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Population fluctuation and changes in the quality of rock ptarmigan in Alaska Theberge, John B.

Abstract

This study attempts to explain changes in abundance of rock ptarmigan (Lagopus mutus) at Eagle Creek, Alaska. It includes an analysis of population data collected from 1960 to 1969, and a test of an hypothesis: that there were no differences in the quality (survival, growth, behaviour) of rock ptarmigan chicks between years that influenced spring densities. Spring densities fluctuated between 1960 and 1969, reaching peaks in 1962 and 1968. This was the result of an orderly and generally synchronous change in the loss of birds (primarily juveniles) in winter, accompanied by changes in the production of young. Both acted together in most years to either increase or decrease numbers. Each contributed approximately equally to changes in total annual loss. Changes in the production of young were primarily caused by parallel changes in both clutch sizes and nest failures. Population regulation by direct extrinsic control appeared unlikely. Other than weasel predation on nests, no environmental factor external to the population itself appeared sufficient to explain changes in winter loss of juveniles or clutch size loss, or their synchrony. These results suggested that some internal process within the population must have been important in changing the abundance of partmigan. I tested the aforementioned hypothesis in 1967, 1968, and 1969 by examining chicks both in the wild, and in captivity. Survival, growth, and behaviour of chicks all varied between years. Changes in survival were apparently determined by unidentified parental influences (genetic or physiological) rather than by the direct influence of the environment. Growth rates were similar between years in captivity, but not in the wild, suggesting that environmental influences must have had some effect. However, neither the changes in survival of chicks in summer, nor in growth rates, could be implicated in altering subsequent spring breeding densities. Levels of agonistic and aggressive behaviour in successive cohorts of aviary chicks differed. In the similar environment of the aviary each year, these behavioural changes were attributed to undetermined parental influences (genetic or physiological). These changes in aggressive-agonistic behaviour offer the best possible explanation of changes in the. population parameters most important in altering spring breeding densities.

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