Náttúrufræðingurinn

						0.6 /km2. A total of 367 kills were found on the

census plots. Most of the kills were made by

Gyrfalcons (Table 1). 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 study plots varied with 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

other ptarmigan census plots in Iceland 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 (Fam= 1.696, 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 bascd on kills found during

censuses was on the average 12% for cocks

(range 0-39%) and 5% for hens (range 0-17%)

(Fig. 5). This is assuming equal sex ratio in the

population in spring and 73% of 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 similar results; of 819 food items

found during 1981 to 1985 on the Gyrfalcon

study area in NE-Iceland 304 were adult ptar-

migan (37%) (Nielsen 1986). Some of the

ptarmigan were undoubtedly scavenged by the

Raven, but I 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 ralio year t+1 (Table

5) (dependent variable) and population change

gave a significant relationship (F, 18=16.994,

p=0,0006). Population ehange was taken 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

Ist 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 between years were re-

flected both in mortality of adults and juve-

niles, but 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 (i=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 (s= 12,58, 33-71%). Most of

the juvenile mortalíty 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).

PÓSTFANG HÖFUNDAR/AuTHOR'S ÁDDRESS

Ólafur K. Nielsen

Náttúrufræðistofnun Islands /

Icelandic Institute of Natural History

Pósthólf/Box5320

IS-105 Reykjavík

Iceland

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