Peak numbers were observed during aerial censuses conducted in December 1974 and 

 March 1976, when 85 and 92 percent, respectively, of the mergansers observed 

 occurred between Billings and Miles City. The concentration of observations 

 in this section may be the result of ice cover on the river downstream from 

 Miles City and the tendency of mergansers to congregate in shallower waters 

 (such as upstream from the mouth of the Bighorn River) where fishing may be 

 more productive. 



Goldeyes [Hiodon alosoides) and stonecats [Noturus flavus) were utilized 

 by mergansers feeding on the lower Yellowstone River (appendix B). Stonecats 

 typically inhabit riffle areas where goldeyes inhabit a variety of water types. 

 The mergansers collected on December 9, 1975, were feeding in deep pools at 

 the downstream ends of two islands. Pools of this type were commonly utilized 

 for feeding areas by mergansers because goldeyes congregate in them during the 

 colder periods of the year (Haddix 1976). Mergansers were also observed 

 standing in and drifting through riffles, as was the drake collected on 

 April 21, 1975 (appendix B), whose stomach contained remains of stonecats. The 

 cooperative feeding behavior of red-breasted mergansers {Mergus serratov) , 

 described by Des Lauriers and Brattstrom (1965), was also exhibited by common 

 mergansers on the lower Yellowstone. Groups of feeding mergansers would float 

 downstream, feeding, then take flight, fly upstream, alight, and begin to 

 drift with the current again and resume feeding. 



Drake mergansers were occasionally encountered when incapable of gaining 

 flight. Of three mergansers collected on December 9, 1975, all exhibited 

 flightlessness and all had recently ingested large goldeyes (appendix B). It 

 is believed that mergansers can become so engorged with fish that they are too 

 heavy to lift off the water. No female mergansers exhibited flightlessness, 

 however, other than those which had begun to molt. This difference is believed 

 related to the ability of drake mergansers to swallow larger and heavier prey 

 than can the females. Latta and Sharkey (1966) stated that the size of the 

 merganser and girth of the food fish limited the size of the prey eaten. The 

 larger male mergansers (Anderson and Timken 1972) were able to swallow larger 

 fish. Latta and Shakey (1966) also stated that the most available and smaller 

 fish were consumed first and that the upper size limit of fish that mergansers 

 could eat ranged between 130 and 165 mm (5 and 6.5 inches) in girth. According 

 to their conclusions, mergansers on the lower Yellowstone which fed in riffle 

 areas were most likely to encounter and consume Cyprinids and stonecats, and 

 those that fed in other water types, goldeyes. 



Although no nests of common merganers were located in the study area, 

 observations of merganser broods showed that mergansers do breed in the area. 

 Nest sites utilized by mergansers in other studies included tree cavities, 

 nest boxes, cliff ledges, and a variety of ground nest sites (Bellrose 1976). 

 Most merganser broods observed during the study were feeding in deep water 

 along riprapped and other steep banks. All broods observed, ranging in size 

 from three to ten young, were between Pompey's Pillar and Hathaway. It is 

 possible that small numbers of mergansers breed in other sections of the river. 



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