Migration reflects the discovery by a species, over time, that there are "easier pickings" in another location at a different time of year. Snow geese did not migrate from Virginia to the Arctic each Spring 18,000 years ago. That's when the glaciers covered Canada, year-round. But as the ice melted, some of the snow geese living south of the glaciers explored the newly-exposed areas. Those who nested in the new habitats south of the glaciers had a relatively high percentage of offspring who survived. Those that continued to nest in the old habitats produced young birds as well, but not as many as the relatives who were migrating. The summer habitat in the north was better - more young birds survived up there, despite the risks associated with flying back and forth twice a year.
Ultimately, the vast population of snow geese in the winter feeding grounds were those who had been born in the north. The non-migratory part of the population disappeared, and the snow geese we know today are all migrants.
The story is actually more complex, of course. Some of the give-and-take of natural selection can be seen in the population trends of Canada geese in Virginia.
Until recently, the "honk" of a Canada goose was a special moment in most of Virginia. Even on the Atlantic Coast, geese were rare except during the winter. But in the last 20 years, Canada geese have discovered two new habitats - golf courses and office parks. Look around the water retention ponds, and you'll see the tell-tale cylindrical droppings of geese. They are stuffing themselves all summer long on grasses, raising crop after crop of goslings. Why migrate, when you have all the goodies in one place?
Both snow geese and Canada geese are now problem species. In Canada and Alaska, the snow geese are eating themselves out of house and home. Snow geese are feeding well in moist soil management units and farm fields during the winter, arriving in large numbers in the tundra each Spring. The geese are in great physical condition. They are breeding so successfully that the wetlands are being destroyed. Snow geese pull up the roots of their food plants - with so many geese arriving now each Spring, the plants are being removed faster than they can regrow. Wetland marshes are being converted to open water, and the food source is disappearing rapidly.
At some point, the snow geese will discover one year that there's not enough food to go around. Few goslings will be produced, lots of adult snow geese geese will die, and the population will crash. That's a natural cycle - except the food provided to snow geese in the winter was un-natural. Humans have been a factor in creating the population boom that is leading to a habitat and population bust, and now humans are examining various proposals for reducing the snow geese population before the wetlands are wiped out.
In Virginia and other states, the Canada geese population has grown so large that the species is now considered to be a pest in many cities. George Mason University brought in border collies from Windchazer in 2006, to harass the geese that were cluttering up the Fairfax campus.
The objective is to shift the population balance so migrating geese are in the majority. This must be accomplished before the "resident" Canada geese are the only ones left. If the migratory instinct is lost, it may never be restored among the wild birds. The circumstances where migration was a good adaptation may have been based upon the retreat of the glaciers. Federal and state agencies are willing to go to extremes now to ensure that "memory" is not lost in the next 5-10 years.