Fróðskaparrit - 01.01.2001, Page 130

Fróðskaparrit - 01.01.2001, Page 130
134 BREEDING DISTRIBUTION AND NUMBERS OF WREN (TROGLODYTES TROGLODYTES) IN THE FAROEISLANDS siskin (Carduelis spinus), but surprisingly few pairs of breeding wrens. This contrasts with the Icelandic wren (Troglodytes t. is- landicus), another endemic, sedentary is- land subspecies, which has attained high densities of breeding pairs in several of the relatively recent plantations of coniferous and some decidious trees, e.g. in Skorra- dalur and Hallormsstaðarskógur (pers. obs.). Elsewhere in Iceland the wren breeds in many of the once much more extensive native woods and shrubs of birch (Betula pubescens), but also on cliffs and gullies as well as in lava fields. The Faroes has not had any native trees or higher shrubs since the wren colonised the islands (i.e. in post- glacial time), which leads one to speculate whether the lack of response to the planta- tions might have a genetic component to it. Genetic aspects have been invoked in dis- cussions of the success of biological inva- sions (Williamson, 1996). All the small islands (< 11 km2), except Koltur for some unknown reason, exhibit relatively high breeding numbers of wrens (Tab. 1). This negative correlation between numbers and island size has previously, on the basis of the survey data from 1981, been associated with the favourable shore- line length to area ratio, and as the wrens disperse to, and defend winter-territories along the shores, small islands can hold rel- atively larger numbers. Furthermore, on a small island the distance to the shore is in- variably short and there are fewer inland, mountainous areas unsuited for breeding than on larger islands (Bengtson and Bloch, 1983). The present study suggests alterna- tive, non-exclusive explanations to the small-island effect. The smaller islands usually have an abundance of cliffs, gjáir and generally craggy terrain, and of breed- ing seabirds (e.g. Mykines, Skúvoy, and Fugloy). The puffin-wren association (see e.g. Williamson and Boyd, 1963) is con- spicuous also in the Faroes, notable exam- ples are Lamba on Mykines, Urðin Mikla on Fugloy, and Urðin on Nólsoy where wren densities are particularly high; some- times 5-10 singing males within a couple of hundred meters. On Koltur seabirds are abundant but wrens relatively few in num- ber. Trøllhøvdi supports dense seabird colonies and the wren breeds there, but in unknown numbers (Bertelsen, 1965; Bengtson and Bloch, 1983). Also, the hu- man populations on these islands are small and the single (two on Fugloy) settlement consists of relatively few, often old-style, sod-roofed houses and lots of stone walls. Such settlements often hold several pairs of wrens in contrast to the expanding commu- nities of the larger islands, where modem buildings and wire fences seem less attrac- tive to the wrens. However, settlements of the former kind also occur on the larger is- lands (e.g. Gjógv, Elduvík, and Funningur on Eysturoy, Bøur on Vágar, Skarvanes and Húsavík on Sandoy, others on Kalsoy, Kunoy etc.) and all these small settlements, irrespective of island size, may or may not have a good number of breeding wrens. The presence of cats has for a long time been considered a menace and threat to wrens at the settlements (see Introduction), for instance by Andersen (1898; 1905) with regard to Nólsoy and Mykines and today in Sumba on Suðuroy (Aksal Poulsen, pers.
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