Effects of Non-Native Shrubs on Caterpillars and Shrubland-Dependent Passerines within Three Transmission Line Rights-of-Way in Southeastern New Hampshire

Abstract

Caterpillars (Lepidoptera, Hymenoptera: Symphyta) are a critical food that breeding passerines use for raising young and serve as an important source of carotenoids that support nestling growth and feather pigmentation. When non-native, invasive plants grow abundant enough to reduce native plant diversity they can cause a reduction in caterpillar abundance. The goal of this study was to determine whether a reduction in caterpillars caused by non-native, invasive shrubs affects nestling health and/or the reproductive success of a shrubland-dependent passerine, Geothlypis trichas trichas (Common Yellowthroat). I first quantified caterpillar abundance on the dominant native and nonnative shrubs comprising Common Yellowthroat territories in shrubby transmission-line rights-of-way and estimated total caterpillar abundance in each territory based on the shrub species composition. I then determined if differences in shrub species composition and caterpillar abundance affected: (1) adult Common Yellowthroat reproductive success, (2) nestling Common Yellowthroat diet composition, or (3) nestling Common Yellowthroat growth rate, plasma carotenoids, and carotenoid-based plumage color. Nonnative, invasive shrubs did not reduce caterpillar abundance until they comprised ≥55% of the shrub volume and significantly reduced the diversity and abundance of native shrubs in bird territories. Differences in caterpillar abundance among sites did not result in differences in Common Yellowthroat productivity, but where non-native shrubs comprised ≥55% of the shrub volume, adults increased their frequency of feeding visits to nestlings and fed nestlings the greatest proportion of non-caterpillar prey. Daily surveys of shrubland bird presence/absence at study sites indicated that most shrubland-dependent bird species were absent where non-native shrubs comprised ≥55% of the shrub cover; such conditions likely equated to habitat loss for bird species less able than Common Yellowthroats to adapt to low shrub diversity and low caterpillar abundance.