Náttúrufræðingurinn - 1995, Síða 103

within ptarmigan breeding habitat and that at
least 1 km separated different sites. A map and
a compass were used to locate the sites out in
the field.
Point counts for ptarmigan cocks were done
in early and late May. The counts were only
conducted in the early morning hours (06:00-
10:00) and in the late afternoon (17:00-22:00).
During the middle of the day the territorial
cocks rest and are difficult to spot. A count
was conducted for exactly 20 minutes at each
site. All cocks seen or heard within that period
were counted and the distance to them meas-
ured with a range finder. The hikes between
the count point sites were used as transects.
Two parameters were measured for each cock
sighted during transects: I. sighting distance
(r.) measured with a range finder; 2. sighting
aiigle (q). Using these parameters perpendicu-
lar distance of the bird from the transect line
was found using the formula: y.= r. x sin q.
The length of the transect was measured from
a map. Only cocks within 500 m of the transect
line (perpendicular distance) were used in the
analyses. The program DISTANCE was used to
estimate density and confidence interval
(Laake et al. 1991). The results from the
transects were used as an unbiased estimator
of how well the point counts reflected density.
The vegetation studies were done in June-
August. Six randomly placed transects were
laid out wilhin a 500 m radius of each count
point site. Each transect was 25 m long. Meas-
urements included cover of 11 plant species
(Table 1) and unvegetated areas under the
measuring tape, and frequency of 16 species or
plant groups (Table 1) measured in three
50x50 cm frames on each transect; the frame
was divided into 25 small 10x10 cm squares.
To have a 100% frequency, the plants had to
be regislered in all 75 small squares on the
transect. Also measured was the height of the
vegetation at 5 cm intervals along the 25 m
transect. The plants measured are either im-
portant food for the ptarmigan or dominant
species in the communities and some are botli.
Average figures for each plant species for the
6 transects on each point count site were used
to describe the vegetation of the site. Not in-
cluded in the analyses were some few rare di-
cotyledons, all monocotyledons, mosses and
lichens.
The data matrix of 27 plant variables x 36
sample points was analysed by the ordination
technique of detrended correspondence analy-
ses (DECORANA) with no downweighting (Hill
1979a) and by TWINSPAN classification (Hill
1979b). Percentage cover and frequency val-
ues were arcsine transformed.
A total of 94 ptarmigan cocks were regis-
tered during the point counts (average 2.2
cocks/site, range 0-9, .v 2.52). The cocks
ranged up to 750 m from the sample points but
most were within 100-400 m (Fig. 3). As rela-
tively few birds were observed during the
point counts, no attempt was made to convert
these numbers to density figures. These raw
figures were used when comparing ptarmigan
abundance and vegetation, under the asump-
tion that they reflected density. The transect
censuses were done to see if this presumption
was valid. Total length of transects was 162.6
km and 234 cocks were observed. Densities
varied according to areas, being highest on the
coastal heathlands and declining inland (range
0.8^t.8 cocks/km2). Linear regression of the
number of cocks observed in point counts on
density derived from transects gave a signifi-
cant relationship (Fig. 4), and the slope of the
regression line was not significantly different
from 1 (1.38±0.74). According to this the
point counts should be a usable index on den-
sity.
Table 1 shows average values and range for
the plants recorded at the sample count sites.
Because of lack of time vegetation analyses
were not done at six of the count point sites
and they were therefore not.used in the vegeta-
tion analyses. The most common shrub and
heather species were Empetrum nigrum and
Betula nana, the most common herbs were
Bistorta vivipara and Thalictrum alpinum.
The results ol the site ordination is shown in
Fig. 5. The closer the points are in the graph,
the more similar are the sites with respect to
vegetation composition. The points arrange
themselves fairly wcll in accordance with geo-
graphic location, with coastal sites and the
sites furthest inland at the opposite ends of the
spectrum. In Fig. 6 some selected variables are
superimposed on the ordination scattergram.
The environmental factors responsible for the
variation according to axis 1 of the ordination
seem to relate to climate and grazing pressure.
It is windy and wet on the coast and precipita-
lion declines as we go inland. Grazing by
sheep is heavier on the lower lying heathlands
than inland. These factors are reflected among
other things in the cover of unvegetated sur-
faces and the height of the vegetation (Fig. 6).
It is not perfectly clear what environmental
factors account for the variation shown by axis
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