Náttúrufræðingurinn - 1995, Side 40

■ SUMMARY
Rock Ptarmigan censuses in
NoRTHEAST lCELAND 1981 TO 1994
I have monitored a Rock Ptarmigan (Lagopus
mutus) population in NE-Iceland since 1981 as
a part of an ongoing study on the population
ecology of the Gyrfalcon (Falco rusticolus)
(Nielsen 1986). The heathlands in NE-Iceland
are important breeding grounds for the Rock
Ptarmigan. Most ptarmigan leave this habitat
in fall and migrate to alpine areas. These trav-
els are both local and regional (Arnþór
Garðarsson 1971, 1988). Only some few birds,
mainly cocks, are found in the heathlands in
winter. The ptarmigan reoccupy the breeding
grounds in April. This paper deals with
changes in spring density of Rock Ptarmigan
in NE-Iceland in 1981 to 1994, and mortality,
sex and age ratio and breeding success of the
ptarmigan.
The main measure of Rock Ptarmigan num-
bers was the annual spring census for lerrito-
rial cocks (Fig. 1). I had six census plots rang-
ing in size from 2.4 to 8.0 km2, total area was
26.8 km2 (Fig. 3). Two of the plots could be
classified as heather heaths, the olher four as
Betula nana heaths. Each plot was censused
once during late May each year. Average cen-
sus day was 22 May, the earliest date was 7
May and the latest 6 June. The census was
most often conducted by two observers work-
ing together and covering the census plot in
the late afternoon (time 17.00-24.00) or early
morning hours (time 05.00-10.00). Position of
all live territorial cocks was plotted on a map
of the census plot (scale 1:15,000), as was the
location of all “kills” found. Number of cocks
arriving on territories in spring was found by
adding the number of new cock kills to the
number of live birds.
A ptarmigan “kill” consists of plucked
feathers left behind by the predator or scaven-
ger, and with them are sometimes found parts
of the skeleton or digestive tract of the bird
(Fig. 2). I tried to collect only new kills, and
discarded kills from the previous winter or
fall. Only 26% of the kills found could be
sexed. Sexing was most often based on col-
oured contour feathers found. The hens moult
before the cocks so using feathers would give a
biased estimate of the actual sex ratio of the
kills. To bypass ihis problem I took all kills as
unsexed and assumed that cocks were 73%.
This was the sex ratio of ptarmigan killed by
150
Gyrfalcons during April and May in 1982 to
1985 (sexing was based on the length of the
humerus; Nielsen 1986). I conclude that this is
justifiable as 84% of thc kills found on the
census plots and identified according to preda-
tor were made by Gyrfalcons.
I tried to determine the predator of each
ptarmigan found eaten on the census plots; if
there were no signs left behind by the predator
(loose feathers, pellcts or tracks), I identified
them by how the prey had been treated. The
Gyrfalcon usually plucks off all primaries and
rectrices, and most of the body feathers. If
skeletal parts are l'ound, they are often broken,
and the keel of the sternum bitten off. The
Raven (Corvus corax) leaves the primaries in-
tact, but will pluck contour feathers. If skeletal
parts are found, Ihey are neatly cleaned and
not broken. The Arctic Fox (Alopex lcigopus)
bites the quills of the larger feathers and oftcn
breaks them; frequently the fox will leave the
whole set of primaries attached together but
with their quills bitten off, or big chunks of
skin with contour feathers. Kills consisting of
only contour feathers or contour feathers and
rectrices were placed in the group “unknown
predator”. This is because I could not be 100%
sure that this was a Raven kill as Gyrfalcons
will sometimes not pick off the primaries, and
also a fox carrying a ptarmigan back to the den
leaves nothing behind at the kill site but some
few feathers. Whole intact ptarmigan were
classified as “accidents”.
It is possible to distinguish between first
year plarmigan and older by colour of primary
#8 and #9 (Weeden and Watson 1967). To de-
rive spring age ratios, I used all kills found out
in the field, also wings with intact primaries
found at Raven and Gyrfalcon nest sites, and
in 1994 birds trapped for ringing.
For assessment of ptarmigan young pro-
duced, I used mainly the census plot Hóll and
adjacent heaths on Tjörnes peninsula. The
covey census was done at the end of July and
the beginning of August by observers covering
the area on foot and trying to find as many
ptarmigan as possible; we l'Iushed the young to
counl them. During the course of other field
work al the end of July and the first week of
August, we noted the size of all family groups
encountered on the Gyrfalcon study area (cl'.
Nielsen I986 page7).
Total numbers ol' live ptarmigan cocks ob-
served on the census plots in 1981 to 1994 was
1929 birds. Average density of live cocks was
6.3 /km2, maximum 27.9 /km2 and minimum
i