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

Table 2. Results of eight iwo-wav analyses of variance to test the
effect ol depth and density on several eonipoiienls of handling time.
for unlifted prey i Fig. SA I, lifted prey (Fig 5B.) and tor all prey (Fig.
6). A hierarchical approach w as chosen in which the higher priority
was assigned to depth.
I nlilted prev
culling lime
eating time
1 i fled prey
culling time
eating lime
All prey
lifting tune
cutting time
eating time
Depth
K'.'i
I2.ti 0,001
7,3 0.001
1.0
31.6
12.2
1.4
0.813
0.0S4
0.001
o.ooi
0.001
OPTIMAL FORAGING AND THE FUNCTIONAL RESPONSE
Density Depth x density
/:• , p «-".', p
15.4 0.001
0.4 0.S5K
14.7
6.1
0.006
0.067
2.1 0.039
13.9 0.001
1.7 0.145
handling nine 22.0 0.001 12.7 O.OOI
2.H
II
4.9
i.'l
0550
0.911
0.484
0.437
4.2 0.035
1.9 0.549
3D 0.289
3.0 0.102
nri nri
esiN thai Oystercatchers, Feeding on Cerastoderma
edule oi Macoma. probe ihe substrate ai random, when
they locate their prey by touch. He calculated the probability
{P-loc) that a bird would hit a ptev al one probe
from the formula:
P-loc = Ds/\& (41
where I) = prey density (n nr-) and s = mean effective
touch area (cm 2 ).
To predict the encounter rate (X) of the Oystercatcher
with prey from a certain depth class, we must
divide P-loc by the lime it takes the bird to insert iis bill
io that depth into the substrate:
X= P-loc/I (5)
where Xt = encounter rate with prey of deplh class /
(prey min' 1 ) and t. = duration of a probe to a depth
needed to locate a prey of depth class /' (min). Since Ploc
is based on the cross section 1 cm below the lop of
Fig. 6. A-I). ITte components of handling time related lo prey density and bur. mg depth. Where handling time is not shown we have no ob­
servations. Four density cla-ses were distinguished, iii I 2 • 24 m ' ll i: 44 + XX m ; 111). 175 + 262 m : (III) and 350 + 437 m " (IV i. and also
four depth classes: (It I em ill. 2 + 3 cm ill): 4 + 5 cm (llli: 6 + 7 cm i IV I. See Table 2 lor staiistic.il analysis.
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