Náttúrufræðingurinn - 1995, Side 41

0.6 /km2. A total of 367 kills were found on the
census plots. Most of the kills were made by
Gyrfalcons (Table I). The ptarmigan popula-
tion was at low levels when the census started
in 1981, then increased in numbers to a peak
and then a subsequent decline (Fig. 4). The
difference in density between the low and the
peak years was on the average 6.3-fold for the
six plots (least 3.4- and greatest 9.0-fold) (Ta-
ble 2). The sludy plots varied willi regard to
ptarmigan density and there was a 5-fold dif-
ference in density between the best and the
worst study plots. The study plots did not show
a perfect match regarding population changes
(Table 3). Peak density was in 1986 on four of
my plots, on one in 1984 and one in 1987. Four
olher ptarmigan census plots in lceland gave
similar results (Table 6).
A total of 527 hens were located with
broods, mean brood size was 8.3 young/hen
(Table 4). Family size was relatively constant
between years (F13494=l .696, p=0.058; fami-
lies of 16 young or larger were not included in
the analyses). Every summer some hens were
observed without young during the covey cen-
sus on Tjörnes in late July, summing up for all
years they numbered 7.4% (n=269).
Spring mortality based on kills found during
censuses was on the average 12% for cocks
(range 0-39%) and 5% for hens (range 0-17%)
(Fig. 5). Tliis is assuming equal sex ralio in the
population in spring and 73% ol' kills being
males.
It is surprising how important a predator on
ptarmigan the Raven seems to be (Table 1).
Studies of food remains collected at Raven
nests gave sintilar results; of 819 food items
found during 1981 to 1985 on the Gyrfalcon
study area in NE-Iceland 304 were adull ptar-
migan (37%) (Nielsen 1986). Some of the
ptarmigan were undoubtedly scavenged by the
Raven, but 1 have found tracts in the snow
where a Raven killed and ate a ptarmigan. The
ptarmigan both ran and flew just before it was
caught and killed. Also I have two independent
eyewitnesses from this area of Raven depreda-
tion of ptarmigan.
Comparing spring age ratio year t+l (Table
5) (dependent variable) and population change
gave a significant relationship (F, 18= 16.994,
p=0.0006). Population change was takcn as to-
tal number of males on all plots arriving in
spring year t+1 devided by total number of
males arring in spring year t. Also included in
this analyses was data from Hrísey 1963 to
1969 (Garðarsson 1971). During decline years
spring age ratios of the ptarmigan population
were on the average 49% (range 37-62%), but
66% (60-81%) during increase years (Fig. 6).
To derive annual mortality of adult and ju-
venile ptarmigan I used the combined number
of cocks arriving on the census plots each
spring (Fig. 4), age ratio in spring (Table 5)
and mean brood size (Table 4). I made the as-
sumption that sex ratio was equal in spring (cf.
Garðarsson 1988), that mortality of hens from
arrival in spring to the end of July was 15%
(Fig. 5 and estimate), and that 7% of females
were without young in late summer. The mor-
tality of the juveniles was calculated from the
lst of August to arrival on the breeding
grounds in spring (c. 20 April), and that of the
adults from arrival on breeding grounds in
spring to arrival next spring. I also used data
from Hrísey in the analyses (1963 to 1969).
Changes in density bctween years were re-
flected bolh in mortality of adults and juve-
niles, bul expecially juveniles. Mortality of ju-
veniles was always high, on the average 87%
during decrease years (,?=4.13, range 79-
93%), but 73% during increase years (í=4.58,
65-79%). Mortality of adults was lower and
more variable, on the average 62% during de-
crease years (s=7,87, 47-78%), and 54% dur-
ing increase years (í= 12,58, 33-71%). Most of
the juvenile mortality hits when the birds have
left the breeding grounds in late summer and
before they return next spring.
Three types of ptarmigan population in-
dexes for combined study plots are compared.
One based on unweighted total sums of ob-
served males (live + kills) on study plots, one
weighted with respect to density and one with
respect to numbers. These three methods all
give essentially the same results (Fig. 7).
PÚSTFANG HÖFUNDAR/AöTHOR'S ADDRESS
Ólafur K. Nielsen
Náttúrufræðistofnun Islands /
Icelandic Institute of Natural History
Pósthólf / Box 5320
IS-105 Reykjavík
Iceland
151