Diagnosing the environmental causes of the decline in Grey Partridge Perdix perdix survival in France

We studied Grey Partridge Perdix perdix mortality during breeding to identi fy the environmental causes of a long-term decline in adult survival. We ra diotagged and monitored daily from mid-March to mid-September 1009 females on ten contrasting study sites in 1995-97. Simultaneously, we recorded habi tat features and estimated the abundance of Hen and Marsh Harriers Circus c yaneus and C. aeruginosus, Red Fox Vulpes vulpes and mustelids. We experime ntally tested whether scavenging could have biased predation rates. We also examined, through the necropsy of 80 carcasses of Grey Partridge, whether disease, parasites or poisoning could have been ultimate causes of high pre dation rates. The survival rate of radiotagged females during spring and su mmer ranged from 0.25 to 0.65 across study areas. Mortality peaked in May, June and July when females were laying and incubating. The direct negative impact of farming practices was low (6%). Predation was the main proximate cause of female mortality during breeding (73%) and determined the survival rate, suggesting no compensation by other causes of mortality. Ground carn ivores were responsible for 64% of predation cases, and raptors for 29%, bu t this proportion varied across study sites. Disease and poisoning did not appear to favour predation, and scavenging was not likely to have substanti ally overestimated predation rates. The predation rate on breeding females was positively correlated with the abundance of Hen and Marsh Harriers, sug gesting an additional mortality in areas where harriers were abundant. The proportion of raptor predation was linearly related to harrier abundance. T he predation rate was not correlated with the abundance of the Red Fox and mustelids. A potential density-dependent effect on the predation rate was c onfounded by the abundance of harriers. We found no convincing relationship between the predation rate and habitat features, but we observed a positiv e relationship between the abundance of Hen and Marsh Harriers and the mean field size. This suggested that habitat characteristics may contribute to high predation rates through predator abundance or habitat-dependent predat ion.