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When the weather gets colder, do you sometimes wish you could pack up and head somewhere warmer? Many species of animals do this – it’s called migration.

Migration is common in the animal kingdom and it’s not just restricted to one group of animals. While there is no simple definition of animal migration, different species’ migrations have some things in common: populations of animals move from one place to another and the movement is cyclical so that over time the animals move backwards and forwards in a regular way.

There are many variations in animal migrations. Animals migrate different distances, at different times and for different reasons. One question that migration raises is: how do the animals know where to go? Animal migrations can cover thousands of kilometres – sometimes the animals have never made the journey before – yet they end up in the right place.

Different animals use different methods to navigate that may include using the Sun, stars, wind and landmarks to help find their way. Another method is to use Earth’s magnetic field. Scientists have shown that some species of turtle use magnetic fields to migrate, and there is a theory that other species, including some birds, do something similar.

The problem is that we don’t know how these animals detect magnetic fields. Until recently scientists have been unable to find cells or receptors that would allow an animal to sense a magnetic field.

A team of scientists recently announced they had discovered iron balls in hair cells of birds. Hair cells are known to be used to detect sounds in the ears of animals with backbones. The team found that each hair cell in birds contained a single iron ball in the same place in each cell. While all the bird species studied had these iron balls, they are not known in any mammal species.

Iron is a magnetic material affected by magnetic fields, including that of the Earth. The purpose of these iron balls is not yet known, however one theory is that they might be used by birds to detect magnetic fields and allow them to navigate.

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