Náttúrufræðingurinn - 1980, Síða 95

Williamson, K. 1959. The September drift-
movements of 1956 and 1958. Brit. Birds
52 (10-11): 334—377.
— 1961a. Aspects of autumn migration,
1960. Bird Migration 1 (5): 218—234.
— 1961 b. Drift migration at Fair Isle. Bird
Migration 2 (1): 51—53.
Witherby, H. F., F. C. R. Jourdain, N. F. Tice-
hurst & B. W. Tucker. 1938. The hand-
book of British birds. Vol. II. H. F. & G.
Witherby. London. xiii + 352 bls.
S U M M A R Y
Willow Warblers and
Chiffchaffs, vagrants in Iceland:
Their frequency of occurrence,
and times of arrival
by Hálfdán Björnsson,
Kvísker in Öraefi, A.-Skaft., 785 Fagurhólsmýri,
lceland.
Willow Warblers (Phylloscopus trochilus),
and Chiffchaffs (P. collybita) are among the
commonest passerine accidentals visiting
Iceland. The present paper examines the
frequency of their occurrence, and times of
arrival at the farm of Kvísker in SE. Iceland.
The surroundings of Kvísker are ideal for
receiving vagrant bird species from the
Palaearctic region. The farm is located, just
south of the large Vatnajökull ice cap, sur-
rounded on other sides by extensive fluvial
plains and moraines, formed by the retreat-
ing Vatnajökull. Vegetation is sparse in this
region, woody areas, which are those most
frequented by passerines, being of very
limitcd distribution.
Three places (a heathland, a birch scrub
area, and an ornamental garden) by Kvísker
attract considerable numbers of vagrant
passerines at certain times of the year. The
small size of these areas make them easy to
survey from day to day. Thus, influx of new
visitors is easily noticed, especially as num-
bers are normally very small at any one time.
The present paper contains data collected
over 36 years, 1943—1978. An effort has
been made to collect as many of the warblers
as possible especially since the species are
difficult to separate in the field. During this
period 59 Willow Warblers were seen, of
which 30 (51%) were collected. Similarly 188
Chiffchaffs were observed, of which 111
(59%) were collected. The collected speci-
mens are preserved as study skins at the
Museum of Natural History, Reykjavík.
Experience of both species now allows me to
separate the species in the field with a high
degree of accuracy, using leg and breast
colour, which are always darker in Chiff-
chaffs.
Most of the warblers have been observed
in late summer — autumn (August—
November). Willow Warblers have never
been seen in spring, or early summer, and
Chiffchaffs only on four occasions, one bird
each time (29. 5—10. 6. 1960, 4. 6.-2. 7.
1967, 12,—14. 4. 1974, 18. 5. 1974).
Fig. 3 shows the annual variation in
numbers during the period of study. Three
peaks are particularly noticeable, i.e. in
1950, 1961, and 1976, with two minor peaks
in 1959, and 1968. One year (1970) is un-
usual in the way that no Chiffchaffs were
observed but 5 Willow Warblers, one of the
highest numbers in any one year.
Fig. 4 shows the dates of appearance in the
area, data from all years combined. There is
a striking difference in the arrival of the two
species.
The difference in numbers of these closely
related species, seems curious in view of: a)
The much greater abundance of Willow
Warblers in the world, and b) The more
northern distribution of that same species,
even though their breeding distributions are
broadly similar (see Figs. 1 and 2).
From literature resources (the collected
material has still to be identified to sub-
species) tentative interpretations are put
forward as to the most likely subspecies
making up the Icelandic material. These are
summarized as follows: Phylloscopus t. trochi-
lus: Breeding distribution too southerly,
leave breeding grounds early. Unlikely to
visit Iceland. P. t. acredula: Northern breed-
ing distribution, migrate later than birds of
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