Náttúrufræðingurinn - 1989, Blaðsíða 30

Density and species composition was
similar on the two mudflats and numbers
changed at the same time. However, the
Knot was much more abundant in Kópa-
vogur and the Black-tailed Godwit was
more abundant in Grafarvogur. We be-
lieve this may reflect differences in food
availability at the two sites.
It is of interest to see how the small
waders - Ringed Plover, Knot, Purple
Sandpiper and Dunlin - use the tidal flats.
In SW Iceland the resident Purple Sand-
piper is only absent from the intertidal
zone during the breeding season, the oth-
er three species are migratory. Though
present in the general area, Purple Sand-
pipers do not use the mudflats in summer
through early winter. They start feeding
on the mudflats on mild days in January-
February, increasing their usage to a peak
in April. Their numbers start declining
when the three migratory species arrive in
late April and Purple Sandpipers are all
but gone when these peak in May. In
spring there is a succession of Calidris spe-
cies using the mudflat, maritima - canutus
- alpina, all feeding on rather similar
foods and dividing the resources at least
partly on a temporal basis. Only the Dun-
lin uses these tidal flats in mid or late sum-
mer to the same extent as in spring. The
main food of the Calidris species while on
the tidal flats are chironomid larvae (Cn'-
cotopus variabilis), small amphipods and
also small molluscs (expecially Knot). To
what extent this usage pattern reflects the
availability of the main prey species re-
mains to be studied.
The core of the Black-tailed Godwit
breeding area in Iceland is in the southern
lowlands, where tidal mudflats are very
limited in area. In the south Godwits go
directly inland in spring and feed in shal-
low freshwater wetlands, mainly on large
insect larvae (Trichoptera, Tipulidae). In
Iceland the staging by Black-tailed God-
wits on estuaries may have developed re-
cently, perhaps during this century, as a
result of breeding range expansion into W
and N Iceland. The timing of the return
migration may be an adaptation to food
availability inland, where most aquatic in-
sect Iarvae have emerged by mid summer,
and thus not related to events in the inter-
tidal zone.
The Whimbrel did not use the tidal flats
or other coastal areas either in spring or
autumn. The Golden Plover was the only
species that used the tidal flats more in
late summer and autumn than in spring.
Both these species feed inland on berries
and large arthropods. More specialized
feeders, such as the Calidris species, may
have difficulties in finding enough food on
the mudflats in autumn.
Oystercatcher, Purple Sandpiper, Red-
shank and Turnstone winter in numbers in
SW Iceland, but only the first two used
the mudflats in mid-winter. Feeding meth-
ods and temperature probably determine
what species can winter. Most Icelandic
tidal mudflats freeze over, at least during
some days in winter. When the mud cools
the invertebrates slow their movements
and it becomes more difficult for the
shorebirds to find them (Pienkowski
1983). The only shorebird species able to
winter are those that can efficiently utilize
tidal habitats other than mudflats, i.e.
rocky shores and open beaches with a rich
driftline of rotting seaweed.
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