Factors Influencing Predation on Ruffed Grouse in the Appalachians
Predation accounts for the majority of ruffed grouse mortality beyond the brooding period throughout grouse range. Most studies of ruffed grouse ecology have taken place in the central portion of the species range including the Great Lakes states and southern Canada where aspen (Populus tremuloides) is dominant. Populations in the central range of grouse distribution exhibit 10-year cyclic patterns of decline that have been associated with the invasions of boreal predators such as great horned owls and northern goshawks during crashes in small mammal populations.
I completed an accuracy assessment of field sign to determine the role of scavenging as it related to diagnosing causes of proximate mortality. Potentially large numbers of grouse are scavenged after death, which can alter or completely falsify the reported cause of death.. My results indicated that stand-level and micro-site habitat variables did not influence scavenger detection of ruffed grouse carcasses (P > 0.05). Scavenging was limited to 24 of 64 carcasses and was entirely attributed to mammalian species. Scavenging behavior was related to the condition of the carcass following death (P = 0.003) and the ambient temperature (P = 0.01). As the temperature increased and as a carcasses entrails and muscle became more exposed, the probability of being scavenged increased. Nineteen percent of whole carcasses placed in the field were scavenged and would have been attributed to mammal predation based on the field sign; 56% of 32 mock avian kills were scavenged would have been attributed to mammal predation.
I related indices of predators and weather patterns to trends in ruffed grouse predation. Using data collected at 10 study sites between February, 1997 and December, 2000, I compared predation rates, and animal and weather indices to predation rates, across sites, years, regions, seasons and month combinations pooled across years. Avian predators were the primary predators of ruffed grouse in the Appalachian region (50% of all predation). Predation rates on ruffed grouse were highest in fall (8.3%), due to high predation on juveniles, and spring (7.4%) in association with raptor migration and pre-breeding activity of grouse. Predation patterns and predator abundance did not indicate the occurrence of predator invasions during the years of this study. Predation rates on ruffed grouse were positively related to the presence of rain and negatively related to the average low temperature and number of rabbits and squirrels observed per hour. Observations of owls and Cooper's hawks per hour were correlated with predation rates on grouse while those of red-tailed, red-shouldered and broad-winged hawks were not.