DABBLING DUCKS 



Observations 



Ground observations of loafing dabbling ducks were recorded at the out- 

 set of the study but later omitted from records because of the unsystematic 

 means by which they were obtained. Aerial censuses were the only reliable 

 estimates of dabbling duck populations on the river. Ground observations 

 were used only as a general indicator of species abundance and distribution. 



Aerial censuses of dabbling ducks were conducted simultaneously with 

 those of geese and other birds. These censuses provided some observations 

 of individual species, but, because not all ducks could be identified by 

 species from the air, total numbers of each species were not obtained. It 

 was possible, however, to distinguish goldeneyes (Bucephala sp.) and common 

 mergansers {Mergus merganser) from the other species. Observations of other 

 diving ducks were included in the dabbling duck figures because they could not 

 be consistently differentiated, although they were believed to be too few in 

 number to appreciably alter the total census. 



Field-feeding observations of dabbling ducks were recorded from late 

 summer through spring. Observations recorded included species and number of 

 ducks, location, and field type. Dabblers and Canada geese were sometimes 

 observed field-feeding together, as the feeding areas of these species often 

 overlapped. 



Although incubating dabbling ducks were encountered during goose banding 

 operations, no systematic attempts were made to survey dabbling duck nesting 

 cover. It was believed that determination of goose nesting requirements would 

 provide a better basis for correlating waterfowl nesting with river structure 

 and vegetation because most geese nest on the river proper. Dabbling duck 

 nesting was believed to occur along oxbows, sloughs, creeks, irrigation 

 ditches, and fields away from the river. 



Sex Ratios 



Sex ratios of mallard flocks were determined within sections of the 

 river where the loafing population size had been estimated during aerial 

 censuses. Flocks were observed until a large number of individuals had 

 been classified as to sex. The sample size was deemed adequate when it 

 exceeded (Snedecor and Cochran 1967): 



4pq 



where: n = sample size needed to attain a given limit of 

 error, L 



p = proportion of drakes observed 



q = proportion of hens observed 



L = limit of error, in this case, 0.1 



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