Immigration in a small population of snow geese

-The Lesser Snow Goose (Chen caerulescens caerulescens) colony on Howe Island, near Prudhoe Bay, Alaska, is the only established nesting colony of this species in the United States. A study was initiated to follow the dynamics of this small population in 1980 when it became clear that oil development would proceed near the nesting colony and in nearby brood-rearing areas. Virtually all brood-rearing Snow Geese in the study area were banded in late July of 1980 through 1993 to determine return rates of banded birds and to determine whether immigration contributed significantly to the maintenance and growth of the population. This was a critical question if, in the future, a local catastrophe (e.g. foul cholera, oil spill) were to affect most or all of the birds in this small population. During this 15-year study, the Snow Goose population expanded from 39 to 412 nesting pairs. Based on markrecapture data from the banding programs, the immigration rates for females and males are estimated to be 12.8 ? SD of 4.39% and 60.7 ? 12.85%, respectively. Exceptionally high immigration rates for males were recorded in 1990 and 1993, when large numbers of twoand three-year-old females (banded as goslings) returned with unbanded mates from other populations. Many females apparently immigrated when they were juveniles. The origins of immigrants is still unclear because of limited banding at neighboring colonies. Resightings of neck-banded Howe Island geese indicate emigration of primarily males to Wrangel Island, Russia, and Banks Island and the Anderson River Delta, Northwest Territories, Canada. Received 1 June 1994, accepted 18 January 1995. THE LESSER SNOW GOOSE (Chen caerulescens caerulescens) nesting colony on Howe Island, near Prudhoe Bay, Alaska (Fig. 1), is the only established colony of this species in the United States (Johnson and Troy 1987, Johnson and Herter 1989, Johnson et al. 1993), and it is the only colony of Snow Geese nesting in proximity to an active oil field (Johnson and Troy 1987). The Howe Island colony has been studied since 1980. The geese nest during June, rear broods in the Sagavanirktok River Delta during July through mid-August, migrate eastward and stage on the North Slope of Alaska and the Yukon during late August through mid-September, and migrate south through the Mackenzie Valley to Alberta and Saskatchewan in late September through October. Most winter resightings and band recoveries of Howe Island Snow Geese are from the Central Valley of California, the Rio Grande and Pecos river valleys of New Mexico, and the highlands of Chihuahua, Mexico (Johnson in press). In these overwintering areas they mix with Snow Geese from the much larger Western Arctic (Canada) and Wrangel Island (Russia) populations (Bellrose 1980). A major objective of this study was to determine whether the Howe Island Snow Goose population is maintained through: (1) internal production supplemented by immigration; or (2) internal production with little or no immigration, as postulated for the La Perouse Bay Snow Goose population in Manitoba, Canada (Cooke and Sulzbach 1978). This was an important question if a major catastrophe were to occur, such as an oil spill or foul cholera outbreak, that could affect the size, growth, and structure of the colony.