Coulson J.C., Monaghan P., Butterfield J., Duncan N., Thomas C. & Shedden C. (1983) Seasonal changes in the Herring Gull in Britain: weight, moult and mortality. ARDEA 71 (2): 235-244

1. The seasonal changes in weight, moult and mortality in the Herring Gull Larus argentatus in northern England and southern Scotland have been investigated over three years. 2. No consistent differences were found in the seasonal weight changes within the study area and birds from the east and west side of Britain showed the same seasonal patterns. In adults, body weight increased from September and reached a maximum in November which was maintained to February, after which there was a decline of about 16 g per month to July. The peak weight in the winter was 9% higher than the post-breeding season low. Amongst first year birds a maximum weight is also reached in November but weight is lost at a faster rate than the adults thereafter (26 g per month). Evidence is presented from recapture data which indicates that this seasonal pattern of weight change is followed in individual birds. 3. Primary moult starts earlier and takes a shorter time to complete in one year old Herring Gulls. The timing and duration is intermediate in older, immature birds whilst adults are last to start and take 41% longer than first year birds to complete the middle 90% of the primary moult. 4. The seasonal distribution of mortality has been examined from the frequency of ringing recoveries in each month of the year. In all birds over one year old, there is a close negative correlation between the mean body weight and the proportion of birds reported dead in that month. A different pattern is evident in first year birds, with a+ marked negative relationship evident between August and December, but the absence of a correlation from January to July. 5. The role of moult in relation to breeding is discussed as two competing energy demands. The problem of the weight changes in the Herring Gull is also discussed in relation to the mortality risk. and the possibility that breeding is a major stress on the Herring Gull is considered, in accordance with other evidence that they have ample food throughout most of the year. It is suggested that the importance of the winter weight increase and deposition of fat is primarily to conserve heat rather than to act as a major food reserve.