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A relation between aggressive behavior and population dynamics in blue grouse Mossop, David Harold

Abstract

This study was a natural experiment to test an hypothesis that blue grouse populations are regulated by aggressive behavior. Three natural populations were studied during two successive breeding seasons. The environments of the three were essentially the same. The density of one remained stable at 5 territorial males per 100 acres. The density of the second was declining 10% annually and was approximately 6 territorial males per 100 acres. The density of the third was 35 territorial males per 100 acres and had risen to this density in the previous three years. It ceased to rise during the study. The annual mortality of adults was similar in all three. Annual recruitment in any one year, was also similar, however the population which was decreasing showed a lower recruitment over the term of the study. In the population which was increasing in density, more chicks were produced per adult, than in the population which was decreasing. Production in the stable population was intermediate. Males in the increasing population, sang longer into the season than those in the other populations. Males in the increasing population hooted less frequently in response to humans and reacted less aggressively to artificial song than those at the other areas. All grouse in the increasing population were less frequently observed on the ground and were harder to capture than those in the other populations. 'Flush distances' were longer, and the time to flush, was shorter than in the other populations. Threatening calls, "head dips" and "neck stretches", all behavior which indicates aggression were observed more frequently in the decreasing and stable populations. "Flutter flights" and "feather spread displays", gestures which probably also serve as threats, were recorded more often in the declining and stable populations. "Grouch and run", a non-aggressive pattern, was observed more frequently in the increasing population. In the declining population, hens showed more vigorous brood defense than those in the increasing. Chicks were further from the hen in the increasing population, flew more readily, and flew further when flushed. Artificial hen calls caused territorial cocks in all populations to alter their songs similarily. Males at the declining and stable populations advanced more quickly toward the sound. Males in all populations courted a dummy female similarily. When reacting to a mirror, males showed fewer aggressive acts per minute in the increasing population. Behavior, including many types of agonistic behavior can vary between populations. Hostile interactions have the potential of being less severe in the increasing population. This may have caused its increase. However, no cause and effect relationship was demonstrated.

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