Náttúrufræðingurinn

Volume

Náttúrufræðingurinn - 1995, Page 40

Náttúrufræðingurinn - 1995, Page 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
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