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

Tahle 2. The intake rate ol < lystereatcheis feeding on eight prey
species during the motet halt of the year (October-Match) and sum
met i vpril-Sepiemhen. The average intake rates (nig AFDW B**)i
SD relei in different studies i sample si/e indicated) summarized hv
Zwarts el,./. m press) A two-way analysis ol variance shows thai
there was i significai iiiiieivenee between intake rate in winter and
summer IR ; = 0.145; p < l).(K) 11, but that the intake rale did not ditler
between the prey species |R- = 0.04; p = 0.58).
wauer MMUM '
X SD V X syi v
Macoma batthka (1 2.42 66 9
Siniliiitiliirti pinna 1.72 0.61 8 2.2U 1
1 ma edule 2.19 12 iss ''I. fi
Mya armaria 2 SJ 1 1
Msulus ,;liihs (XI 0.61 27 250 1 23 12
erricohr 1.74 li 1 •
FOOD SUPPLY HARVESTABLE BY WADERS
0.91
8
Arenicola marina ii 3.10 1
hum ma linnrea 1.40 1 o
all species 1.92 0.71 52 2.ftl 0.92 18
Scrobicularia and Mya, ihat allowed their depth to be
monitored continuously, revealed that they scarcely
changed their position. Even more importantly, when
attacked, they did not move their position but withdrew
their foot and siphon(s) within the shell and closed the
valves firmly (unpubl. data). Burying depth of the four
bivalve species studied is thus a good measure of their
accessibility to waders.
Clearly, an Oysiercaicher with a bill length of 7 cm.
cannot take Scrobicularia living at 8 cm or more, but
where exactly is the limit between prey that have
reached the depth refuge and those that are still in
danger? There are two uncertainties in answering this
question. First, the burying depth of bivalves has been
defined as the distance between the surface of the
substrate and ihe upper edge of the shell, but the birds
must probe more deeply, cither to grasp the prey before
being able to lift it to the surface or to eat the llesh in
situ. Second, the probing depth of birds mas vary. The
probing depth exceeds the bill length when birds probe
up to their eyes in mud. although usually the probing
depih is less than the bill length. For instance.
63
Ha. II. Seasonal variation in the accessibility Ol Scrobicularia
plana. (35 mml over 4 years. The proportion of the population living
in the upper 4.6 and 8 cm is shown.
Oystercatchers feeding on Macoma or .Si rohu idaria
probe their 7.5 cm long bill on average 3 to 4 cm into
the substrate | Hulscher 1982, Wanink & Zwarts 1985).
and the 2.7 cm long bill of Sanderling Calidris alba is
pushed only 2 cm into the substrate on average
((ierritsen&Meiboom 1986).
Three studies have exactlv deiermined the depth at
which prey are taken: Sanderlings preying on three
crustacean species. Curlews Numerous arquata
feeding on Mya and Oystercatchers on Scrobicularia.
Captive Sanderlings were offered fro/en isopods or