An efficient method for sampling aerial arthropods at nest sites of an insectivorous songbird in steep decline

The decline of aerial insectivorous birds has been hypothesized to stem, in part, from a decline in aerial arthropod prey, underscoring the need for long-term monitoring of both bird and arthropod populations. However, trapping arthropods can be time consuming and efficient methods are required. Our primary goal was to identify the optimal combination of insect traps to collect taxonomic orders of prey in the diet of a songbird in steep decline, the Olive-sided Flycatcher (Contopus cooperi). Our secondary goal was to sample the arthropod community as broadly as possible, and thereby monitor for general changes in arthropod assemblages, which may affect multiple species of migratory insectivores. In the boreal forest of central Alaska, we compared captures of canopy Malaise traps and three types of near-ground pollinator traps (blue vane traps, yellow vane traps, and chemically baited wasp traps) at 22 breeding territories where adult flycatchers actively foraged and bred. Combined, traps collected 11,193 specimens from 12 arthropod orders, of which moths (Lepidoptera, 36%), flies (Diptera, 34%), and wasps (Hymenoptera, 18%) were the most abundant, and also known components of flycatcher diets. General linear mixed models determined that canopy Malaise traps collected the greatest overall ordinal richness (11 orders) with a significantly greater abundance of specimens from six orders, two of which are aquatic specialists linked to breeding success of Tree Swallows (Tachycineta bicolor), another declining aerial insectivore. The three types of near-ground pollinator traps overlapped in taxa collected, but blue vane traps captured the most flies, bees, and yellowjackets, all of which were not well represented in Malaise samples. All trap types failed to collect dragonflies (Odonata), a known prey item of breeding Olive-sided Flycatchers. We therefore conclude that a combination of canopy Malaise traps and blue vane traps, plus hand-netting of dragonflies, is an efficient combination for quantifying prey abundance on Olive-sided Flycatcher territories, while simultaneously monitoring the broadest possible number of arthropod orders near flycatcher nests within the boreal forest.