waders and their estuarine food supplies - Vlaams Instituut voor de ...

Petersen 1973. Warwick & Price 1975. Moller &
Rosenberg 1983. Winther & Gray 1985. Emerson et
al. 1988). although the summer weights in Massachuscits
(Edward & Huebner 1977) were lower (Fig.
15). There was also a reasonable similarity in the
seasonal variations in the body weight of
Cerastoderma in Sweden (Moller & Rosenberg 1983).
E. England (Goss-Custard et al. 1977a). S. England
and Wales (Hancock & Franklin 1972. Warwick &
Price 1975. Hibberi 1976. Jones 1979. Sutherland
1982a. 1982c) and the Wadden Sea (Fig. 16).
Weight change and scope for growth
II" Ihe negative relation between winter temperature
and change in body weight (Fig. 10) is a general
phenomenon, very low body weights are to be expected
in southern areas during winter. The average
winter sea water temperature is close to 0 °C in SW.
Sweden (Moller & Rosenberg 1983), 2-4 °C in the
Wadden Sea, 6-7 °C in Wales. Ireland and France and
14 °C in Portugal (Hughes 1972. Bachelet 1980.
Desprez el al. 1991). Despite this large variation, there
is no evidence, however, of a latitudinal variation in
change in body weights during winter (Figs. 13-16).
The annual variations in the weights of the soft
parts of the bivalves depend on the energy budget, and
thus on the balance of total energy intake and energy
demands. The intake rate depends on the food
consumption and thus the available food supply, while
the energy demands depend primarily on ambient
temperature. The decrease in body weight oi Macoma
in mid summer can be explained by the decline in food
supply at a time when energy demands reach a peak
because of the high temperatures (Beukema et al.
1985. Hummel 1985c). The weight reduction in July
may be as great as 20'i in years when the sea water
temperature is between 18 and 20 °C. but with July
SEASONAL VARIATION IN BODY WEIGHT OF BIVALVES
44
temperatures of 15 or 16 °C. Macoma are able to
maintain iheir body weight I Beukema et al. 1985).
Laboratory experiments (Hummel 1985b) confirm
that a change in body weight depends on a
combination of temperature and food supply. The
apparent absence of a latitudinal variation in winter
weight (Figs. 13 to 16) would suggest that the higher
eosis of living in the more southern areas are offset by
a higher fcxxl intake (and food supply) during winter.
Implications for birds
Winter is a difficult period for waders. Firstly, the
fraction of prey which is accessible to them is (much)
lower than in the summer (Zwarts & Wanink 1989).
The fraction of large Scrobicularia accessible for
(iv stercatcbei Haematopus ostralegus is 40% in
summer but nearly i)% in winter (Zwarts & Wanink
1989 and 1991). Secondly, they have to eat more prey
io compensate for the poor condition of the prey. The
data summarized in this paper show dial waders have
to find 1.5 to 2 times more prey in winter than in
summer to maintain the same level of daily food
consumption. For Scrobicularia. it has been shown
that the seasonal variation in the weights of the prey
actual I \ accessible to Oystercatchers is even larger,
because prey within reach of the bill has less-thanaverage
body weight (Zwarts & Wanink 1991).
Thirdly, the energy demands of waders increase when
wind forces increase and temperatures decrease: the
daily food consumption of an Oystercatcher increases
by 40-50%, from 30-35 to 45-50 g AFDW, as the
ambient temperature drops from 10 to 0 °C (Kersten &
Piersma 1987). The effect of severe winter conditions
on the energy demands of ihe birds themselves is thus
not fully compensated by the better body condition of
the prey that results from low temperatures (Fig. 10).