Ecological correlates of mortality of roe deer fawns in a predator-free environment

The mortality of roe deer (Capreolus capreolus) fawns in summer was studied on Storfosna Island (10 km(2)), a predator-free island 2 km off the west-c entral coast of Norway, during a period of rapid population increase. Betwe en 1991 and 1994 the population density increased from about 10 to 40 roe d eer/km(2). During this period 285 live and 36 stillborn fawns were found an d the live fawns were all subsequently radio-collared. All observed mortali ty occurred within the first 35 days of life. The main causes of death were stillbirth, starvation/hypothermia, drowning, car accidents, and falls. Th e average annual total mortality for all 4 years was 18%. Although there wa s a large increase in mortality between 1991 and 1992 (from 10 to 18%), the re was little subsequent increase in the last 3 years, resulting in an over all weak density dependence in total mortality. However, the proportion of stillbirths (5% in 1991 to 15% in 1994) showed a clear and significant tren d toward an increase with increasing density. Low temperatures in April, wh ich strongly affect the timing of the spring flush of high-quality forage d uring late gestation, also tended to contribute to higher mortality. Fawns that died were found to have reduced individual growth rates prior to death compared with fawns that survived. Fawns born to relatively light mothers or in triplet litters had higher mortality rates than those born to heavy m others or in smaller litters. Maternal age and date of birth or sex of fawn had no effect on mortality.