Fluctuations in the Numbers of British Tetraonids

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Abstract
Data on red grouse, Lagopus scoticus, are given for some 80 moors and the oldest records go back to 1834. Peaks are shown, and some records of disease outbreaks back to 1797 are noted. Data on other tetraonids, Tetrao urogallus, Lyrurus tetrix, Lagopus mutus, from a number of estates in central Scotland from 1866 to 1949 are given. L. scoticus fluctuates throughout its range, the period being irregular, between 3 and 10 years but mostly 5 or 6. There is a general synchrony which is not however exact. Peak years may differ even on adjoining moors. It is suggested that this synchrony is brought about because, while crashes on each moor are due to the interaction between the birds and their immediate environment, there is occasionally a very good or very bad year which brings big areas into line. The critical density may differ in the same area in different periods and varies widely from place to place. Where density is high, strongylosis (infestation by the nematode, Trichostrongylus pergracilis) is often but not always associated with crashes. Cycles with much the same rhythm and dates occur in areas where it is unknown, when bad nesting seasons are thought to be the limiting factor. Some of the worst crashes occur when they are associated with strongylosis. A possible explanation is that once the critical density is exceeded or resources are reduced below requirements (perhaps only temporarily) the particular agency which reduces numbers may vary. The other three tetraonids show rises and falls more or less synchronously with grouse. Duplicate peaks sometimes occur at intervals of 2 years only. Species other than grouse sometimes have interpolated peaks in long intervals. The only common factor likely to cause these events is food, which is affected by weather. Peak periods of 2 or 3 years are suggested, during which most peaks occur. Blackgame bags from south Scotland from 1850 and 1949 are given and agree in general with data from central Scotland. An increase in warmth and wetness in the northern climate may be the ultimate cause of the long-term variations.