Ecosystem function of insectivorous birds in California vineyards
Diet choices of high order predators affect not only trophic interactions and predator-prey population dynamics but impact food webs around the globe. Insectivorous birds are abundant in most environments, but their ecological function is poorly understood primarily because they prey upon relatively small organisms. Establishment of songbird nestboxes in vineyards increases insectivorous bluebird populations yet it is unknown whether direct consumption by these predators is lowering pest or beneficial insect populations, providing growers with ecosystem services or disservices, respectively. Moreover, insectivorous birds may be functioning as intraguild predators (by consuming predaceous arthropods such as spiders) with uncertain net top-down trophic effects on plant production. To evaluate predator diet breadth and ecosystem function of Western Bluebirds (Sialia mexicana) nesting in vineyards, we sampled birds and arthropods from seven vineyard blocks with active nest box programs. Arthropods were also collected from adjacent oak woodland forest patches to compare prey availability across habitats. The field research was conducted during the bird-breeding season from April-July 2012. We implemented non-invasive avian sampling methods by gathering fecal samples from bluebirds, extracting and amplifying prey DNA, and applying next-generation sequencing on the Illumina Miseq platform. We compared these data to arthropod sequences in the BOLD database.
Over 4500 arthropods were collected, sorted, and identified to order or family. Arthropod abundance and diversity were high both in vineyard and adjacent woodlands, indicating avian populations had sufficient access to food sources over the breeding season. A comparison of over 200 bluebird fecal samples revealed that adult and nestling birds did not differ in the species of prey consumed, nor were there differences between male and female adults. Avian fecal samples contained an average of 6.9 ± 5.7 unique prey sequences. Bluebirds consumed a diverse diet of prey orders including Diptera, Hemiptera, Hymenoptera, Lepidoptera, and Trichoptera. Several insect families that contain significant agricultural pests were consumed regularly by bluebirds including leafhoppers and sharpshooters (Cicadellidae : Hemiptera,) and torticid moths (Tortricidae: Lepidoptera). Additionally, birds were found to feed on important arthropod natural enemies including Zelus sp. (Reduviidae: Hemiptera) and ichneumonid wasps (Ichneumonidae: Hymenoptera). Bluebirds foraged across arthropod guilds, demonstrating a complicated ecosystem function that likely responds to annual and seasonal variation in arthropod community abundance and composition.