Fluctuations of observed breeding Rough-legged Hawks and Gyrfalcons: regularity reconsidered.

We recently assessed regularity in fluctuation of annual numbers of breeding pairs observed for several raptor species, and found no indication of reputed regular or cyclic fluctuations in our Colville River, Alaska sample for Gyrfalcons (Falco rusticolus) or Rough-legged Hawks (Buteo lagopus) (Mindell et al. 1987). Discussions with colleagues have prompted us to expand our analysis and clarify key points. We present time series analyses, using the Colville River sample size and interval (13 surveys during a 27 year period) for a data set on lynx (Lynx canadensis) (Elton and Nicholson 1942) and for a hypothetical Gyrfalcon population. Indication of regular 10-year fluctuations (P<0.05) in these analyses supports the adequacy of our Colville River sample size and interval in assessing a possible 10-year cycle for Gyrfalcons. We provide dates and methods used for the annual Colville River surveys in discussing applications and limitations of the survey findings. We use the annual Colville River surveys as an index of fluctuation in actual numbers of Gyrfalcons and Rough-legged Hawks present in the study area at the time, based on similarity in timing of surveys between years, use of boat surveys in 12 of 13 years, evidence indicating similarity in sampling error between years, and demonstration of the adequacy of the sample interval. Whether or not numbers of breeding Gyrfalcons and Rough-legged Hawks fluctuate regularly remains an open question. We have provided a first assessment of regularity in fluctuation of observed numbers of breeding pairs, and find regularity for this parameter unsupported considering the Colville River study area as a whole. If other researchersare stirred to test "conventional wisdom" presuming regularity, our study will have served its purpose.