Birds Feeding on Herring Eggs at the Yaquina Estuary, Oregon

Abstract

-I observed 17 bird species feeding on herring eggs throughout high and low tides at high rocky intertidal and low intertidal areas in an Oregon estuary. At low tide gulls fed directly on eggs, but at high tide they pirated eggs from diving birds or picked up eggs drifting in water. Brant, wigeon, and coots picked up eggs while walking, or tipping up or through piracy; in deeper water coots dove for eggs. Diving ducks obtained eggs by diving, by piracy, or by picking up eggs while swimming. Less than 25% of the gulls, coots, or Buffleheads, but as many as 45% of the scaups and 83% of the scoters observed in the lower estuary were in groups feeding on herring eggs. The species composition and abundance of birds varied within the estuary and probably reflected: 1) the onset of spring migration; 2) immigration of birds into the estuary to feed on eggs; 3) the presence of birds near a site of egg deposition; and 4) the domination of herring egg deposits by gulls in the upper intertidal zone. Along the eastern Pacific Ocean, herring (Clupea harengus) are present from northern Baja California to the Beaufort Sea, spawning in estuaries and other coastal areas (Hart 1973). The fish aggregate and spawn in a California bay 4-7 times per spawning season at sites that vary in area from 42 to over 1 million m2 (Hardwick 1973). Water-hardened eggs are about 1.3 mm in diameter and are laid in masses attached to eelgrass (Zostera marina), algae, rocks, piers, or other relatively immobile objects (Hart 1973). Because herring spawn in late winter and early spring, their eggs may be important to birds as a source of nutrition for fat deposition prior to or during spring migration. Although spawning is infrequent, the amount of potential food available to birds may be as great as 1.3 x 105 kg eggs/site, with mean densities as great as 1.37 kg/m2 (Hardwick 1973). Several studies have described the species composition and measured the extent of bird predation on herring eggs (Munro and Clemens 1931, Munro 1941, Outram 1958, Taylor 1964, Steinfeld 1972, Hardwick 1973). Little attention has been given to changes in group composition or to the role of gulls in predation of eggs. Here, I report the techniques whereby birds obtain herring eggs at the Yaquina River estuary, Oregon. I also relate species composition to sites where eggs are laid, seasonal changes in species abundance, and domination of egg deposition sites by gulls. Finally, I discuss the impact of bird predation on herring eggs and on macrophyton distribution. STUDY AREA AND METHODS The Yaquina River estuary, on the central coast of Oregon (Fig. 1), is a drowned river valley with an area of about 15.8 km2 (Oregon State Land Board 1973). Tides have a range between Mean Lower Low Water (0.0 m, MLLW) and Mean Higher High Water of 2.55 m; Mean Tide Level is +1.40 m (Oregon State Land Board 1973). All elevations in this paper are relative to MLLW. I found herring eggs on intertidal macrophyton as early as 26 December (in 1974); generally 3-7 spawnings per year occur from mid-January to mid-April (Steinfeld 1972; J. Butler, Oregon Department of Fish and Wildlife, pers. comm.). Spawning usually occurs in patches and is not synchronous throughout the estuary (Steinfeld 1972). Using a 20x telescope, I searched for any birds feeding on herring eggs two to five days per week from 8 February to 31 April 1979. Birds feeding on herring eggs were observed at only five sites (Fig. 1). They were found at these sites throughout the day and during all tide stages unless they were disrupted by fishermen, boats, or Bald Eagles (Haliaeetus leucocephalus). Bird groups were relatively undisturbed at sites 1, 2 and 5, and groups generally reassembled within 10 min after a disturbance. Fishermen so often disturbed birds at sites 3 and 4 that large groups seldom formed, and birds moved frequently between areas where fishermen were absent. Because of frenzied bird activity and subtle interspecific plumage differences, not all of the birds in groups could be identified to species and counted. I divided gulls into two categories: large (Western, Glaucous-winged, and Western x Glaucous-winged hybrids [Hoffman et al. 1978]) and small (Ring-billed and Mew). White-winged and Surf scoters, as well as Greater and Lesser scaup, could not always be distinguished. Using my telescope from a car, I censused selected aquatic birds over the area shown in Figure 1 by diagonal lines, at the time of high slack tides. The possibility of counting birds more than once was mini-