Foraging behaviour of four avian species feeding on the same temporarily available prey

Abstract

Low tide events provide terrestrial predators with ephemeral, but predictable and abundant sources of prey. Understanding the relationships between tidal cycles, prey availability, and predator abundances is vital to characterizing the ecological relationship between terrestrial predators and their marine prey. Here, we describe the foraging tactics of four common bird species in western North America — Bald Eagles (Haliaeetus leucocephalus (Linnaeus, 1766)), Great Blue Herons (Ardea herodias Linnaeus, 1758), Glaucous-winged Gulls (Larus glaucescens J.F. Naumann, 1840), and Northwestern Crows (Corvus caurinus S.F. Baird, 1858) — feeding on the same transiently accessible fish species, the plainfin midshipman (Porichthys notatus Girard, 1854). We conducted avian predator surveys at breeding beaches of plainfin midshipman across multiple years and sites. Our census data showed that Bald Eagle and Great Blue Heron abundances were higher when the tides were receding than incoming at Ladysmith Harbour, British Columbia, Canada, but the opposite trend was found for total predator abundance at a second site in Dabob Bay, Washington, USA. Glaucous-winged Gull abundance decreased over the course of the plainfin midshipman breeding season (April–July), whereas the abundances of the other three species remained stable. Our data suggest that the foraging activities of birds in the intertidal zones of western North America are linked with the tidal cycles, corresponding to periods of high prey vulnerability.