Náttúrufræðingurinn - 1972, Page 84

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NÁTTÚRUl7 RÆÐINGURINN
Lewis, E. 1938. In search ol the gyrfalcon. An account of a trip to northwest
Iceland. London.
Manniche, A. L. V. 1910. The terrestrial mammals and birds of northeast
Greenland. — Medd. om Grönland 45: 1 — 199.
Stevens, E. 1953. Laggard. London.
Wayre, P and G. F. Jolly. 1958. Notes on the breeding of the Icelandic
gyrfalcon. — Brit. Birds 51: 285—90.
Witherby, H. F. el al. 1958. The Handbook of British Birds. Vol. III. London,
S U M M A R Y
Observations on nesting gyrfalcons (Falco rusticolus)
in the Lake Myvatn area in 1960 to 1969
by
Sven-Axel liengtson,
Department of Animal Ecology, Zoological Institute,
University of Lund, S-223 62 Lund, Sweden.
Nesting gyrfalcons on about 700 km2 near Lake Myvatn, N.E. Iceland,
were studied intermittcntly during May to September 1960 to 1969 (except
1967). Population size, breeding season, and nesting success are described. Tlie
census work was restricted to old and well known nest sites, lience population
figures may fall short of the true number. Each year 2 to 4 pairs were known
to breed except in 1968 and 1970 when only one pair bred (Table 1).
Reproductive performance and population size of gyrfalcons are thought to
be related to fluctuations in the density of prey populations (Dementiev 1951,
Cade 1960). The Icelandic gyrfalcon population seems to fluctuate consider-
ably, the fluctuations being associated with the ten-year cycle of the ntajor
prey species, ptarmigan (Lagopus mutus) (cf. Guðmundsson 1960). Extensive
non-breeding of gyrfalcons occurred in 1968, a year of low ptarmigan numbers,
and Stevens (1953) failed to find breeding gyrafalcons in 1948, also a year of
low ptarmigan numbers.
Since nests with eggs were only rarely found, the date of hatching was
estimated from the date of fledging (Fig. 1). The broocl size when the young
were about one week old was established in 16 cases and avcraged 3.4 young
(range 2—5). From 12 nests 39 young liatched ancl of these 29 were fledged
(i. e. 74% or 2.4 young per nest). In two of the above nests the complete
broods disappeared and may have been taken by man. If these two broods
are excluded, the fledging success was 94% or 2.9 young per nest. Of 13
fledged young 12 remained in the vicinity of the nest site after one week,
and after two weeks 10 remained. These observations indicate that there was
little mortality both among the nestlings and among juveniles immediately
following fledging.