Energy Content of Some Spiders and Insects on Branches of Spruce (Picea abies) in Winter; Prey of Certain Passerine Birds

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Abstract
Spiders and some insects were collected from branches of spruce, P. abies, in Sept.-Oct. and Feb.-March in southwest Sweden. At least 19 spp. [including Dictyna sp., Hyptiotes paradoxus, Clubiona sp., Diaea dorsata, Xysticus sp., Philodromus aureolus, P. margaritatus, Cryphoeca silvicola, Theridion tinctum, Araneus gibbosus, A. diadematus, A. sturmi, A. cucurbitinus, Araneus sp., Cyclosa conica, Erigoninae, Linyphiinae and Theridiidae] of spiders were present. Spiders of family Thomisidae were most common with 37-45% by number, and the species-group P. aureolus alone made up 35-39%. Family Linyphiidae came 2nd with 27% and Araneidae 3rd with 11-20%. The taxonomic composition was remarkably similar in Sept. and March. Birds which severely reduce spider density over winter apparently prey on spiders with little or no selectivity as to taxonomy. A fitted power function of fresh body mass vs. length had the exponent 2.39. This indicated that spiders of different size were geometrically dissimilar. In autumn the energy content/g of dry mass was 24.0 kJ for spiders, 21.7 kJ for Psocoptera and 22.0 kJ for Curculionidae. Average energy content per g of dry mass of spiders decreased by 5.8% in 4 winter months. If this reduction was caused by metabolism of stored fat only, the corresponding reduction of dry mass per spider would be 8.1% and the reduction of energy content per spider 13%. The energy cost of standard metabolism of spiders over winter was calculated to be a considerably larger proportion of the energy content of spiders in autumn.