. The Canadian field-naturalist. 1973 Notes 307. ^5^*^-'-*--~*--**'" , , , RING-BILLED GULL, MIQUELON LAKE, CANADA •^-—° = RING-BILLED GULL, KAWINAW LAKE, CANADA o = o HERRING GULL, KAWINAW LAKE, CANADA . .. HERRING GULL, SKOMER ISLAND, ENGLAND • - • HERRING GULL, WALNEY ISLAND, ENGLAND * ' HERRING GULL, GRAESHOLM ISLAND, DENMARK • • • HERRING GULL, RHODE ISLAND, USA, DAYS AFTER FIRST CLUTCH Figure 2. Cumulative distribution of clutch initiation of 473 and 167 Ring-billed Gull clutches at Miquelon and Kawinaw Lakes respectively; and 130, 729, 300, 91, and 88 Herring Gull clutches at Kawin
. The Canadian field-naturalist. 1973 Notes 307. ^5^*^-'-*--~*--**'" , , , RING-BILLED GULL, MIQUELON LAKE, CANADA •^-—° = RING-BILLED GULL, KAWINAW LAKE, CANADA o = o HERRING GULL, KAWINAW LAKE, CANADA . .. HERRING GULL, SKOMER ISLAND, ENGLAND • - • HERRING GULL, WALNEY ISLAND, ENGLAND * ' HERRING GULL, GRAESHOLM ISLAND, DENMARK • • • HERRING GULL, RHODE ISLAND, USA, DAYS AFTER FIRST CLUTCH Figure 2. Cumulative distribution of clutch initiation of 473 and 167 Ring-billed Gull clutches at Miquelon and Kawinaw Lakes respectively; and 130, 729, 300, 91, and 88 Herring Gull clutches at Kawinaw Lake, Canada, Walney Island (MacRoberts and MacRoberts 1972) and Skomer Island (Harris 1964), England, Graesholm Island (Paludan 1951), Denmark, and Rhode Island, (Erwin 1971), respec- tively. Figure 2 is that the clutch initiation of Ring-billed Gulls is more abrupt than that of Herring Gulls. Discussion Kruuk (1964) and Patterson (1965) suggested that synchronized egg-laying in Black-headed Gulls, Larus ridibundus, had an anti-predator function. Patterson found that predation mainly by foxes was relatively higher for late-hatched broods than those hatched during the peak of hatching. As Kruuk and Patterson suggested for Black-headed Gulls, the synchronized clutch initia- tion at the start of egg-laying in Ring-billed Gulls may also be influenced by predation. Ring-billed Gulls are susceptible to avian predators such as California Gulls and Herring Gulls in western and eastern North America respectively. Ring-billed Gulls nesting on islands in shallow lakes of the prairies are susceptible to mammalian predators such as coyotes (Vermeer 1970a, 1970b). Her- ring Gulls are restricted to the larger and deeper lakes in the Canadian prairie provinces (Vermeer, in press) which are less accessible to mammalian predators. The abrupt start of egg-laying in Ring-billed Gulls could be influenced by social stimulation. Ring-billed Gulls nested considerably closer t
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