Winter philopatry in migratory waterfowl

Philopatry in migratory species can apply to any location used during the a nnual cycle. The degree of philopatry influences the genetic structure of p opulations, but only at the stage of the annual cycle when pair formation a nd gene exchange occur. Because pair formation in birds typically occurs du ring the breeding season, most studies have focused on breeding-site philop atry. Waterfowl (Anseriformes) are an important exception to this pattern b ecause pair formation often occurs during the winter months. Yet, surprisin gly few studies have examined winter philopatry in waterfowl. To serve as a n impetus for future research, we summarize published information on winter philopatry in waterfowl and examine these patterns in light of current hyp otheses proposed to explain philopatric behavior. Our analyses indicate tha t geese, swans, and sea ducks show high levels of winter philopatry, with h oming rates varying between 49 and 98% to small study areas. In contrast, r eturn rates (0 to 20%) and homing rates (35 to 85%) to large study areas pr obably are comparatively lower for dabbling ducks and pochards. Unfortunate ly, detailed comparisons among groups are hindered by variation in the scal e 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 stud ies of winter philopatry would benefit by the adoption of a more standardiz ed methodology. Many of the hypotheses proposed to explain breeding philopa try apply equally well to winter philopatry. In particular, both genetic an d ecological mechanisms may play a role in the evolution of philopatry to t he wintering ground. Additional field studies are needed to test these hypo theses, and we suggest future directions for a more detailed examination of this neglected area of research.