Variation of body mass in moorhens Gallinula chloropus wintering in a temperate area

The variation of mass during the winter season was studied in a Moorhen (Gallinula chloropus) population wintering in North Italy. The influence of 19 short- and long-term time and climatic parameters was evaluated both by single factor regressions and by multivariate analyses. Mass was related positively to the temperature of the days preceding capture and the 71-year mean temperature, and negatively to the date, 71-year temperature variability and photoperiod. Moorhens lost more mass after days with low temperatures and in periods with a high temperature variability. The composition of the body was evaluated by measurement of its electroconductivity with the Tobec system. This device gives an indication of the lean body mass, since the contribution of lipid tissue to conductivity is negligible. During the winter season the Moorhens had a small decrease of lean body mass index suggesting that the consistent weight decrease was due to lipid depletion. By experimentally exposing 36 Moorhens to different overnight temperatures we observed a higher decrease of body mass with lower night temperature. The weight changes in Moorhens did not follow the pattern of winter fattening found in many passerine species. Instead there was a noticeable seasonal decrease of mass (-30 %) linked to long-term climatic factors and an adjustment of body mass in relation to short-term temperature.