Zebra jumping spiders opportunistically forage on different sized midges

Abstract Prey preferences influence predator diets and therefore the structure and function of food webs. Yet preferences may arise for several reasons, including the maximization of energy gain or the acquisition of specific nutrients. Detecting prey preferences requires a robust estimation of foraging rates from the functional response, but these are unavailable in most prey preference studies. Here we assess preference for small or large midges (Chironomidae) by zebra jumping spiders (Salticus scenicus). We conducted functional response experiments for both small and large midges to determine expectations for foraging rates on each. We then conducted foraging trials to assess preferences, predicting that jumping spiders would have a preference for large midges because of their greater energy content. We found that zebra jumping spiders had a higher functional response when foraging on small midges than when foraging on large midges. Rather than indicating a preference for small midges, this difference sets the correct null expectation for choice experiments, and when offered a choice of prey types, spiders showed no preference for large or small midges. In contrast, the Manly's α choice metric suggested a preference for small midges, and we therefore argue that this approach provides a distorted depiction of prey preferences in this spider.