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

Prey switching
Waders feeding on tidal flats, face a huge variation in
the relative occurrence of different prey species, this
being partieularlv large when the fluctuations in the
numbers of harvestable prey alone are considered. Waders
would be expected continuously to adjust their
diet as the food supply available varies. Goss-Custard
(1969. 1970a. b) was the first lo describe the reduction
in ihe prey accessibility associated with short-term
changes in an environmental factor and how the
waders responded. He found that Corophium did not
emerge from their burrows when the mud temperature
was below 6 °C and lhat Redshank then switched to
less preferred prey. Nereis and Macoma. these still
being available. Similarly. Smith (1975) showed that
when the temperature of the substrate dropped below
3 °C. Arenicola became inactive and Bar-tailed Godw
lis siarted to eat a smaller worm. Scoloplos armiger.
Consequently, the intake rate decreased at low temperatures,
and approached nil as the temperature came
close to 0 °C. More recently. Pienkowski (1983a. b)
found thai, as mud temperature decreased, small worm
species were less active al ihe substrate surface, lie.
fewer outflows of water from the hole) and ihe intake
rate of plover species decreased.
At a larger lime scale, variation in diet also occurs
as will be illustrated by six cases. (1) Blomert el al.
(1983) studied the prey selection of individually
marked Oystercatchers along the Frisian mainland
coast in July to October. The birds took Macoma and
Scrobicularia on a mudflat and Mytilus on a nearby
mussel bed. The females wilh bill lengths of 7.5 to 8.5
cm, took twice as many Scrobicularia as Macoma. In
contrast. Scrobicularia occurred in the diei of ihc
males (bill length 6.5 to 7.5 cm) as much as Macoma.
The long-billed birds thus seemed more specialized at
taking deep-living prey than the short-billed ones. The
expectation was that the birds would switch from
Scrobicularia to Mamma in late summer, since the
densit) of accessible prey would drop much more in
Scrobicularia than in Macoma (Figs. 7. 10, 11). This
was not the case, however. Instead, many males left the
mudflats and started to feed on the nearby mussel
bank. The result was that the total predation by Oystercatchers
on Scrobicularia and Macoma was higher in
late summer than in autumn. The intake rate of birds
feeding on the mudflats decreased from July to
FOOD SUPPLY HARVESTABLE BY WADERS
77
October. In contrast, ihe intake rate of mussel-eating
Oystercatchers was low in summer, but increased later
on. The birds switched from mudflats to the mussel
bank when Mxiilus provided a higher intake than
Scrobicularia and Macoma.
(2) The majority of Scrobit alalia larger than 3 cm
live out of reach of the Oystercatcher's bill (Fig. 11).
bui the smaller size classes burrowed less deeply, and
were even accessible in winter (Fig. 6). Habekotte
(1987), who studied Oystercatchers on the Frisian
island Schiermonnikoog in winter, found thai birds
taking Scrobicularia about 2(1 mm long were able to
achieve an intake rate sufficient for them to attain the
required daily consumption. The feeding rale strongly
depended on the density of Scrobicularia. increasing
from 0.2 to I prey min -1 within the range of 100 to 600
nr 2 at which Scrobicularia occurred. The majority of
these prey lived at 4 to 7 cm below the surface.
Macoma of suitable size occurred in the area at a
density of 70 to 120 prey nr 2 , but were hardly taken.
They were accessible, but Scrobicularia were
preferred above ihe smaller Macoma because ol their
greater profitability.
(3) Macoma. and to a lessei degree Nereis, were
the major prey of Oystercatchers on ihe modflats near
Schiermonnikoog in spring and summer (Bunskoeke
et al. 1996). Nereis were taken less during receding
tide than during low tide, ihe reverse being the case for
Macoma (de Vlas ei al. 1996). This change in diet may
be wholly attributed to a shift in the feeding behaviour
oi Nereis through the tidal cycle; they are filter feeders
in ihe burrow during the receding tide, hut feed on the
surface around the burrow at low tide (Esselink &
/watts 1989). The Oystercatchers switched from
Macoma lo Nereis during the summer (Bunskoeke et
al. 1996). Macoma did not start to increase their
burying depth before August (Fig. 7), hence this shift
in diet could not be explained by a reduced accessibility
of one of the prey. It is more likely that the
decrease in the body condition of Macoma from June
onwards (Beukema & de Bruin 1977. Zwarts 1991),
made them less profitable. Indeed, the intake rate of
Oystercatchers feeding on Macoma did decrease from
May-June to August (Bunskoeke etal. 1996).
(4) Boates & Goss-Custard (1989) studied Oystercatchers
wintering in the Exe estuary, SW. England.
They observed a switch in October from Nereis to