Náttúrufræðingurinn

Volume

Náttúrufræðingurinn - 1995, Page 41

Náttúrufræðingurinn - 1995, Page 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
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