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The breakdown of the transhumant grazing system in the Spanish Pyrenees has led to a severe decline in the area of pastures. However, in the high mountain zone there are still large areas of species-rich grasslands. The aim of this study was to assess the oviposition preferences of the shrub-feeding Blue-spot hairstreak, Satyrium spini (Denis & Schiffermüller, 1775), in montane common pastures in the Spanish Pyrenees and recommend a way of managing these grasslands that favours this species. Our study showed that females of S. spini laid their eggs on Dwarf buckthorn (Rhamnus pumila Turra) and Alpine buckthorn (R. alpina L.), which are novel host plant records for Spain. A warm microclimate was of crucial importance for egg-laying. Occupied plants grew mostly at sparsely vegetated grassland sites where there were large patches of bare rocks, stones or gravel. Most egg batches were laid close to the ground and 75% consisted of only one egg. The number of batches per R. pumila plant was higher on east-, southand west-facing slopes than on north-facing slopes. Presence of eggs and the number of egg batches per R. pumila plant were best explained by a long sunshine duration. At high altitudes particularly warm microhabitats seem to be more important for S. spini than at lower altitudes in Central Europe. We assume that the preference for unusually warm microhabitats is explained by the cold climatic conditions near the altitudinal range limit of the species. That most of the eggs were laid singly and not in small batches as in Central Europe might be a risk-spreading strategy to cope with the harsh climatic conditions and the high inter-annual variation in weather conditions in the high mountain zone in the Pyrenees. The best way to maintain open grasslands for S. spini and other thermophilous grassland species in the high mountain zone of the Pyrenees is to use the traditional combination of sheep and cattle grazing. [ABSTRACT FROM AUTHOR]