Collar, S., D. D. Roby, and D. E. Lyons. Top-down and bottom-up interactions influence nesting success at the world's largest colony of Caspian terns (Hydroprogne caspia) in the Columbia River estuary.
Our study investigated the influence of bottom-up and top-down drivers on the declining productivity at a once thriving breeding colony of Caspian terns (Hydroprogne caspia). Situated at the mouth of the Columbia River, Oregon, East Sand Island (ESI) is home to the largest Caspian tern breeding colony in the world. Since 2001 the decline in reproductive success of Caspian terns at ESI has been associated with a significant increase in average river discharge during May and June. The abundance of forage fish for terns in the estuary was inversely related to river discharge, which also apparently affected the reliance of tern nest predators on the tern colony as a food source, resulting in increased disturbance and decreased reproductive success at the tern colony. There was a significant increase in disturbance rates by bald eagles (Haliaeetus leucocephalus) during June for terns nesting at the ESI colony, and eagle disturbance rates were positively associated with May river discharge. We found a significant increase in kleptoparasitism rates of terns by glaucous-winged/western gulls (Larus glaucescens x L. occidentalis) since 2001, and a significant negative relationship between average annual rates of gull kleptoparasitism and Caspian tern nesting success at ESI. Although correlational, our results support the hypothesis that the decline in Caspian tern nesting success at this large estuarine colony was primarily driven by the interaction of bottom-up and top-down factors, influencing tern productivity through the food supply, and triggering potential predators to identify the tern breeding colony as an alternative source of prey.