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New insight to old hypotheses: Ruffed Grouse population cycles.

Article Excerpt
The Ruffed Grouse (Bonasa umbellus) is a widespread and important game bird in North America (Rusch et al. 2000). Population size of grouse is known to fluctuate through time and, in some areas, these fluctuations follow a cycle of ~10 years (Keith 1963, Rusch et al. 2000). Their population fluctuations and cycles have been studied extensively (Bump et al. 1947, Keith 1963, Gullion and Marshall 1968, Keith and Rusch 1989, Small et al. 1991). These studies have generated many hypotheses about mechanism(s) responsible for grouse cycles. Ideas advanced as mechanisms controlling grouse fluctuations range from the bizarre (e.g., rabbit [Lagomorpha] seasons were too long; Bump et al. [1947]) to the plausible (e.g., predation; Rusch et al. 2000). Most recently, some researchers have presented correlative data that suggest predation by Northern Goshawks (Accipiter gentilis) and Great Horned Owls (Bubo virginianus) are most likely causing cyclic population fluctuations in Ruffed Grouse (Keith and Rusch 1989, Lauten 1995). However, there have been no studies that simultaneously evaluated competing hypotheses about factors (e.g., predation, weather, competitors) that could potentially influence grouse numbers through their effect on survival and reproduction. We used model selection to compare relative support for multiple hypotheses representing potential factors that could influence Ruffed Grouse population fluctuations. Our objective in this study was to revisit a phenomenon (the 10-year grouse cycle) that has been assumed to be resolved scientifically (i.e., predation causes these cycles, Rusch et al. 2000) using the comparative model approach. We believe this approach can be used to guide future research about Ruffed Grouse population cycles.

METHODS

Study Area.--Our study area encompassed a 20,358-[km.sup.2] area around Grand Rapids, Minnesota, USA (47[degrees] 13' N, 93[degrees] 31, E), defined by a circle having a radius of 80.5 km. This was the approximate maximum distance traveled from Grand Rapids by hunters who collected grouse for our study. This region was within a transition zone between the deciduous hardwood and boreal forests. The weather was variable over the study period and was characterized by warm, moist summers and cold, snowy winters.

Response Variable.--We were interested in factors that might influence Ruffed Grouse cycles and used spring counts of male Ruffed Grouse drumming displays (a gross index to population fluctuations; Rusch and DeStefano 1989, Zimmerman and Gutierrez 2007) as our response variable. Male Ruffed Grouse use several displays to attract or court females during the spring, one of which is called the "drumming display" (Rusch et al. 2000:9). This display consists of a male grouse beating its wings in a manner that produces a distinct low frequency nonvocal sound (Rusch et al. 2000). We used data from surveys within the study area that were conducted annually by the Minnesota Department of Natural Resources. Surveys occurred along non-randomly selected, but consistently surveyed, routes that were approximately 14.4 km in length along maintained roads. Wildlife biologists, land managers, and volunteers attempted to survey each route once during the spring (early Apr-early May). Volunteers conducted surveys by stopping every 1.6 km along each route for 4-min intervals to listen for drumming displays. The number of drumming displays heard per 4-min interval was recorded at each stop (Minnesota Department of Natural Resources, unpubl. protocol). Grouse were not surveyed on rainy or windy days. We recognize this index may not truly represent population size and may have represented only drumming display activity of male Ruffed Grouse (Anderson 2001).

Development of a priori Hypotheses.--We evaluated hypotheses (models) about factors that influence Ruffed Grouse population dynamics that exist in the literature (Bump et al. 1947, Gullion and Marshall 1968, Rusch et al. 2000) and our own experience working with grouse. We limited our set of hypotheses to those for which data were available. We considered 13 hypotheses, a priori to analysis, which represented predictions about how goshawk abundance, weather during the breeding season, weather during the previous winter, exploitative competition (forest tent caterpillars, Malacosoma americanum), color phase ratios, mass of male grouse, and age ratios of both genders during the autumn correlated with Ruffed Grouse population indices that were conducted from 1983 to 2004 (Table 1). We limited our analysis to the interval 1983-2004 because we did not have individual grouse data prior to 1982; the 1982 data were used to predict the population index for the following spring (i.e., spring 1983 counts).

Predation Hypothesis.--Ruffed Grouse are killed by many predators (Bump et al. 1947). Correlative evidence supports a substantial, if not controlling, effect of goshawk and Great Homed Owl predation on the Ruffed Grouse cycle (Keith and Rusch 1989). We did not locate relevant data about Great Horned Owl predation or owl abundance on our study area, but we did acquire population indices of goshawks, which are important predators of Ruffed Grouse in Minnesota (Eng and Gullion 1962). We estimated an annual index of goshawk abundance from counts of migrating raptors at Hawk Ridge Nature Center in Duluth, Minnesota (~110 km...

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