sweden, 1971-1977, the percentage of clutches that hatch- 

 ed ranged from 16 to 38% (Eriksson 1979c). 



Fledging Success Bellrose (1976) reported that Common Goldeneye broods 

 suffer unusually high losses. He noted that an average clutch of 9.2 eggs 



394 



