SEASONAL AND ANNUAL VARIATION IN BODY-WEIGHT, NUTRIENT STORES AND MORTALITY OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS

The average body weight of Oystercatchers captured in the Dutch Wadden Sea and ajoining breeding areas varies between 520 g in mid summer an d 620 g in mid winter, but the individual differences remain large: be tween 450 and 650 g in summer and 500 and 725 g in winter. A part of t his variation can be explained by body size. Yearlings weigh, on avera ge, 30 g less than adults and 23 g less than subadults. However, after body weights are standardized for birds with the same body size, defi ned as the same wing length, yearlings actually weigh more than adults . Besides season and age, four variables explain a further part of the variation in the standardized body weight: (1) females are 20 g heavi er than males; (2) during the breeding season, inland birds weigh 20 g less than coastal birds; (3) adults captured on their nest weigh 25 g less than non-breeding adults, and (4) in late summer, birds undergoi ng wing and body moult are 15 g heavier than non-moulting birds. Since the majority of the birds are sedentary, there are no premigratory pe aks in weight. Carcass analysis shows that the seasonal weight variati on may be attributed to a < 10 g increase in lean dry weight in winter , and to a large seasonal difference for fat: coastal birds have, on a verage, 45 g of fat in summer compared with 100 g of fat in winter. Ye arlings weigh more in winter than adults because they store more fat. Also females deposit more fat than males in winter as well as in summe r. The variation in nutrient stores is thought to be a compromise betw een avoidance of predation and avoidance of starvation. This explains the seasonal variation in body weight, but also the individual and yea r-to-year variation, as follows: (1) Body weight in Oystercatchers win tering in the tropics is very low compared to conspecifics wintering i n the temperate zone, where a high body weight may serve as insurance against cold spells; (2) body weight in birds breeding inland is lower than in coastal birds, presumably because grasslands offer a more sta ble, predictable food resource than tidal flats that are exposed, and thus available as feeding area, during a variable part of the day; (3) the large year-to-year variation in the body weight of coastal breedi ng birds can be attributed to variation in the food supply: if the adj oining feeding areas are poor and the birds have to feed in the lower tidal zone, the nutrient stores need to be larger; (4) boy weight is r elatively high in estuaries with a small tidal range where due to wind , there is a larger daily variation in the length of time for which fe eding areas are exposed; (5) yearlings deposit more fat than (sub)adul ts and females more than males, because adults are dominant over yearl ings and males over females and the subordinate categories suffer more when food resources are limited due to ice covering most of the tidal flats. Despite laying down more stores, mortality during severe winte rs is higher in yearlings than in adults. Winter mortality does not di ffer much between the sexes, but since more males than females leave t he Wadden Sea at the onset of a cold spell, more males than females ar e killed by French hunters.