Ponds, lakes, and rivers — at least those with open water —are filling with courting ducks. Why did the birds arrive what amounts to early, when there is lots of ice, little water, and no good place for a nest?

It's called zugunruhe --zoo-gun-roo -- a German term for the restless feeling birds experience at migration nears. The birds' endocrine system releases hormones in response to longer daylight hours. The birds at first increase their activity level, spending more time flying, movements that orient them to their migration direction.

And then, off they go, here they come.

Birds wintering near the equator, where daylight hours are constant throughout the year, act the same way. Scientists are uncertain how this happens, speculating it could be because of the changing angle of the sun.

Before the birds begin migration they will eat heavily to gain weight. They need the calories for the upcoming long flights.

Once breeding season ends the birds' reproductive organs shrink, reducing weight to make flight easier. The reverse is happening now, those hormone signals causing the reproductive system to grow again to accommodate breeding.