aspect of the river. Statistical correlation of the number of eagles observed 

 during aerial censuses versus total number of islands, sinuosity, gravel bar 

 area, areas of agricultural land, area of cottonwood-grassland, and area of 

 dense cottonwoods showed no significance. 



During the study period, two juvenile bald eagles which had been wing- 

 tagged by Jon Gerrard (1976) in Saskatchewan were observed in the study area. 

 One apparently wintered for a few weeks along the river near Rosebud (LaBree 

 1976) and the second was observed on one occasion near Kinsey in late March 

 1976 (Coleman 1976). Gerrard believed the second eagle may have been marked 

 as a nestling at Besnard Lake in north central Saskatchewan in the summer of 

 1974. These observations, as well as six other similar reports from other 

 locations in eastern Montana, show that some of the bald eagles which frequent 

 the study area are migrants from breeding grounds 1,100 km (670 mi) north. 



On a few occasions, bald eagles were observed attempting to capture ducks 

 loafing on the river. One eagle was observed attacking a flock of feeding 

 common mergansers, one of which regurgitated a goldeye when threatened. The 

 eagle quickly scooped up the fish and flew with it to a streamside cottonwood 

 perch. Although no eagles were observed actually capturing waterfowl, they 

 reportedly do capture molting (Swenson 1975) and crippled waterfowl, including 

 Canada geese (Wentland 1974), when available. 



Patterns of roost and feeding perch attendance observed along the lower 

 Yellowstone appeared to be similar to those described by Shea (1973). Bald 

 eagles were observed hunting and roosting along the river at all hours of 

 the day, although some were observed hunting away from the river, usually at 

 midday. The streamside cottonwoods served as night roosts as well as feeding 

 perches. Bald eagles were observed hunting in sagebrush-grassland areas north 

 of the river on several occasions. They also frequented lambing grounds, feed- 

 lots, pastures, and other areas where dead livestock were available. One lambing 

 ground near Kinsey was frequented by golden eagles (Aquila ehrysaetos)vih-\ch 

 were reported by local observers to have killed some newborn lambs. Bald 

 eagles were also attracted to the grounds and to sites where the lamb carcasses 

 were discarded. Carrion appeared to be the principal food source of the bald 

 eagles on the lower Yellowstone, judging by aerial and ground observations 

 of feeding eagles. During aerial censuses, 14 instances of eagle utilization 

 of large mammal carcasses were observed, as well as one of fish, one of skunk, 

 and three of other unknown small animal carcasses. Use of carrion by wintering 

 bald eagles was also reported by Hancock (1964) in his studies in the Southern 

 Gulf Islands of British Columbia. 



GREAT BLUE HERONS 



Great blue herons (Ardea herodias) were present in the study area from 

 March to October (figure 23), and single birds were occasionally observed as 

 late as December. Aerial censuses revealed the highest numbers of herons 

 during spring migration and after adults and young had left the rookeries to 

 feed along the river. Eighty-nine percent of the great blue heron observations 

 recorded during aerial censuses occurred in sections of the river where rookeries 

 were present (67 percent of the length of the river included in the study). 



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