山 階 鳥 研 報(J.Yamashina Inst.Ornith.),20:71-81,1988 Regional Difference in the Diet of Slaty-backed Gulls Breeding Around Hokkaido Yutaka Watanuki* Abstract Regional difference in the diet of Slaty-backed Gulls, Larus schistisagus was studied at the colonies around Hokkaido, Japan. They fed their chicks with sardines Sardinops melanosticta , rock fish Sebastes spp., and seabird chicks on Teuri Island, sardines on Daikoku Island and sardines and sandlances Ammodytes sp. on Yururi Island. They preyed upon chicks of seabirds such as Black-tailed Gulls L. crassirostris, Rhinoceros Auklets Cerorhinca monocerata and conspecifics on Teuri Island, while they preyed upon adults of Leach's Storm-Petrels Oceanodroma leucorhoa on Daikoku, Yururi and Moyururi Islands. Although availability of conspecific chicks was higher on Daikoku, Yururi and Moyururi Islands than on Teuri Island, they preyed upon conspecific chicks more frequently on Teuri Island than on Daikoku, Yururi and Moyururi Islands. Thus , availability of conspecific chicks did not explain the regional difference in the occurrence of predition attempts on conspecific chicks. Introduction Regional differences in the diet have been reported in many species of Larus gulls. These differences have been attributed to regional differences in the food availability (Harris 1965, Andersson 1970, Spaans 1971, Trapp 1979, Vermeer 1982 and Gotmark 1984). Among Great Skuas Catharacta skua, regional difference in food preference also affected regional difference in the diet (Furness 1979). In order to better understand of the factors responsible to regional diet difference among gulls, an additional study was made on Slaty-backed Gulls Larus schistisagus breeding around Hokkaido, Japan. They are ecological generalists that forage fish , marine invertebrates, garbage and seabird chicks (Watanuki 1984a). This paper describes the regional difference of their diet that was determined by food-loads delivered to chicks and pellets and food remains. Study Area and Methods 1. Study area The study was conducted on Teuri Island (44°25'N,141°19'E) in 1980,1984 and 1985,on Daikoku Island (42°56'N,144°52'E) in 1982 and on Yururi Island (43°13'N , 145°36'E) in 1983 (Fig.1). Additional data were obtained on these three islands in 1981 , on Teuri Island in 1979 and 1987, on Daikoku Island in 1986 and on Moyururi Island (0 .8 km east of Yururi Island) in 1981. Received 1 February 1988 * Institute of Applied Zoology , Faculty of Agriculture, Hokkaido University, Kita-9, Nishi-9, Kita-ku, Sapporo 060, Japan Present address: National Institute of Polar Research, 9-10, Kaga 1-chome, Itabashi-ku, Tokyo 173, Japan 71 72 Y. Watanuki Fig. 1. Map of Hokkaido showing the locations of the study colonies of Slaty-backed Gulls. Teuri Island is located 28km off Haboro. About 3km of a total of 12km coast line is comprised of steep cliffs rising 80-100m above sea level. About 200-400 pairs of Slaty-backed Gulls, 20,000-30,000 pairs of Black-tailed Gulls L. crassirostris,172,000 pairs of Rhinoceros Auklets Cerorhinca monocerata and 500-700 pairs of Japanese Cormorants Phalacrocorax filamentosus bred on the island. Daikoku Island is located 1.8km off Tokotan. A total of 6.1km coast line of the island is comprised of cliffs, about 40-80m in height. About 3,500 pairs of Slaty-backed Gulls, 300 pairs of Japanese Cormorants and 415,000 pairs of Leach's Storm-Petrels Oceanodroma leucorhoa bred on the island. Yururi Island is located 3km off Konbumori, Nemuro together with Moyururi Island. Circum- ference of Yururi and Moyururi is 7.5km and 3km, respectively. The coast lines of both islands are comprised of cliffs of 30-40m height. About 800 pairs of Slaty-backed Gulls and 90 pairs of Japanese Cormorants bred on Yururi and about 1,200 pairs of Slaty- backed Gulls and 250-300 pairs of Japanese Cormorants bred on Moyururi. Further details of these islands can be found in Kuroda (1963) and Watanuki et al. (1986) for Teuri, in Kushiro Municipal Museum (1981) for Daikoku and in Fujimaki et al. (1976) and Kondo et al. (1986) for Yururi and Moyururi. Diet of Slaty-backed Gulls 73 2. Study plot On Teuri, one study plot containing 42-71 nests was set in a center of a subcolony on a gradual slope with sparse vegetation of Artemisia and Calamagrostis, and boulders. On Daikoku, I set four study plots; three were on relatively steep slopes covered with dense stands of Artemisia and Elymus and one was on a rocky slope with sparse vegetation. Each plot contained 14-42 nests. On Yururi, I set two study plots containing 45 and 42 nests, respectively on a beach strewn with boulders and several clumps of vegetation and one plot containing 15 nests on a maritime slope of a small headland covered with Elymus. Individual nests in the plots were marked with numbered wooden stakes after the first egg of a clutch was laid. 3. Diet analysis Every 5 days, I visited all the nests in the study plots and collected pellets, food remains and food-loads regurgitated by chicks or parents found in each territory. Following types of pellets and food remains were separated: fish, marine invertebrates (seashell, crab, seaurchin, starfish and squid), eggs of seabirds, adults and chicks of seabirds, rodents, insects, chicken meat bones, papers and seeds. Species of seabirds were recorded if possible. Food-loads were preserved in 10% formaldehyde and weighed in a laboratory. Pellets and food remains that were collected from one nest per one visit, were combined. Then the combined sample was defined as a "nest-sample". One or more remains of single food type that were found in a nest-sample was counted as one point of that food type. Then, % occurrence of each food type was calculated as the proportion of the sum of the point of it. Gulls usually regurgitate larger portions of inedible parts of foods as pellets and food remains when they eat marine invertebrates or seabirds than they do when they eat fish or earthworms (Spaans, 1971; Watanuki, 1984b). Hence, the occurrence of marine invertebrates and seabirds in pellets and food remains shows a larger value than that in food-loads. Foods for chicks differed from those for parents own in Great Skuas (Furness & Hislop, 1981). However, it would be reasonable to compare the diets among colonies using occurrence of food items in the pellets and food remains. As no food-loads was collected on Daikoku Island, I recorded the occurrence of food items in food-loads by observing about ten breeding pairs with binoculars during the chick raising period in 1986. I also recorded the predation attempts by Slaty-backed Gulls ad libitum. I assumed a "predation attempt" when the gull attacked potential prey at least one time. I excluded attacks on adults of Black-tailed Gulls and conspecific chicks in the context of territorial aggressions. Differences between predatory behaviors on conspecific chicks and attacks by neighbor adults on chicks in the context of territorial aggression are described in Watanuki (in press). The number of remains of seabird species including wings, feet and bones was counted. Then % composition of the remains of esch species was calculated. The prey preference was evaluated by Ivlev's Elective Index (Ei), calculated from % composition of i th seabird species (or type) available (Ni) and % composition of the i th species in the 74 Y. Watanuki pellets and the food remains (ri), Ei=(ri-Ni)/(ri+Ni), : (Ivlev, 1965). Positive Ei (<1.0) suggests that the gulls kill i th prey more frequently than expected from the % composition of available prey (or the gulls "prefer" i th item). Furness and Hislop (1981) reported that Great Skuas tore the body of prey into three or four portions when they killed large seabirds, hence possibly regurgitate three or four pellets. Slaty-backed Gulls swallowed eggs and small chicks of gulls and chicks of Rhinoceros Auklets, while they tore large chicks or fledglings of gulls. Hence, the number of large chicks killed by Slaty-backed Gulls might be overestimated. Although I could not determine the relationship between the number of pellets and the size of prey in this study, I assumed that there was no regional difference in this relationship and believe that it is reasonable to compare the Ei s between colonies. The number of available prey was estimated from the number of breeding pairs, the mean clutch size and the brood size of the species (Watanuki et al., 1986; Kondo et al., 1986; Watanuki, 1987). 4. Body size Morphometric measurements were made on adult Slaty-backed Gulls on Teuri and Daikoku. Breeding adults were captured with box traps on Teuri during their incubation period. Sexes were determined from their courtship and copulation behaviors following Tinbergen (1959) and Burger and Beer (1975). Additional data were obtained from dead birds collected on Teuri and Daikoku, in which the sexes were determined by dissection. Food load size and body size of Slaty-backed Gulls are shown as the mean±SD and sample size in parentheses. Difference between values were examined with Mann-Whitney U-test. Results 1. Diet The seabird remains occurred frequently on Teuri and Daikoku, while rarely occ- urred on Yururi(Table 1). Most of items found in the pellets and food remains on Yururi were bones and scales of fish. Marine invertebrates were minor foods on every islands. Among invertebrates, crabs (Pugettia sp. and Pagurus sp.) and molluscans (Tugalina spp. and Cellana spp.) occurred frequently on Teuri, and chiton Cryptochiton stelleri and a crab Telmessus sp. occurred frequently on Daikoku and Yururi. Cryptochiton and Telmessus are distributed along the eastern coast of Hokkaido but do not along the north west side of it.