Although the dance is entertaining, the birds' motivation is purely practical.

The foot stamping vibrations trick the subsoil worms into believing that it is raining and so they head for the surface, only to be eaten by the seagulls and waders.

Worms come to the surface in the rain because worm tunnels can flood and drown their occupants. They may also surface in the rain so they will not risk drying out in the sun.

What is the meaning of the term 'one-parent family'?

This form of family consists of one parent and his or her children. One- parent families may be formed through widowhood, divorce or separation. They may also be formed when an unmarried person, usually a woman, raises children on her own.

In many Western societies, the one-parent family, especially the single-mother family, is becoming more common and tolerated. But whether it is as successful as the nuclear family is not clear.

It is becoming increasingly common, however, for a mother to retain her children and raise them. This is done with the help of her own parents or of government social agencies. In many countries this type of arrangement is more socially acceptable than formerly. In some parts of Africa, female-headed one- parent families are actually more common than nuclear families as the basic household unit.

How do accents develop and change?

Accents and dialects develop and change for two distinctive reasons, one phonetic and the other social. On the phonetic side, speech sounds change because of the way they are produced and perceived.

Phonetic changes do not take place continuously because language is used for communication. If your pronunciation is suddenly different from that of the people around you, you will not be understood. The communicative function of language provides a social brake on the phonetic cause of change.

In any community, however, phonetic changes can take hold from one generation to the next.

When communities are relatively isolated, as Australia was from England during its development, they may adopt different phonetic changes. This is how Australian pronunciation and England's pronunciation have diverged.

This kind of divergence brings a more subtle social effect into play. The information you convey when you talk is not limited to the linguistic meaning of your words, but includes many things about yourself, such as regional origin and level of education.

Graphic: SPBRAIGLO