Population fluctuations in spruce grouse: what determines their numbers in spring?

Abstract

The number of spruce grouse (Falcipennis (Canachites) franklinii) present in the spring population on a 247-ha block of pine-dominated habitat in southwestern Alberta was counted annually over a 21-year period. During the first 5 years of this study, the population increased steadily from 7.3 to 16.6 grouse/100 ha. Subsequently, for a period of 11 years, numbers were relatively stable at higher densities (19.0–29.1 grouse/100 ha). During the last 5-year period, the population declined to levels lower than at the onset of the study (16.6–4.9 grouse/100 ha). Year to year fluctuations in the size of this population were considered in the light of two hypotheses put forward to explain such fluctuations (spacing behaviour and breeding success). We concluded that both hypotheses were needed to explain our observations. During those years when spring population size fell below the mean number of grouse recorded over the 21-year period (17.8/100 ha), there was some evidence that breeding success in one year may have influenced population size the following spring but we found no evidence that spacing behaviour was important in limiting numbers under these conditions. On the other hand, when the size of the spring population was greater than the mean there was no evidence that breeding success influenced changes in population size the following spring, but there was evidence that spacing behaviour was limiting recruitment. The long-term change in population size appeared to be tied to the status of the fire sere inhabited: maturation of the forest (principally an increase in the height) was accompanied by a decline in population size.