Bird predation as a sex- and size-selective agent of the arboreal spider Pityohyphantes phrygianus

1. The solitary sheetweb spider Pityohyphantes phrygianus (Araneae: Linyphiidae) lives in canopies of coniferous trees. Earlier studies in Scandinavia have shown that bird predation affects the spider abundance negatively.
2. In a large-scale, 2-year field experiment, passerine birds were excluded from foraging on spruce branches. The experimental branches were net-enclosed, which prevented bird predation but left spiders free to leave and enter. Samples were taken in the autumn and spring for 2 years.
3. Removal of bird predation increased the mean number of subadult P. phrygianus per branch between 2·1 and 10·6 times the control number for each sex. Bird predation had approximately similar effects on the sexes, except in the second experimental spring when males were affected twice as much as females, and there was a significantly lower proportion of males on control branches exposed to bird predation than on net-enclosed branches.
4. The mean size of females was larger on net-enclosed branches than on controls in both spring samples. In males, this was the case in the first spring sample only.
5. This study shows that there are also mortality costs for both sexes associated with large size in the non-mating season. In certain winters, however, there is an additional mortality risk to males, presumably caused by a higher activity level than in females.