A swirl of wild geese

First you hear them: a rippling, overlapping chatter. Then you see that the field is covered in geese: dark gray geese in front, a sea of white birds behind. A wave of wings lifts up from the mass and blurs the sky above. The wave spreads as more birds take off. The white geese reveal striking black on their wings.

There are so many of them. It baffles the eyes. It sends a startle of delight to your chest.

Every winter, thousands of snow geese, Ross's geese, cackling geese, and white fronted geese migrate south from their nesting grounds in Canada and Alaska. Huge flocks spend the winter in the Central Valley, including at the San Joaquin River National Wildlife Reserve where my parents and I saw them from the Beckwith viewing platform just before New Year.

The dark gray geese we saw were cackling geese. Cackling geese look almost exactly like Canada geese but smaller, and until recently the various populations were considered subspecies of Canada goose. Those we saw were Aleutian cackling geese, the subspecies of cackling goose that nests in the Aleutian Islands, the arc of volcanic islands trailing off to the southwest of mainland Alaska. They travelled over 2,000 miles from their nesting grounds to this muddy field near the San Joaquin River.  Through binoculars, I spotted a few bright orange goose feet. These belonged to greater white-fronted geese, just a few of them tucked in among the masses of cackling geese. The white geese behind were snow geese and Ross's geese, two species that are nearly identical: almost all white when on the ground, with just a touch of black near the rear, bold black on the wings in flight. The snow goose has a larger bill than the Ross's and some have a slight yellow tinge on the head and neck.

Our understanding of species has changed based on genetic evidence, and continues to change as scientists gather and analyze more of this data. Genetic evidence revealed that cackling geese are not just smaller Canada geese, but rather their own species. Cackling geese look so similar to Canada geese (and Ross's geese look so similar to snow geese) because their evolution into separate species is relatively recent. There is still some interbreeding among the species and subspecies where they overlap. Changes in land use, hunting, conservation and climate over the last several centuries has altered the ranges of these goose species and likely affected the ongoing subtle evolution of the shifting populations.

Watching the flocks of geese fly through the chill December air, the groupings  form, shift, and reform. Threats or disturbances I can't see send swaths of bird into the air. Something sends them slanting toward the river, while another group settles to the ground. The complex patterns mirror the changes and shifts in their populations on a much larger time scale.

Cornell Lab of Ornithology:
Cackling goose
Canada goose
Snow goose
Ross's goose
Greater white-fronted goose

San Joaquin River National Wildlife Refuge