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

Abstract

Spiders and some insects were collected from branches of spruce, Picea abies, in September-October and February-March in southwest Sweden. At least 19 species of spiders were present. Spiders of family Thomisidae were most common with 37-45% by number, and the species-group Philodromus aureolus alone made up 35-39%. Family Linyphiidae came second with 27% and Araneidae third with 11-20%. The taxonomic composition is remarkably similar in September and March. This suggests that the birds, which severely reduce spider density over winter, prey on spiders with little or no selectivity as to taxonomy. A fitted power function of fresh body mass versus length has the exponent 2.39; this indicates that spiders of different size are geometrically dissimilar. In autumn the energy content per gram of dry mass is 24.0 kJ for spiders, 21.7 kJ for Psocoptera, and 22.0 kJ for Curculionidae. Average energy content per gram of dry mass of spiders decreased by 5.8% in four winter months. If this reduction is assumed to be 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.