Zwarts L., Hulscher J.B., Koopman K., Piersma T. & Zegers P.M. (1996) Seasonal and annual variation in body weight, nutrient stores and mortality of Oystercatchers Haematopus ostralegus. ARDEA 84 (A): 327-356

geen Nederlandse samenvatting beschikbaar op internet The average body weight of Oystercatchers captured in the Dutch Wadden Sea and adjoining breeding areas varies between 520 g in mid summer and 620 g in mid winter, but the individual differences remain large: between 450 and 650 g in summer and 500 and 725 g in winter. A part of this variation can be explained by body size. Yearlings weigh, on average, 30 g less than adults and 23 g less than subadults. However, after body weights are standardized for birds with the same body size, defined 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) ~~ are 20 g heavier than ##; (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 undergoing wing and body moult are 15 g heavier than non-moulting birds. Since the majority of the birds are sedentary, there are no premigratory peaks in weight. Carcass analysis shows that the seasonal weight variation 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 average, 45 g of fat in summer compared with 100 g of fat in winter. Yearlings weigh more in winter than adults because they store more fat. Also ~~ deposit more fat than ## in winter as well as in summer. The variation in nutrient stores is thought to be a compromise between avoidance of predation and avoidance of starvation. This explains the seasonal variation in body weight, but also the individual and year-to-year variation, as follows: (1) Body weight in Oystercatchers wintering in the tropics is very low compared to conspecifics wintering in 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 stable, 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 breeding birds can be attributed to variation in the food supply: if the adjoining feeding areas are poor and the birds have to feed in the lower tidal zone, the nutrient stores need to be larger; (4) body weight is relatively 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 feeding areas are exposed; (5) yearlings deposit more fat than (sub)adults and ~~ more than ##, because adults are dominant over yearlings and ## over ~~ 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 winters is higher in yearlings than in adults. Winter mortality does not differ much between the sexes, but since more ## than ~~ leave the Wadden Sea at the onset of a cold spell, more ## than ~~ are killed by French hunters.