Winter philopatry in migratory waterfowl

Philopatry in migratory species can apply to any location used during the annual cycle. The degree of philopatry influences the genetic structure of populations, but only at the stage of the annual cycle when pair formation and gene exchange occur. Because pair formation in birds typically occurs during the breeding season, most studies have focused on breeding-site philopatry. Waterfowl (Anseriformes) are an important exception to this pattern because pair formation often occurs during the winter months. Yet, surprisingly few studies have examined winter philopatry in waterfowl. To serve as an impetus for future research, we summarize published information on winter philopatry in waterfowl and examine these patterns in light of current hypotheses proposed to explain philopatric behavior. Our analyses indicate that geese, swans, and sea ducks show high levels of winter philopatry, with homing rates varying between 49 and 98% to small study areas. In contrast, return rates (0 to 20%) and homing rates (35 to 85%) to large study areas probably are comparatively lower for dabbling ducks and pochards. Unfortunately, detailed comparisons among groups are hindered by variation in the scale at which philopatric behavior is evaluated (ranging from <1 km(2) to 10(5) km(2)), and by confounding of return rates with homing rates. Future studies of winter philopatry would benefit by the adoption of a more standardized methodology. Many of the hypotheses proposed to explain breeding philopatry apply equally well to winter philopatry. In particular, both genetic and ecological mechanisms may play a role in the evolution of philopatry to the wintering ground. Additional field studies are needed to test these hypotheses, and we suggest future directions for a more detailed examination of this neglected area of research.