Gene Flow between Breeding Populations of Lesser Snow Geese

Abstract

THE measurement of gene flow between natural populations of animals in the field has rarely been achieved in population biology. The nesting colonies of Lesser Snow Geese (Chen c. caerulescens) in the Hudson Bay area provide a favorable situation for such measurements-they are relatively discrete, often separated by hundreds of miles (Fig. 1), and, at many of the colonies, large numbers of geese have been marked with leg bands, facilitating the detection of movement between colonies. The Lesser Snow Goose is dichromatic, comprising blue and white (snow) phases; this dichromatism is controlled by a single gene or tightly linked group of genes (Cooke and Mirsky 1972). In this paper the names Lesser Snow Goose or Snow Goose will be used to denote the species, and blue and white to denote the color phases. The ratios of blue to white phase individuals differ significantly from colony to colony and have changed within colonies in the recent past (Cooch 1963). A westward spread of the blue phase led Cooch to postulate some exchange between colonies. The lack of morphological differentiation between Lesser Snow Geese from different colonies (in contrast to that of Canada Geese (Branta canadensis) nesting in the same region that show significant regional morphological variation (Maclnnes 1966)) is consistent with the considerable gene exchange between different Snow Goose colonies. The two color phases mate assortatively (Cooch and Beardmore 1959). Cooke and Cooch (1968), on the basis of a genetic analysis of the Boas River colony, postulated that individuals select mates according to the color phase of their parents. Mate selection might be modified by the relative availability of each color phase at the time of mate selection. Lemieux and Heyland (1967) and Cooch (1961) showed that white phase birds from the Koukdjuak and Boas River colonies, respectively, tended to have a more westerly fall migration route and wintering range than blue phase birds from the same colonies, although separation was far from absolute. This phenomenon is hereafter referred to as differential phase migration. The migration and wintering distributions of the two colonies shared large areas of overlap, indicating that birds from at least two colonies mixed at this time. If pair formation occurred during this period, interchange between colonies would result unless the geese possessed a special means of recognizing birds from their own colony or unless birds from a single colony maintained exclusive flocks. The purpose of this paper is to determine the amount of gene flow