“High up in the air, sometime a mile or more above the earth, if the weather be clear, travel flocks of migrants where they can obtain a bird's-eye view of the country to be traversed. Geese have been detected four miles high. Rivers running like silver threads across the map, {{w|mountain range}}s, {{w|valley}}s, and the seacoast line, must be far more familiar to the birds that follow them systematically than to Macaulay's schoolboy.”