Foraging effort by Surf Scoters at the peripheries of their wintering distribution: Do foraging conditions influence their range?

I measured foraging effort of surf scoters at the northern and southern peripheries of their winter distribution (Southeast Alaska and Baja California, Mexico, respectively), and compared those to foraging effort previously documented at the wintering range core (Strait of Georgia, British Columbia, Canada). I predicted that if foraging conditions influenced their winter distribution, foraging effort would be higher at the peripheries of the distribution and that scoters, which are typically diurnal foragers, might be forced to extend their foraging activities into nocturnal periods. Overall, scoters in Baja California foraged over twice as much as scoters in Alaska, with intermediate foraging effort at the range core. Scoters also foraged nocturnally in Baja California but not in Alaska or British Columbia. I conclude that foraging opportunity may limit winter distribution at the southern periphery, but other factors limit distribution at the northern periphery.