and in late summer (figure 24). Although two cormorant breeding colonies are 

 known in southeastern Montana, one on the Tongue River Reservoir (Knapp 1975) 

 and one on a reservoir near the lower Powder River (Mincoff 1976), no colonies 

 of breeding cormorants were observed on the Yellowstone River. One pair did 

 establish a nesting territory in the heron rookery near the mouth of Armells 

 Creek in 1976. However, when part of the rookery collapsed into the river as 

 flows increased in May, the tree which contained the nest was lost. 



The cormorants summering on the lower Yellowstone appeared to be non- 

 breeding birds, some of which were lighter-colored juveniles. Flock sizes 

 commonly ranged from one to ten birds, although one group of 30 was observed 

 downstream from the refineries east of Billings. Cormorants commonly roosted 

 on narrow tips of barren islands near the water's edge. They also roosted in 

 Cottonwood trees near the refineries east of Billings and below Intake diversion. 



On two occasions, single cormorants incapable of flight were observed. 

 This phenomenon was believed due to their ingesting a large enough quantity of 

 fish to render them temporarily flightless, as occurred with drake common 

 mergansers. 



The low numbers of cormorants present during the high-water period in 

 June of both years may have resulted from poor fishing success in the turbid 

 flood waters. Absence of cormorants during the colder months of the year is 

 believed due to their inability to withstand cold temperatures, a characteristic 

 also exhibited by the other large piscivorous species observed on the river. 



OTHER MIGRATORY BIRD STUDIES 



OSPREYS 



Ospreys {Pandion haliaetus) were observed in the lower Yellowstone Valley 

 from mid-September to mid-October in 1974 through 1976, and in late April in 

 1975. Hackman and Henny (1971) reported that the highest rate of osprey 

 passage over White Marsh in Maryland occurred during September in eight years 

 of observation of fall osprey migrations. Sixty-five percent of the ospreys 

 observed during the present study occurred along the Yellowstone in Rosebud 

 and Custer counties. Osprey migrations through the study area occurred when 

 turbidity and flow of the river were low, which probably aided ospreys in 

 their fishing efforts. 



LESSER SANDHILL CRANES 



These cranes {Grus canadensis canadensis) were observed in the study area 

 only during their fall migrations. Spring observations recorded in western 

 North Dakota on two occasions indicate that cranes may follow a more direct, 

 easterly migration route to their northern breeding grounds. In the fall, 

 their migration appeared to span greater lengths of time and east-west distance 

 through eastern Montana. Migrations which preceded extended periods of cold, 

 snowy weather were the most extensive and represented passage of the largest 

 segment of the observed population. 



63 



