Pellet and bone accumulation at a colony of Western Gulls (Larus occidentalis)

Abstract

Accumulation of vertebrate remains in recent and fossil deposits by avian and mammalian predators is well documented (e.g., Moon, 1940; Mellett, 1974; Mayhew, 1977; Dodson and Wexlar, 1979; Emslie, 1988; Hoffman, 1988). Seabirds with this behavior include albatrosses and skuas (Clarke et al., 1981; Ryan and Fraser, 1988), the latter of which can accumulate up to a hundred carcasses of petrels in a concentrated midden (Furness, 1987). Cormorants, murres and other diving seabirds that prey on molluscivorous fish can transport deep-water molluscs into shallow-water deposits and may account for anomalous occurrences of molluscs in the fossil record (Lindberg and Kellogg, 1982). Western Gulls (Larus occidentalis) are large larids (similar in size to Glaucous-winged and Herring Gulls) that also accumulate bones of fish, birds, mammals and shell or other hard parts from invertebrates on their breeding colonies (Moore, 1975). This behavior can result in mixed assemblages of vertebrate and invertebrate remains that include species from local and distant environments. We studied pellet and bone accumulation by Western Gulls on Southeast Farallon Island, California, with the objectives to (1) determine the rate of bone accumulation by gulls, (2) identify patterns in bone representation and breakage in gull pellets, and (3) document the percent of non-native vertebrate remains that occur in regurgitations and pellets as a result of long-distance foraging by gulls. Southeast Farallon Island is located 42 km west of San Francisco and is the southernmost of three island groups in the Gulf of the Farallones. It was formed of uplifted granitic rock and has an area of 44 ha that includes flat marine terrace and numerous cliffs and hills; the highest peak has an elevation of 109 m. Vegetation is dominated by the endemic Farallon Weed (Lasthenia minor maritima) and at least 38 other native and introduced species of grasses and plants (Coulter, 1971). Southeast Farallon Island has the largest breeding colony of seabirds in the continental United States, which includes 12 species (3 cormorants, 5 alcids, 2 stormpetrels, 1 gull and 1 oystercatcher). Because the Gulf of the Farallones is an upwelling system, annual variation in the marine environment and ocean productivity affects prey selection and breeding success of these seabirds (Ainley and Boekelheide, 1990). The population of Western Gulls approximates 25,000 breeding adults, or 40% of the world population (Spear, 1988). Breeding territories occur across a multitude of habitats on the island, ranging from exposed level marine terrace to rocky slopes and cliffs, so that most available nesting areas are used. Gulls attend breeding territories nearly year round with lowest attendance in the fall after breeding. At this time, gulls disperse along the Pacific coast in correlation with the period of lowest food availability in the Gulf of the Farallones (Spear, 1988). Territory attendance increases in winter and spring and egg laying commences in late April (Ainley and Boekelheide, 1990). Gulls consume vertebrate prey whole but, like most avian predators, are unable to digest bone. Pellets containing feathers, bone and other hard parts are regurgitated 24-48 hours after consumption and may include parts from more than one meal. Gulls also have an expanded esophageal sac (crop) in which food is stored and then regurgitated to feed chicks, mates or to break apart and reconsume. Relatively large prey items can be stored in this sac including complete adult Cassin's Auklets (Ptychoramphus aleuticus). This nocturnal species (average weight 170 g) is the most abundant bird on Southeast Farallon Island, where there are up to 105,000 breeding adults (Manuwal, 1974); it excavates burrows or occupies rock crevices for nesting, and is frequently killed and eaten by Western Gulls (Fig. 1A, B). Diets of Western Gulls on Southeast Farallon Island were studied by Ainley and Boekelheide (1990) who collected and analyzed pellets and feces. Gulls feed primarily on marine fish (60-90%), but also on invertebrates (5-7%), birds and human garbage. In addition, gulls can be diet specialists and select one prey species consistently over others (Pierotti and Annett, 1987). On Southeast Farallon Island, specialists on Cassin's Auklets or gull chicks are rare but obvious (Fig. 1 A, B). The ground surface on Southeast Farallon Island is presently littered with thousands of bones of fish, birds and mammals (Fig. lA). In some areas of the marine terrace bone density is as high as 1000 or more bones per m2, but density varies considerably throughout the breeding colony. The most apparent, and probably most abundant, taxa represented by these bones are domestic animals (chicken, turkey, cattle) whose remains were brought to the island primarily by Western Gulls, the only seabird on Southeast Farallon Island that feeds on garbage. Bones of cattle include fragments of vertebrae, ribs and limb bones that were butchered (with obvious saw and cut marks) for human consumption. It is unlikely that these bones were left from earlier human occupation of the island fresh regurgitations containing paper and other garbage and bones of domestic animals that still have tissue adhering to them are observed frequently near gull territories during the breeding season. To investigate this bone accumulation, we surveyed 51 breeding territories of gulls during a 40-44 day period from mid June to late July, 1989. Most territories we surveyed were occupied by gulls with chicks; a few were incubating eggs or had lost their chicks