Náttúrufræðingurinn - 1993, Page 43

NE-Iceland. The winter observations were
made incidental to visiting Gyrfalcon ter-
ritories to collect pellets and record occu-
pancy (Nielsen & Cade I990a,b). In win-
ter I travelled extensively on foot or on
skies through lavafields, heathlands and
shrublands. I noted down all Rock Ptarmi-
gan I saw and habitat type. I recognized 3
types of habitats, namely heathlands,
shrublands and lavafields. I also censused
regularly a 4.5 km2 plot in Hofstadaheidi
for ptarmigan during winter and spring
1983-1984. Special attempts were ntade to
establish when the ptarmigan cocks be-
carne permanently territorial in spring.
In winter most ptarmigan were seen in
shrublands or lavafields (Table 1, Fig. 1).
Very few were observed in the heathlands
(Table 2). In spring 1984 males estab-
lished territories all over the heathlands
synchronously on 28 April (Fig. 2). Tliis
happened 24 April 1985, 22 April 1987,
25 April 1988, 20 April 1990 and 22 April
1991. The influx out onto the heathlands
had started somewhat earlier and sporadic
territorial activity or sntall flocks were
observed already on 17 April 1984 and 10
April 1985.
Gardarsson (1971) has described habitat
selection and travels of the Icelandic Rock
Ptarmigan. Most ptarmigan leave the
heathlands in fall and migrate to alpine
areas. These travels are both local and
regional, the cohorts taking part in longer
iravels are mainly females and juvenile
males. The proportion of males during the
open season in NE-Iceland was 69-71%
(Gardarsson 1971, Nielsen 1986). Winter
snow drives the ptarmigan from the moun-
tains in mid-winter and they gather, de-
pending on snow cover, in wind exposed
areas or shrublands. Only some few birds,
mainly cocks, are found in the heathlands
in winter. It is not known when the nti-
grants return in spring but influx of birds
out onto the heathlands starts in early
April (cf. Gardarsson 1971, this sludy).
The Gyrfalcons start courtship feeding
at least 15 days before laying of the lst
egg (Nielsen & Cade 1990a). So most of
the Gyrfalcon population in NE-Iceland
has started preparation for laying long
before the ptarmigan cocks become terri-
torial. Observations of prey remains left
by the fentale during courtship and incu-
bation show almost only ptarmigan
(Nielsen 1986), but pellet analyses indi-
cate that the male is also taking other prey
(Nielsen & Cade I99()b). It is clear frorn
pellet studies that ptarmigan become in-
creasingly important as food items in
March and April compared with mid-win-
ter and this happens before the cocks be-
come territorial (Fig. 4). I believe that the
reason for this change in vulnerability re-
sides with the ptarmigan and is explained
by:
1) Increasing density of ptarmigan as
birds return from wintering areas in other
parts of Iceland.
2) Changes in habitat selection. Ptarmi-
gan presumably become easier prey at the
beginning of April as they switch from
mountains and shrublands and move onto
the heathlands.
If the timing of the Gyrfalcon’s breed-
ing season is an adaption to the life cycle
of the ptarmigan, what is then the critical
time for the falcons? The ptarmigan hunt
is reliable during the territorial phase last-
ing from late April through mid-June in
Iceland. The food requirements of the
Gyrfalcon family peak 2-3 weeks post
hatch (Poole 1986). Falcons hatching their
young before 30 May are past this period
when ptarmigan become scarce ar'ter mid-
June and the falcons are forced to turn to
other prey. Late nesting falcons residing in
wetland areas and on the coast can ntake it
through by hunting waterfowl and alcids,
but falcons nesting up in the heathlands
must depend on ptarmigan all through the
summer and starvation of young has been
documented. Another important period for
the Gyrfalcon is when the young leave
their parents. The optimum time for this
should be in early August when the ptar-
migan hunt, now directed at the juveniles,
is improving after the annual summer low.
The fledgling falcons stay with their par-
ents for at least 3 weeks post hatch. Ac-
cording lo Fig. 3 most of the Gyrfalcon
young are making this important step to-
ward independence in early August.
37