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

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profitability (mg s-' handling)
Fig, IS. The intake rate (mg s ' (coding) as a function ot profitability
(mgs' 1 handling) woeOynercatdtera to feed on Mytilus, I eras
toderma and Nereis of differenl weights. The graphs are based upon
the relationships between profitability and prey weight (Table I) and
between intake rate and pre) weight (Figs, 12, \s& 15). Pot Mytilus
a Selection is made of birds using ilic slabbing technique.
ity (Fig. 18). All else being equal, a doubling of profitability
will have little impact on the intake rate when
the birds spend most of their time searching, but will
nearly double intake rate when the birds spend most of
their time handling. It is therefore not surprising that in
Fig. 18 profitability differences strongly affect intake
rate in Cockles and Mussels, in which handling lime is
very long, but have much less effect when the birds
search for Nereis, in which most time is spent in
searching.
To investigate the relationship between profitability
and intake rate between prey species, we plotted the
average intake rate per species (Table 3) againsi ihe average
profitability (Fig. 19A). This comparison shows
lhat. even when profitability increases sixfold, the intake
rate remains the same. This implies that when the
prey species are compared, the average lime spent handling
as proportion of total feeding time, i.e. handling
and search time combined, decreases with profitability
(Fig. I9B). Soft-bodied prey are so profitable that.
even when 959?- of the feeding time is spent in searching,
the intake rate remains at 2 mg s'. If Oystercatchers
were also to search for 95% of the feeding lime
when they take Mussels, their intake rate would reach
PREY PROFITABILITY AND INTAKE RATE
30
200
the extremely low level of 0.1 mg s '. and they would
starve as shown in the next section.
Since average prey weighl differed so much be-
Ivveen species, we standardized the profitability and intake
rate to prey of similar weight choosing 2(X) mg
(Fig. 9). We calculated ihe intake rate for each species
with the same weight, using the multiple regression
equations for soft-bodied and armoured prey with different
intercepts but parallel slopes. When prey species
with similar body weight are compared, the intake rate
doubles within the range of profitabilities observed
(Fig. 19C) and the handling time as proportion of the
total feeding time halves (Fig. 19D). Thus, when prey
species of similar weight are compared, iniake rale
strong I v depends on the profitability, such as was also
found within the species.
Profitability and minimal intake rate
What is the minimal intake rate of food on which Oystercatchers
can sustain themselves? Oystercatchers in
the wild need at thermoneutmlity 36 g a day to keep
their body weighl constant (Zwarts et al. 1996b). If
they feed for all the five to six hours per low water period
during which the feeding areas are usually exposed,
the intake rate must be at least about 1 mg s '. If
it is less than this, they have to collect extra food either
on the upper shore during (he incoming and receding
tides and/or on inland grasslands al high tide. The iniake
raie on ihe high shore is low. however (Sutherland
1982, Ens et at. 1996c, Meire 1996b). because the
huge prey taken by Oystercatchers do not occur there
or only at very low densities (e.g. Zwarts et al. 1996c)
and their condition is often poor (Goss-Custard 1977,
Sutherland 1982a). Inland grasslands may provide
some compensation, but these opportunities are only
available locally. Thus, an intake rale of 1 mg s ' may
be considered as a limit below which food consumpiion
will in the long term usually be too low.
If the average weight and handling time of the prey
taken are known, it is possible to calculate the lengih of
the searching time at which the intake rate reaches the
lower acceptance level of 1 mg s~*. Since the relationships
between prey weight and handling time are
known (Figs. 2-6. Table 1). we can easily calculate the
search time per prev needed to achieve an intake rate of
I mg s '. The resulting Fig. 20 shows that if Oystercatchers
feed on prey containing more flesh, they may