Common qoldeneyes, like common mergansers, were most numerous on the lower 

 Yellowstone from late fall throuqh early spring. Unlike the mergansers, how- 

 ever, there was no evidence obtained during the study that goldeneyes nest along 

 the Yellowstone. 



Bald eagles were present in the study area from September through April 

 in 1975 and 1976. Peak numbers during the study exceeded 110 birds. Most 

 of the eagles observed during censuses occurred between Billings and Miles 

 City, partially attributable to concentrations of eagles during winter in 

 these upstream areas where open water was available. One pair of bald eagles 

 attended a nest site near Sanders both years of the study but was not known 

 to have raised any young. Eagles commonly perched in streamside cottonwoods 

 near flocks of ducks and dived at them, but most eagles observed feeding utilized 

 deer and livestock carcasses. 



Great blue herons were present in the study area from March through October 

 in 1975 and 1976. Most herons occurred near rookeries, 14 of which were located 

 in the study area in the tops of mature cottonwoods, usually on large islands 

 but occasionally on the main banks. The breeding population of herons in these 

 rookeries was estimated to be 222 pairs in 1975 and 176 pairs in 1976. Herons 

 began nesting in April, and the first young hatched in early June. Fledglings 

 began to leave the rookeries in mid- to late July. 



White pelicans, double-crested cormorants, ospreys, and sandhill cranes 

 were also observed and data collected. 



IMPACTS OF WATER WITHDRAWALS 



Generally, if the reduced flows are insufficient to alter the existing 

 channel morphology, the impact on resident dabbling duck and goose populations 

 would probably be small. Migrant populations of field-feeding ducks and geese 

 would probably benefit from the increase in feeding areas resulting from 

 increased irrigated acreages. Flows low enough to be detrimental to existing 

 fish and invertebrate populations would probably reduce the numbers of herons, 

 cormorants, pelicans, mergansers, and goldeneyes which feed along the Yellow- 

 stone River. 



The increase in agricultural acreages in the Bighorn Valley coincident 

 with further irrigation development could attract more ducks and geese to 

 that area. However, large concentrations of these species induced to overwinter 

 on the Bighorn could suffer high mortality from disease or severe winter 

 weather. Flow reductions on the Bighorn River which accelerate changes in the 

 channel morphology which have occurred since the closing of Yellowtail Dam 

 could be detrimental to duck and goose breeding populations. Fish concen- 

 trations resulting from flow reductions could temporarily benefit fish-eating 

 birds as long as the reductions did not permanently reduce the fish populations. 



77 



