Doctoral Dissertations

Date of Award

12-1984

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major Professor

Herman H. Shugart

Abstract

Much of the bald eagle's (Haliaeetus leucocephalus) range has been subjected to anthropogenic disturbance of greater magnitude than the natural regimes of pre-European settlement times. Consequently, many eagle populations are depauperate. Eagle populations are large and stable, however, along the relatively pristive Pacific Northwest Coast of North America. This study examines (1) the behavior and ecology of bald eagles along the northwest coast and (2) the effects of environmental disturbance and resource dynamics on the ecology and evolution of eagles.

The ephemeral nature of food supplies along the northwest coast apparently results in eagles being limited primarily by food stress. Studies in the Chilkat -Valley, Alaska from 1979-83 revealed that the number and distribution of eagles correlated with food availability. Also, non-breeding eagles made migratory movements between food patches. Within the Chilkat Valley in winter, habitat was used preferentially, possibly to minimize the cost of thermoregulation. The use of gravel bars declined and use of streamside trees increased as ambient temperature dropped. During winter storms and at night, eagles sought refuge in dense stands of conifer trees.

The foraging behavior of eagles was analyzed using evolutionary game theory as a theoretical construct. Eagles obtained food by searching for unclaimed carcasses and by stealing from conspecifics. For a sample of 14 eagles, the rewards (food intake) and costs (incidence of injury) were similar for each tactic. This suggests that eagles hunted and pirated at rates which maximized fitness and that the population was in an evolutionarily stable state. The outcomes of contests for food were influenced by size, hunger level, possibly position (in the air or on the ground), but not by age. Pirating eagles assessed the size and hunger level of feeders and tried to steal from those that were most likely to retreat without fighting. Hungry birds capitalized on the assessment capabilities of others; they dissuaded attackers by using displays to advertise that they were hungry and willing to fight. A result of assessment was that contests were usually settled without physical fighting. Food availability influenced the foraging behavior of eagles. Frequency of display and escalated fighting increased as food levels decreased. A theoretical model showed that the rates of hunting and stealing across the population may or may not change with fluctuating as food levels.

These studies offered new perspectives on information transfer during animal contests. Contrary to current thought, evidence is provided that displays may function to signal fighting ability, expected gain in victory, or intensions.

Productivity was found to be variable and generally declining in southeast Alaska. Reproduction was influenced by food abundance and habitat quality but not by chemical contaminants. Food levels in spring influenced if or where eagles nested and when they laid eggs. Active and inactive nests differed in habitat quality. Food supplies during incubation and rearing regulated offspring survival.

Food shortages probably limited eagles over evolutionary time and several adaptations for survival and reproduction are apparent. Eagles maximized energy input for survival by: feeding opportunistically; making broadscale movements to find food patches; locating food within a patch by searching for prey or for conspecifics with prey; assessing prey profitability; acquiring food by hunting and stealing; and by defending food through threat displays or fighting. The light wing-loading of subadults may be an adaptation for making distant flights in search of food. The cryptic plumage of subadults may serve to make them less conspicuous to potential pirates when the subadults are feeding. Eagles conserve energy by minimizing nonessential activities and by maximizing time in habitats that offer favorable microclimates. Injury is minimized by avoiding dangerous prey or dangerous opponents and by using habitat to maintain a buffer to danger. Eagles obtain food for reproduction by defending feeding territories and by storing food in their nests. Possible adaptations for nest defense include heavy wing-loading in adults which may increase flight agility and bright adult plumage which may advertise that a territory is occupied.

These strategies and adaptations translate up scale and influence characteristics of the regional population. The mobility and the broad feeding niche of eagles result in the birds being distributed over most of North America. The dispersion of non-breedering birds reflects the patterning of their food supplies; they are dense where food is abundant and scarce elsewhere. An important consequence of eagles being drawn together at food patches is sociality. Interactions between eagles in feeding aggregations may be cooperative or competitive. A result of intraspecific competition is that low status individuals may be inhibited from feeding and starve. Thus social behavior in bald eagles is an important factor in the regulation of survival. Territoriality may also result in endogenous population regulation. Nest site quality in the region is determined primarily by food availability. The best competitors may claim and defend disproportionately large shares of the suitable habitat and other eagles may be forced to forego breeding that year. A surplus of non-breeders is probably a natural feature of ephemeral food supplies and territorial behavior. Delayed maturation may be an evolutionary consequence of surpluses of non-breeders. Young eagles, being poor competitors for nest sites may maximize lifetime reproduction by avoiding the risks of breeding too early. Another possible consequence of non-breeder surpluses may be a long-term population cycle resulting from competition for food between breeders and floaters.

This work on eagles illustrated some of the general principles which govern how environmental dynamics influence organisms. Natural disturbance drove the dynamics of the food and habitat resources that limited eagles. Eagles responded at several scales including the organismal, population, and evolutionary levels. These responses were felt at organizational levels ranging from individuals and breeding pairs up to species. This landscape perspective on the nature of environment/animal interactions has important implications for behavioral ecology and conservation biology.

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