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

lahle- I. Numenius arquala. Search rate predicted bj ihe 'und
method' i explanation in the text) compared io the search rate .is derived
from ihc product ot pace length (as measured Irom the prints
in the muill anil pace frequency. All dala were collected for one
Curlew observed an entire low water period
Search rule
Grid method icin s ')
Pace rate x pace lengih (cm s ; i
pace rate (pace s'')
pace length (cm)
VERSATILITY OF CURLEWS FEEDING ON NEREIS
Mean ± SE »
22.6*0.02 180
22Ja
1.8*0.02 63
13.6 ±0.68 20
length or size of colour ring as a reference), (4) duration
of 4 types of activity io die nearest second: handling
time (the Curlew catches and swallows a prey; if
handling is preceded by probing this is always a part of
the handling lime): probing time ithe Curlew probes a
greater part of iis bill into the substrate without finding
a prey): searching time (feeding time without probing
and handling: single pecks (duration < I s) are considered
as part of the searching limci: non-feeding time
(preening, resting and aggression, bin kleptoparasitism
is considered as part of the feeding time).
Additional information was collected for Cut lew s
50
4;;
30
ST 20
10
observer
ABCD
DDII
- 4 - J 3 - 2 - 1 0 li JL
1 2 3
deviation from observer E (cm)
Fig. 1. Comparison between observers who estimated lengths of
Vi reis diversii aim- taken by a Curlew. Frequency distributions show
agreement of 4 different observers t A: n = 32: B: n = 32: C: n • 56;
D: n = 111 with 0*M H worms were *' lo |4cm.
319
feeding within the grid of 5x5 m plots around the
hides, viz. the habitat from which the prey was taken
(water film or dry surface) and coverage by water film
(estimated horn the tower as '< of the surface in the
5x5 mplot visited).
All observations were pooled in 3 min periods. The
analyses for this paper are confined to 3 min periods
during which < 20% of the total biomass of prey taken
by male Curlews (bill lengih < 12.5 mm) consisted of
prey other lhan Nereis. After these 2 restrictions a total
feeding time of 6335 min was left for analysis.
The search rate was estimaied by measuring the
pace frequency (time to make 50 Steps during searching)
and pace length (from prints in the mud). The
search rate of Curlews feeding within the grid of 5x5
m plots could be estimated from the path length as predicted
with the grid method (Reddingius et al. 1983).
The path length equals the number of grid crossings
multiplied by TC/4 times the side lengih of the plots (in
our case 5 m). On one day both methods of measuring
search rate were used simultaneously and showed corresponding
results (Table I).
1 he estimation of worm size by most observers differed
little: 39*3! ol the worms taken b) a Curlew were
6 8 10 12 14
estimated worm length (cm)
Kig. 2. Nereis div, rskohtr Worm size measured in the laboratory
i maximal length of creeping worms, see Hsselink & Zwarts 198
a function of estimated worm size, averaged for 5 observers using a
lelcseopc al a distance of I00 m. Worms were held near the bill of 2
smiled Curlews iO: bill lengih 11.7 cm. n = 136 and •: 14.7 cm.
n = 74).