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

4 6 8 10 12
worm length (cm)
Fig. 16. Numenius arquata feeding on Nereis diversicolor. A. Accessihilnv
of N ^ worms ipercenc of burrows found in the upper
12 cm of the substrate: clav content < 5%) as a funeiioii nl worm
length during late summer i same data as Rg, 2 in F.ssehnk .V Zwarts
1989). B. Relative si/e •ricctiOfl on Np,,^. as a function ot norm
leiigth in late summer, averaged for the 3 years (tame data H Rg.
9C). Lower graph show s the selecliv itv without taking into account
the increase of hiirrnw depihs w nh si/e. Upper graph lakes into account
the relative number of worms found in the upper 12 cm of the
substrate (Fig. 16Aj. C. Proliuhilitv img s ' handling) of fi?ntK. including
broken worms and probing time (cf. Table 3) as a function
ol worm lengih. Iniake rale img s ' Heeding) is indicaled.
VERSATILITY OF CURLEWS FEEDING ON NEREIS
a;
energy gain during handling is below the general intake
rate during feeding and has to take each profitable
prey encountered, i.e. prey for which mg s' 1 handling
is above mg S"' feeding (Hughes 1980. Krebs et al.
1983).
The profitability ofN-^ and Npnihv. incicascs wilh
si/e because although Curlews handle small worms
relatively rapidly, this is not fast enough given the
small amouni s\\ flesh taken (Fig. 10). The prey value
curves, shown in Fig. IOC. concern intact prey only.
The profitability decreases 9% for N^and 12% for
Nprc.iv '•' broken prey are included and again 2N'.' Im
Nprotv '''*'" unsuccessful probing limes are reckoned as
handling limes (Table 3). The adjusted profitabilities
of NptoN. (Fig. 16C) are Uius 36% below the prey values
for iniaci worms and successful probes only (Fig.
IOC).
The iniake rate of male Curlews in late summer and
autumn was, according to our data. 1.9 mg s' 1 feeding
on average. The profitability of all size classes of Np^
is above this lower limit: the yield of an Npivk of 5 cm
is 5 mg tr', and making an extrapolation downward
worms of 4 cm should still be profitable, but in fact
the] .nv not i.iken.
For all si/e classes Nprobc is less profitable than
iNpeck- The rare Np^^. < 6 cm is below the predicted
lower limit and 2.2 mg s"' for an Nprobc of 7 cm is just
above the iniake rate of 1.9 mg s~' (Fig. 16C). According
to the simple optimal diet equation there should be
a step change in the selecliv iiy at a worm of C. 7 cm. As
in many other empirical studies (Krebs et al. 1983).
this is not the case - rather there is a gradual increase of
selectivity with size.
One of the assumptions of the classical optimal diet
model is that predators know the energy value of all
encountered prey, whereas it is more realistic to suppose
that predators can only estimate prey values and
thus must make mistakes (Rechten el al. 1983). A
Curlew which detects a worm at the surface must derive
its size from the prey width, since usually only a
part of the body emerges from the burrow. N lhc is extracted
from the substrate. Its si/e can be estimated
only from die width of the entrance of the burrow. Although
we have not quantified the relationship between
width of the corridor and worm length, we are
conv inced diat there is a close relationship, also because
body width and body length are well correlated