First Advisor

Richard B. Forbes

Term of Graduation

Fall 1972

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Biology






Birds -- Oregon -- Sauvie Island, Bird populations, Birds of prey



Physical Description

1 online resource (46 pages)


This study is an analysis of the various factors affecting the population of raptorial birds on Sauvie Island, Oregon. A census of diurnal and nocturnal raptors was carried out along with an analysis of food habits. Once the major prey species were determined they were censused, and the effects of the land management practices on their numbers was investigated. In addition, 100 randomly selected nest boxes were sampled so as to determine the extent to which raptors were making use of them. Red-tailed hawk and Great Horned Owl nests were located.

It was found that four diurnal raptors and three nocturnal raptors were present in sizeable numbers on Sauvie during the winter and spring of 1972. In addition here were less common sightings of four diurnal and one nocturnal raptor.

The most common food item of raptors in general was the vole, Microtus townsendi, which was later found to be present in extremely high numbers.

The common practice of planting fields of grains and the intentional flooding of them (for waterfowl use) was found to be a major factor in the numbers and vulnerability of Microtus townsendi.

Ducks were a common food item for all raptorial species during and immediately after duck hunting season, but not at any other time of year which seemed to indicate that the raptors were feeding on carrion or wounded ducks.

It was found that Barn Owls (Tyto alba), Screech Owls (Otis asio) and Kestrel (Falco sparvarius) made use of the nest boxes on Sauvie Island.

The overlapping of food habits of Sauvie Island raptors was discussed and an attempt was made to reconcile the apparent contradiction to Gause's Rule.


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