e. Navigation/ migration in adults

If you take an adult bird anywhere in the world, it is thought that it will be able to find its way back to its breeding grounds. This is known as true navigation, and cannot be explained by a bird's following a simple compass direction alone (as seen in section C.i.2).

The difference is illustrated elegantly by Perdeck's experiments. When he took adult starlings from the Netherlands to Switzerland he found that they compensated for the displacement and migrated towards their original destination. Some birds reached their usual wintering sites through true navigation¹². These starlings had used environmental cues to work out where they were on Earth in relation to their migration destination. We still don't know how birds manage to do this, but it's possible that they use cues such as the Earth's magnetic field.

Once adults know where they are, they use one of the compasses above to identify the direction their flight will take. When birds reach a familiar area, they may use other cues to reach their nest site. For instance, some birds follow their noses and smell their way home. Papi showed that birds with an impaired sense of smell were less likely to successfully return to their roost, although many still found their way home^{2, 15}.

Birds are able to use more than one type of navigation and are likely to use whatever reliable cues are available to them. Different cues may reliable at different spatial scales. For instance, new research by Dr Tim Guilford at the University of Oxford, suggests that pigeons use a magnetic compass to navigate through unfamiliar territory, but used the roads in Oxford to navigate close to home. Some flew along the Oxford bypass and even turned off at particular junctions¹⁶!

So whilst much has been discovered about how birds orientate themselves, many questions remain. For instance, how do adult birds know where they are when they have been displaced, which cues are used during the different stages of the journey and how do the different stimuli and navigational tools interact? These are just some of the questions that continue to challenge researchers today.