They have been colonised by flocks of green ring-necked parakeets, an Indian bird that has mysteriously gone feral in much of south-east Britain.

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The next day in the five-million acre Pacaya-Samiria National Reserve, the sun burned bright, giving an electric glow to the throbbing canopy where parakeets swooped, sloths napped and tamarind and red howler monkeys played.

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And although the parakeets required human assistance to get from Asia to Britain, their arrival is only an extreme form of the constant, natural shuffling of species brought about by the movements of adventurous individuals which studies such as this seek to monitor.

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