You can take a pigeon from its loft and release it up to 1,000 miles away from its loft and it will fly home! How does the pigeon know which way to fly? How does it know where home is? How does it keep flying in a straight line? These are all questions we've been trying to answer for the past 30 years!
We began this process by putting tiny radio transmitters on the pigeons and then following them in an airplane on their return flight. That meant we had to build tiny radio transmitters and figure out how to attach them to the pigeon in a way that didn't bother the pigeon. Then we had to find a way to follow the radio signal from an airplane. Finally, I had to learn how to fly the airplane. It turns out that biologists have to learn all sorts of things in addition to biology.
Over the years we have learned that pigeons use many cues to find their way. They can use the sun to tell them direction. But when the sun is covered by clouds they seem to switch and use the earth's magnetic field. But knowing where north is won't allow you to find your way home. Imagine that you are in a small boat in the middle of the ocean. I give you a magnetic compass so you know the direction of north, south east and west. Which way do you steer your boat?
You can't tell. Which way you want to steer depends on where you are and where you want to go. You need to know more than which direction is which; you need to know where you are and where you want to go. Once you know that you are in the Atlantic Ocean and home is west, then a compass is very useful in steering west.
We call this two step system "The Map and Compass." The compass is a way of telling direction, the "map" is a way of telling where you are in relation to where you want to go. A road map is one kind of map, a nautical chart is another. We have no idea what kind of map the pigeon uses, nor do we all agree on what the pigeon's map is based. A group of biologists in Italy led by Floriano Papi believes that the map is based on smell; other biologists suspect that the earth's magnetic field may be important. I simply don't know. Tell me your ideas.