I raised my eyes to an eminence of about 15ft, and there, looking down at me with a curiously detached expression, a goose. Not just any goose: an Egyptian goose, a bird unexpectedly at home in trees. I was beside the Thames, rather than the Nile, for I am in London covering Wimbledon for this newspaper. It was odd to have this moment of eye-contact with the gyppo.

It took me back to my beloved Luangwa Valley: Lord, the racket those geese made, carrying the notion of territory to an insane level, cackling and honking in demented duet at everything that flew past. These are pan-African birds, rather than exclusively Nilotic: you have a right to ask, then, what this statuesque and mercifully silent bird was doing at a reservoir just short of the most beautifully named landmark in London, Barnes Bridge. It was probably an escape from a wildfowl collection: though the birds breed here, notably in Norfolk, and if you care about such matters, you can tick it. And so, musing on the mighty Luangwa and all things Zambian, I walked on. Inevitably, I was interrupted by the screeching of parakeets.

Ring-necked parakeets, take me back to India, to that time in Mysore, sitting on the balcony with a beer and watching the evening unfold, the night shift of fruit bats clocking on as parakeets in their thousands screeched in cacophonous roosting squabbles. They are part of life in southeast England now, and have been sighted in Scotland: escaped pets who found the ever-balmier climate of this island to their liking.

I turned back to the reservoir and there before me a pair of ducks, gleaming in copper with an absurd clown's beak of blue. Ruddy ducks: and this time, my mind went back to a traumatic cricket tour that fell apart in Antigua, when I found occasional relief in a patch of wetland near the hotel. There, I saw ruddy ducks in the place where they are supposed to be. They got out from collections in this country and bred.

In the middle of the reservoir, there is a series of rafts for the convenience of breeding birds. But it was being used for the convenience of terrapins. There were half a dozen of them, the size of dinner-plates, red-eared terrapins to be precise. This is a North American species. They make cute pets the size of a 50p piece, but they are rather good at growing. People bought them by the bucket during the craze for Teenage Mutant Ninja Turtles. They, or their descendants basked in the sun, revelling in the joys of England in Wimbledon week.

I am supposed to be angry about all this, I know. After all, the introductions of exotic species is one of the greatest bringers of extinctions, after habitat destruction. But I can't help feel a sneaky admiration for the creatures that adapt to an alien place and make it their own. I can't help but relish the incongruity: and feel strengthened by the indomitable nature of life.

Hatred of exotic species seems sometimes to be the blood-brother to xenophobia, and besides, I am devoted to the little owls that whoop and yap around my place in Suffolk. Extreme reactions to exotic species sometimes seem more like ethnic cleansing than conservation. All the same, I know I am in the wrong here: but we are humans and cannot help but humanise our response to non-human life. Some see alien invaders: I see a glorious assertion of life, and stirring success against the odds.

Wimbledon highlights

But you will be agog to hear how the annual Times/Guardian Wimbledon Bird Race is going.

Well, it's a caucus race, really, Stephen Bierley, Guardian tennis correspondent and a damn good birder - better than me, anyway, though that's no great mastery - and I annually see how many species we can rack up at the All England Club in the fortnight.

It's not easy: the tennis can be a serious distraction and on Centre Court you are restricted to a small patch of sky.

So far we have reached the dizzy heights of five: feral pigeon, pied wagtail, lesser black-backed gull, black-headed gull and swift. We'll have to hope to recapture past glories such as heron and kestrel next week. We might even get ring-necked parakeet.

Nadal to Descartes

So I bought a glass of lemonade for a tennis-loving schoolgirl, daughter of an old friend, and she cheered me up immensely by talking with equal enthusiasm of Rafael Nadal, the Spanish tennis player, and René Descartes, the French non-tennis player. So naturally we turned to Peter Singer: and here she spoke with still greater delight.

The following day, I read that Spain, of all places, was preparing to grant rights to apes. Singer, an ethical philosopher, has proposed the notion of expanding circles of concern: saying that over history, human notions of concern and responsibility have moved progressively and relentlessly beyond family, beyond tribe, beyond nationality, beyond race - and that the next logical step is the expansion of our circles of concern beyond the barrier of species. Gloriously, Spain leads the way.

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