Breeding population fluctuations in some raptors

Summary

We investigated differences in annual breeding population stability and its relationship to diversity in food habits for several species of raptors. Chi-square tests showed no significant differences between observed and expected (based on logistic growth equation) breeding population sizes for Peregrine Falcons (Falco peregrinus) during recovery from pesticide induced declines in 4 Alaskan populations. This indicates that no major Peregrine Falcon population fluctuations occurred aside from the recovery itself. On the Colville River, Alaska, Rough-legged Hawk (Buteo lagopus) and Gyrfalcon (F. rusticolus) breeding populations experienced several declines and increases during the Peregrine Falcon recovery. Serial correlation analysis does not indicate consistent regularity in fluctuation for either the Rough-legged Hawk or Gyrfalcon on the Colville River during 1967–1985. Possible occurrence of regular fluctuations in sub-populations or over shorter time periods, however, could not be investigated. Population fluctuations for the Rough-legged Hawk and Gyrfalcon are significantly correlated during 1971–1985, with largest declines for both occurring synchronously. This correlation is consistent with the possibility that shared environmental stresses, among other factors, are responsible for population limitation. Regularity, with a 3 year interval, is indicated, however, for extremely small fluctuations in an Idaho, USA population of the Red-tailed Hawk (Buteo jamaicensis), suggesting that investigations of raptor population fluctuation and regelarity should not be limited to arctic or subarctic environments. Low diversity in food habits is not a good predictor of degree of annual population fluctuation in the study groups, while greater diversity in food habits is associated with relatively stable annual populations. An association was found in the Rought-legged Hawk between reduced annual population variability and reduced climatic severity. Neither food habits diversity nor annual population variability is constant across species ranges. Food habits diversity was also variable between years in local populations of Peregrine Falcon, Rough-legged Hawk and Gyrfalcon.