Consequences of arthropod community structure for an at-risk insectivorous bird

Abstract

Global declines in bird and arthropod abundance highlights the importance of understanding the role of food limitation and arthropod community composition for the performance of insectivorous birds. In this study, we link data on nestling diet, arthropod availability and nesting performance for the Coastal Cactus Wren (Campylorhynchus brunneicapillus sandiegensis), an at-risk insectivorous bird native to coastal southern California and Baja Mexico. We used DNA metabarcoding to characterize nestling diets and monitored 8 bird territories over two years to assess the relationship between arthropod and vegetation community composition and bird reproductive success. We document a discordance between consumed prey and arthropod biomass within nesting territories, in which Diptera and Lepidoptera were the most frequently consumed prey taxa but were relatively rare in the environment. In contrast other Orders (e.g., Hemiptera, Hymenoptera)were abundant in the environment but were absent from nestling diets. Accordingly, variation in bird reproductive success among territories was positively related to the relative abundance of Lepidoptera (but not Diptera), which were most abundant on 2 shrub species (Eriogonum fasciculatum, Sambucus nigra) of the 9 habitat elements characterized (8 dominant plant species and bare ground). Bird reproductive success was in turn negatively related to two invasive arthropods whose abundance was not associated with preferred bird prey, but instead possibly acted through harassment (Linepithema humile; Argentine ants) and parasite transmission or low nutritional quality (Armadillidium vulgare; "pill-bug"). These results demonstrate how multiple aspects of arthropod community structure can influence bird performance through complementary mechanisms, and the importance of managing for arthropods in bird conservation efforts.

Fig 1. Overview of experimental design.

The map identifies the location of the eight studied Cactus Wren territories within Orange County. The flow chart indicates the different data sources and sample sizes and how they are integrated in analyses. Details on each data stream and analyses procedures are provided in Methods text.

Fig 2

Arthropod orders and suborders in (a) Coastal Cactus Wren diet samples and (b) nesting territories. Arthropod taxa are ranked by the frequency of occurrence among diet samples. The total estimated biomass is given at the territory-scale for canopy and ground arthropods. Symbols show the variation in estimated biomass among territories (color) and site (shape). Grey bars show the mean biomass (+ SE) across all territories (n = 8). Biomass estimates are drawn on a log scale.

Fig 3. Territory- and habitat-element-specific estimates of arthropod density among habitat elements from canopy sampling (top row) and ground pitfall traps (bottom row).

Grey bars show the biomass of non-native arthropods, Isopoda and Hymenoptera. White bars show the biomass of arthropod orders included in diet samples. Points are shown for each bird nesting territory, with shape referencing the study site and each territory a different color. Arthropods were sampled from native plant species; Artemisia californica (ARCA), Erioginum. fasciculatum (ERFA), O. littoralis (OPLI), Rhus integrifolia (RHIN), Sambucus nigra (SANI), and grasses (NAGR).; non-native Brassica nigra (BRSP) and grasses (EXGR); and bare ground (BARE). Bare ground was sampled by both pitfall sampling and vacuum collecting any arthropod observed within 0.5 m of the pitfall trap, referred to as canopy sampling above for consistency with plant sampling.

Fig 4. Mean biomass (mg ±SE) of prey orders from field-sampled arthropods across focal habitat elements in the canopy (top row) and on the ground (bottom row).

Arthropods were sampled from native plant species; Artemisia californica (ARCA), Eriogonum fasciculatum (ERFA), O. littoralis (OPLI), Rhus integrifolia (RHIN), Sambucus nigra (SANI), and grasses (NAGR).; non-native Brassica nigra (BRSP) and grasses (EXGR); and bare ground (BARE). Bare ground was sampled by both pitfall sampling and vacuum collecting any arthropod observed within 0.5 m of the pitfall trap, referred to as canopy sampling above for consistency with plant sampling.

Fig 5. Relationships between the first egg date and territory-level estimates of prey (mg) (left column), first egg date and Hymenoptera biomass (mg) (middle column) and territory-level estimates of prey (mg) and Hymenoptera biomass (mg) (right column).

Results presented for plant canopies (top row) and on the ground (bottom row).

Fig 6. Territory-level relationships between arthropod prey composition and first egg date in plant canopies and on the ground.

PC loadings are provided in the first row for PC axes representing variation in arthropod prey composition. Variation in composition is considered separately for canopy (columns 1 & 2) and ground (columns 3 & 4) arthropods. Trend lines indicate significant (P < = 0.05) correlation between axes. PC1 and PC2 for canopy composition explain 50% and 29% of variation and for ground composition explain 58% and 29% of variation (see S4 Fig in S1 Appendix).