..geese came winging down the coast from Iceland and Spitzbergen each October, in great skeins that darkened the sky and filled the air with the rushing noise of their passage

d. Orientation

It might seem amazing to us that a swallow can find its way thousands of miles from England to Africa, and then return the next summer to the barn in which it was raised. However, if you take a closer look, a bird's eye view, at the Earth, it is covered with cues and reference systems that birds can tap into, which help them find their way. Many cues in the environment occur in predictable patterns and can provide a directional reference system. These are used by many different creatures, including birds, as compasses.

i. Sun compass

In 1952 Gustav Kramer showed that birds use the sun as compass to orient themselves toward their migration destination. When he kept starlings in cages with a view of the sun, they orientated themselves successfully in their migration direction. However, when he blocked the view of the sun, the birds showed no directional preference¹³. The next year, Kramer used mirrors to change the apparent direction of the sun and the starlings changed direction accordingly.

From earth, the sun appears to be on the move constantly: arcing across the sky during the day, and rising and setting at different points throughout the seasons. In order to use the sun as a compass, birds need an internal clock to calibrate its movements. Kramer proved the existence of such a clock by artificially decreasing day length in a captive population of birds, thus fooling them into thinking that the seasons had moved on. When the birds were released, they orientated themselves using their adjusted clock and headed in the wrong direction¹.