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There are as many different bird songs as there are birds. But some birds use sight as well as sound to attract a mate.

Male frigate birds, for example, use an inconspicuous shrivelled patch of red skin on their throat as a visual signal. They inflate it into a bright red throat balloon - a feat which can take around 20 minutes to achieve.

The females, who do not have a throat pouch, fly overhead to assess the male displays below. The size of the pouch gives the females a good idea of the male's vigour. When a female comes in for a closer look, the other males who do not yet have a nest will keep an eye on the proceedings.

If the female leaves, then one of the other males may come in to challenge the male on the ground and try to steal his nest.

When fights break out, the throat patch is an obvious target - if that is torn then the owner will not be able to attract anyone. If the interloper manages to steal the nest, then he has a chance of attracting a female.