Anthropogenic impacts on water quality indirectly alter the diet of Louisiana Waterthrush (Parkesia motacilla) nestlings via disruption of aquatic-terrestrial trophic subsidies
Riparian songbird communities are ecologically delicate systems that are vulnerable to water pollution due to their dependence on aquatic insects. These songbird communities are specialized for breeding in the forested habitat surrounding a freshwater stream and are primarily comprised of forest-obligate migrants. The long-term decline of these migrants has been well documented for over 40 years and is believed to be primarily due to anthropogenic impacts such as abandoned mine drainage, acid precipitation, and agricultural run-off. These impacts indirectly affect breeding riparian songbirds by reducing the abundance and diversity of aquatic insects, which disrupts the aquatic-terrestrial trophic subsidy on which they depend. Nevertheless, the aquatic insect foraging specialist Louisiana Waterthrush (Parkesia motacilla) have been documented breeding and successfully fledging young along impacted streams. We hypothesize that waterthrush compensate for low aquatic insect availability by diversifying their diet and targeting terrestrial prey. To answer this question, we utilized DNA barcoding and Next-Generation Sequencing to identify residual prey DNA from the fecal sacs of Waterthrush nestlings.
Our preliminary data suggests that waterthrush nesting along impacted streams (agricultural run-off) supplement their diet by foraging for terrestrial insects such as moths and caterpillars. In fecal sacs collected from 16 nests, a single genus of mayfly, Epeorus (Ephemeroptera: Heptageniidae) was the only taxon present in every diet sample but accounted for less than 4% of individuals in Surber samples from these streams, which indicates a high degree of prey selectivity. The diet of waterthrush nesting in the most impacted catchment contained prey items that were not present in higher-quality habitats, including other mayfly genera (e.g., Maccaffertium and Isonychia) and terrestrial prey (Geometridae: Lomographa and Tortricidae: Pseudexentera). Furthermore, the diet of waterthrush nestlings was more taxonomically rich than those collected from non-impacted catchments, which suggests a diversification of target prey in the absence of preferred taxa. This phenomenon may cause a ripple effect throughout the riparian food web, which may alter the trophic niches and foraging strategies of competing songbirds within the riparian ecotone.