The Periodic Invasions of Goshawks

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Abstract
Goshawk [Accipiter gentilis] migration was studied each autumn during 1950-74. Two major invasions, composed largely of adults, were seen, one in 1962-1963 and the other in 1972-1973. The 1972 southward movement was prabably the greatest in history. Comparisons of interannual fluctuations in age and sex ratios with those derived from a model population suggest that major invasions of Goshawks indicate a massive reproductive failure and a precipitous decline in population, approaching 70% for 1972-73. The magnitude of an invasion is determined by the synchrony of the decline of prey populations in time and space, and by the size of the goshawk population. Agonistic interactions between goshawks play an important role in producing both invasions and massive reproductive failure. The decline in population in 1972-73 was sufficiently great that recovery will take more than 12 yr of optimal reproduction. Several 10-yr cycles will pass before another major invasion of goshawks will occur.