So what happens on Mauritius , or indeed any island, is that when endemic vegetation or animals are destroyed for any reason, the exotic forms leap into the breach and take over. It's hard for an Englishman to think of something like privet as being an exotic and ferocious life form - my grandmother has neatly trimmed privet bushes lining her front garden - but in Mauritius it behaves like a bunch of marauding triffids. So does the introduced guava and numerous other foreign invaders, which grow much more quickly and produce many more seeds.

Black ebony comes from the lowland hardwood forests of Mauritius , which is why the Dutch first colonised the island. There's hardly any of it left now. The reasons for the forest being cut down include straightforward logging, clearing space for cash crops, and another reason: deer hunting. Le Chasse.

Vast tracts of forest have been cleared to make game parks, in which hunters stand on short wooden towers and shoot at herds of deer that are driven past them. As if the original loss of the forest were not bad enough - and for such a reason - the grazing habits of the deer keep the fragile endemic plants from regrowing, while the exotic species thrive in their place. Young Mauritian trees are simply nibbled to death.

We passed through huge fields of swaying sugar cane, having first negotiated our way past the sugar estate's gate keeper, an elderly and eccentric Mauritian called James who will not let anybody through his gate without a permit, even someone he's let through every day for ten years but who has accidentally left his permit at home that day. He did this to Carl recently, who since then has been threatening to superglue the gate shut in revenge, and it's quite possible that he will. Carl is clearly the sort of person who will get as many laughs as he can from a situation by threatening to do something silly and then try and get a few more by actually doing it.

There was a more serious confrontation a little while earlier when Carl and Wendy arrived with a party of officials from the World Bank from whom they were negotiating some financial support. James wouldn't let them in on the grounds that they had two cars and he was only authorised to let in one.

James also reports to Carl and Richard regularly about the movements of the kestrels, not because they've asked him to but just because, other evidence to the contrary, he likes to help. If he hasn't actually seen any kestrels he'll still, in a friendly and encouraging sort of a way, say that he has. This means that now, whenever Carl has to change the coloured bands the kestrels wear round their legs, he makes a point of putting on a different colour so that he will know James is lying if he claims to have seen a kestrel with a band that doesn't exist.

The kestrel we were going to see had been trained to take mice in 1985. The purpose of feeding kestrels in the wild was to bump up their diet and encourage them to lay more eggs. If a kestrel was well fed then Carl could take the first clutch of eggs the bird laid from its nest and back to the breeding centre, confident that the kestrel would simply lay some more. In this way they were increasing the number of eggs that might hatch, but there was a limit to the number of birds available to sit on them, so they had to be incubated artificially. This is a highly skilled and delicate task and requires constant monitoring of the egg's condition. If an egg is losing weight too rapidly, by evaporation of liquid through its shell, then portions of the shell are sealed. If it is not losing enough, then portions of the shell are meticulously sanded to make it more porous. It is best if an egg can have one week under a real bird and the other three in the incubator - eggs which have been swapped around like that have a much higher success rate.

Richard yanked the Landrover to a halt on the edge of the forest near the bottom of the gorge and we piled out of it. The air was brisk and clear, and Richard strode about the small clearing making an odd assortment of calling noises.

Within a minute or two the kestrel came zipping through the forest and perched itself up in a high tree overlooking a large hemispherical rock. Since the bird is adapted to living in the forest rather than the open land, it does not hover like many falcons, but can instead fly at great speeds unerringly through the forest canopy, where it catches its food of geckos, smaller birds and insects. For this it relies on having fantastically keen and fast eyesight.

We watched it for a while and it watched us intently. In fact it watched everything that moved, glancing rapidly in one direction after another with constant attention.

`See the way it's so interested in everything it can see? said Richard. `It lives by its eyes, and you have to remember that when you keep them in captivity. You must make sure they have a complex environment. Birds of prey are comparatively stupid. But because they've got such incredible vision you've got to have things that will keep them occupied visually.

'When we originally started breeding birds of prey in captivity we brought in some very, skittish birds and whenever anybody went past the aviary the birds just went mad, and we thought they must be upset by the disturbance, so someone came up with the bright idea of what's called a skylight and seclusion aviary. All four walls were opaque and just the roof was, open so that there was no disturbance for the birds. But what we found was that we'd overdone it. The offspring that were born in those environments were basket cases because they hadn't got the sensory input they needed. We'd got it completely the wrong way round.

`I mean, animals may not be intelligent, but they're not as stupid as a lot of human beings. You look at the primate areas in some zoos which are equipped with metal green architect-designed `trees' which, in a minimalist sort of way, reproduce the shape of the tree, but don't actually include any of the features that a monkey might find interesting about a tree: leaves and bark and stuff. It may look like a tree to an architect, but architects are a lot more stupid than monkeys. We just got a brochure through from the States for exactly this - fibreglass trees. The whole brochure was designed to show us how proud they were of what they could sell us here in Mauritius, and showing the particular paints they had for painting lichen on trees. I mean it's bloody ridiculous, who are these people? OK. Let's feed the bird. You watching?

The bird was watching. It's hard to avoid saying that it was watching like a hawk. It was watching like a kestrel.

Richard swung his arm back. The kestrel's head followed his movement precisely. With a wide underarm swing Richard lobbed the small mouse high up into the air. For a second or so, the kestrel just watched it, jittering its legs very slightly on the branch as it engaged in monumental feats of differential calculus. The mouse reached the top of its steep parabola, its tiny dead weight turning slowly in the air.

At last the kestrel dropped from its perch, and swung out into the air as if on the end of a long pendulum, the precise length, pivotal position and swing speed of which the kestrel had calculated. The arc it described intersected sweetly with that of the falling mouse, the kestrel took the mouse cleanly into its talons, swept on up into another nearby tree and bit its head off.

'He eats the head himself,' said Richard, `and takes the rest of the mouse to the female in the nest.'

We fed the kestrel a few more mice, sometimes throwing them in the air, and sometimes leaving them on the hemispherical rock for it to dive for at its leisure. At last the bird was fed up and we left.

The term `fed up' actually comes from falconry. Most of the vocabulary of falconry comes from middle English, and zoologists have adopted a lot of it.