Date of Award:

5-1-1997

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Committee Chair(s)

Kimberly A. Sullivan

Committee

Kimberly A. Sullivan

Committee

Edward W. Evans

Committee

James A. Gessaman

Committee

Barrie K. Gilbert

Committee

Mark E. Ritchie

Abstract

Generalizations regarding the relationship between age and foraging proficiency in birds may be incomplete and potentially misleading because most were formulated from studies of seabirds and shorebirds. Therefore, I examined three aspects of Bald Eagle (Haliaeetus leucocephalus) foraging behavior: numerical and functional responses, patch residence time, and producing-scrounging behavior. The study was conducted during autumn migration when hundreds of eagles congregate at Hauser Reservoir, Montana, to feed on spawning kokanee salmon (Oncorhynchus nerka). I determined eagle numerical and functional responses from 1991-1995. Eagles tracked the number of salmon during four of five autumns, and immigration and emigration rates were similar across years. Subadult eagles showed closer synchrony with salmon density than adults. Eagles had a Type II functional response. Handling time remained constant across salmon density, whereas daily attack rates increased. Functional responses were age-specific, with subadult eagles having a Type IV response and adults a Type I. Handling time of adults was constant, but that of subadults increased with salmon density. Attack rates of both age groups increased with salmon density. Consumption rates differed between age groups and among eagles using the three foraging modes. I determined whether body condition and foraging behavior of eagles affected patch residence time. I radio-tagged 68 eagles from 1992-1994. Juveniles had longer residence times than older eagles. Body condition was similar between age groups and did not correlate with patch residence time. Older eagles had higher daily consumption rates than juveniles. I found no evidence that eagles responded to environmental conditions. Departure from Hauser Reservoir was also independent of local resource abundance. I developed a three-player game to analyze producing-scrounging behavior of foraging eagles. Stoopers had higher daily intakes than pirates, who had higher intakes than scavengers. Salmon density affected the proportion of both producer strategies while the proportion of scroungers remained constant. Older eagles stooped more and scavenged and pirated less than younger eagles. Juvenile and 1.5-year-old eagles had consumption rates near the minimum needed to maintain neutral energy balance, suggesting that existence of a foraging group is needed to reduce starvation.

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