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

20 30 40 50
shell length (mm)
PREY PROFITABILITY AND INTAKE RATE
B
10 50 100
prey weight (mg)
o field
A lab; 2cm deep
Fig. 6. Handling lime as a function of A. prev length or B. prey weighl in .V/v.i in captive Ovsiercaichcrs feeding on prey lying al a depth of 2
Ctn I Wanink &. Zwarts 1996) Of wild birds feeding on clams from which lite llesh is eaten in sim (Zwarts & VVaiiink 19S4. Bunskoeke el al.
I9')d & unpubl.). The regressions are calculated without the lour largest cl.mis since llieir handling times were unlypicallv long. Ihe handling
limes as a function ol prey weighl are not significantly different Ip = 0.33 in panel Bl. Resulis ol covariance analyses: A. R-' = OXs) tor prev
lengih: B. R-' = 0.869 for prey weight and R- = 0.002 lor ihe two groups: Table I gives more details.
weight of the flesh extracted from Mya of unknown
size, so more field data were available when handling
time was plotted against ihe prey weight (Fig. 6B). As
Fig. 6B shows, the relationship between handling time
and prey weight was the same as in the laboratory,
where all the llesh from shallow prey was eaten, and
field studies, where a variable amount of llesh was extracted
from deep-living prey.
In conclusion, the handling time of all armoured
prey species increases wilh pre) si/e. An additional,
and substantial, part of the variation in handling time
may be explained by four other variables: (I) depth
from which prey are taken: (2i whether prey are hammered
or stabbed: (3) whether prey are opened in situ
or opened after being pulled up from or off the sediment:
and (4) portion of the prey eaten.
Handling time of bivalves in relation to prey
condition
Figures 2-6 show the handling times as a function of
prey length and of prey weight. If most of the handling
time is spent in eating the llesh. we expect that the handling
time would depend on the amount of llesh ingested
and lhat the close relationship between han­
:m
500
dling nine and prey size is due only to the high correlation
between prey si/e and llesh weight. On the other
hand, if handling time consists mainly of lime speni in
breaking ihc shell, handling lime would be primarily
determined by size-related strength ol the shell and mn
by the amount of llesh ingested. Prey weight and prey
size are so highly correlated, that it is hardly surprising
that it is not possible to tell from Figs. 2-6 whether
handling time depends on prey weight, and thus indirectly
on si/e. or on prey si/e. and ihus indirectly on
the amount of flesh to be ingested.
Although llesh weight and prey size were highly
correlated when both were plotted on a log-log scale.
the weight variation within each si/e class was huge
enough to investigate whether the amounl of llesh. independent
of prey size, affected the handling time.
Most of the species preyed upon by Oystercatchers
contained in late winter40*3" less llesh than specimens
of similar si/e in early summer (Chambers & Milne
1979. Zwarts 1991, Zwarts & Wanink 1993. Ens et al.
1996b). Prey condition had been defined as percent
dev laiion of the average prey weight, such as obtained
by regressing loglweighn againsi log(si/ei. using all
data given in Figs. 2-6. Multiple regression analyses