Date of Award

Fall 8-1-1987

Document Type


Degree Name

Master of Science (MS)




I studied predation ecology of Northern Harriers (Circus cyaneus hudsonius) on Mallard Island in McLean County, North Dakota, during May-August, 1984-1985. Harrier population parameters, productivity, food habits, and feeding rates were examined. I located 15 nests in 1984 and 14 in 1985. Nesting chronology, clutch sizes, hatching and fledging success, and productivity were similar to values previously reported. Flooding, predation, starvation, and parental abandonment were responsible for egg loss or pre-fledging mortality. Chick survival to fledging was independent of brood size which suggested that quality of parental care, not a limited prey resource, was an important influence on survival. The island had a high density of nesting harriers, which coupled with average productivity, resulted in high harrier production. Activity patterns of harriers were determined by driving pre-determined census routes at random times throughout the day and recording behavior. Harriers hunted during daylight periods, and to a lesser degree, at dusk and dawn. They primarily utilized tame grass/iegume fields, native prairie, and shelterbelts as foraging areas. I determined abundance, activity, and habitat utilization of prey species with trapping, counts, and

visual assessment. Because of their activity and habitat usage was coinc dent with that of harriers, meadow voles (Mlcrotus pennsyxvanicus), thirteen-lined ground squirrels (Spcrmophilus tridecemllneatus). Ring-necked Pheasant chicks (Phasianus colchicus), and young eastern cottontails ."rir ’l’"1' ,J" 1 1 r"rirr"’ ’ ■ . . (Sylvilagus f 1 oridanus) were believed to be most vulnerable prey species. I determined food habits by examining pellets and prey remains at nest sites. The most common prey species were voles, ground squirrels, pheasants and cottontails. Harriers utilized prey species in different proportions than would be predicted based upon their occurrence, suggesting that factors other than availability govern prey selection. In both years, biomass contributed by voles and ground squirrels decreased and that by rabbits and pheasants increased as the brood period progressed. I propose that these changes were related to increased energetic demands of nestlings, increased hunting effort by females, changes in prey vulnerability associated with changing cover, and increased availability of juvenile cottontails and pheasants. Nestling feeding rates were examined via remote timelapse movie photography. Feeding rates increased with age and brood size. Ratios of prey biomass per chick were smaller in large broods. Smaller food rations did not affect fledging success.