Survival rates are necessary for calculating allowable harvests and for modelling population performance. Survival rates for 93 black grouse Tetrao tetrix equipped with radio tags in two populations of the southern French Alps were estimated. Annual survival rates of hens (0.68, SE = 0.075, 0.84, SE = 0.105), but not of cocks (0.56, SE = 0.09, 0.66, SE = 0.122), tended to be higher than those of juveniles (0.52, SE = 0.071, sexes combined). The lower survival of juveniles was due to higher predation, mostly by raptors, in autumn and winter. Although both autumn and winter were seasons of peak mortality in juveniles, most adult deaths occurred either in spring (hens and cocks), or in early summer (cocks). Causes of mortality were as follows: raptors 51%, mammalian predators 27%, unidentified predators 11%, and hunting 11%. Principal raptors were golden eagltAquila chrysaetos and goshawk Accipiter gentilis. Mammalian predators included fox Vulpes vulpes, pine marten Maries maries and stone marten M. foina. Only males could be legally shot, and hunting was a minor mortality factor on our study areas. Nevertheless, the distorted adult sex ratio favouring hens that we observed on our study areas was probably due to hunting of males on and around the study areas. Sensitivity analysis showed that population growth is influenced more by adult survival than by either reproductive success or juvenile survival to one year of age. To prevent shooting from causing highly distorted spring sex ratios, which might compromise reproductive success, harvests should be reduced in years of poor reproduction, and shooting should be oriented towards juvenile males.
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Vol. 3 • No. 3/4