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

shell length (mm)
FIR. 3. Peringia. Size frequency distribution of 450 individuals
found in nine gizzards of Knot collected on Vlieland, Dutch Wadden
Sea. during the period 15 August to 24 October 1982. Dark
shading: snails for which ihe lengih could be determined. Light
shading: crushed snails for which width at the base, but not lengih.
could be measured and for which length was derived from ihe relationship
between width and length in undamaged snails isee righl
inset), l-eli inset ShOWl thai in more than half of the snails width nor
length could be measured (IIKK^ = 10221.
esiimates of shell length give values of average flesh
w eight of prey taken by Knot which differ by 50%, due
io the fact that llesh weight aUometrically relates to
shell length with a power of 2.14 (Fig. 21.
Prey selection
According to our observations, not all prey taken from
the mud were actuallv swallowed: 63 of the 84 Macoma
brought to the surface, were ingested, and only
one in four of the Cerastoderma. Two shrimps Crangon
crangon 5 cm long were handled for several seconds
but finally not eaten. Rejection of the latter two
WHY KNOT TAKE MEDIUM-SIZED MACOMA
273
prey species was unexpected as Cerastoderma is a major
prey elsewhere (Goss-Custard el al. 1977b. Boere
& Smit 1980. Nehls 1992) and faecal analysis showed
that Knot can \'ccs\ on shrimps 43 to 63 mm long (own
unpubl. data).
Both the Held obsct vations and faecal analysis indicate
that Macoma was the main prey and that Cerasioderma.
Nereis and Comphium were rarely taken
(Table 2). Even though very few of the prey seen eaten
were too small to identify. 360 Mud Snails Peringia 11
to 3 mm) and six very small bivalves 11 to 2 mm) were
found in the faeces. However, all were intact and contained
flesh, so they must have been accidentally
scraped from the surface while the faeces were being
collected.
One of the Knot observed produced 15 faeces during
21 min of continuous observation. All but one of
the intervals between faeces depositions varied between
58 and 88 s. The exception was 174 s long, suggesting
that we missed one faecal packet during one
interval. If so. Knot produced one faecal packet per
80s. Therefore the faeces that were collected reflected
a period of 20 x 80 s. or 26 min, feeding. Since the lotal
time spent watching the birds was 83 min. the number
of prey seen taken should have been 83/26 or 3.2
times as large as the number found in the faeces, and
Table 2. Prey taken by Knot during 82 min (left column i and prev
found in 20 faeces from the same individuals t right column I.
W'leuuiier Hats. Dutch Wadden Sea. 4 August 1983. Among the
prey taken, three were so small that the species could not be identified,
The number ol prev in the faeces was determined by counting
unique fragments (jaw S ol the Ragworm Nereis, second antennae of
•niiirn. umbos of Macoma and Cerastoderma); since each animal
has two such fragments, the numbers found were divided by
two. Animals accidentally scraped from the surface arc indicated in
parentheses
v.. prey Ho pies in faeces
Macoma >9 mm 63 19
Cerastoderma 12 mm 1 0.5
Nereis I 0J
Corophium 3 05
Macoma spat 1 I2i
• derma spa! » ,4,
Peringia J (360)