The Spring Phenology of Arboreal Beetles and Spiders and Their Potential as Alternative Prey for Birds Mistimed With Peak Caterpillar Abundance

The match/mismatch hypothesis (MMH) predicts that consumers that are asynchronous with a key ephemeral resource will have reduced fitness. However, by diet‐switching to an alternative resource with which it is synchronous, a consumer may be able to buffer the negative fitness consequences of asynchrony. The trophic interaction between spring arboreal caterpillars and insectivorous passerine consumers is a well‐established MMH study‐system, though caterpillars are not the only prey guild used. Here, we examine the phenological distribution of beetle and spider abundance, two alternative prey guilds utilised by many breeding passerines. Using data collected across a 220 km transect of Scotland, encompassing 44 mixed deciduous woodland sites, we address four questions: (i) Do beetle and spider abundance show a peaked phenological distribution during spring? (ii) Do the phenological distributions of different taxa vary among years and tree taxa? (iii) Does the overall abundance of either taxon vary among tree taxa or (iv) with temperature? We also compare the average phenological distribution of beetles and spiders to the caterpillar peak, considering whether there is potential for diet‐switching for birds mistimed with the peak in caterpillars. We found beetle abundance consistently peaked in early June, while spider abundance decreased linearly throughout spring. Overall, our results demonstrate both tree taxon‐specific and annual variation in the abundance and phenological distributions of both alternative prey guilds, with spider abundance also increasing with spring temperature. Our results demonstrate the potential for diet‐switching for mistimed birds, as the abundance of beetles was consistently higher than caterpillars throughout spring and spider abundance was higher earlier in the season, prior to the peak in caterpillars.