If you hear a high, yelping chatter somewhere overhead, look up for long wavering lines of white birds with black wingtips. Those are snow geese, on their way from Arctic nesting grounds to farm country far to the south, and the fall passage is fast: the heaviest movement fits inside about a month, often hundreds of birds at a time. A few birds in the line are dark grey with white heads, and for about two hundred years those were counted as a separate species, the blue goose (the dark form was named first, which is why the scientific name caerulescens means "bluish"). They are one species in two colours, and a young goose usually picks a mate the colour of the family that raised it. Tundra swans follow a week or so behind, bigger, all white, and higher pitched.