Sirocco, the kakapo, has been taking the world by storm and made this endangered flightless parrot from New Zealand a household name. In case you missed the fifth programme, we had what can only be described as an intimate encounter (Sirocco tried to mate with my head) and the resulting two-minute clip somehow appeared on news programmes from America to Australia. Now, it has become an internet sensation - apparently, it is the most-viewed video on the BBC website and, at the last count, has been seen by more than a million people on YouTube.

We knew at the time that Sirocco's little outburst would make good television, but had absolutely no idea that it would attract quite so much attention. Meanwhile, Stephen has taken to calling me the Kakapo Porn King and I am slowly getting used to people coming up to me in the street and asking if the scars have healed.

The good news is that, as a result, the kakapo project in New Zealand has been swamped with donations and offers of help. That makes it all worthwhile.

This week also marks the end of the series - with our search for the blue whale in Baja California, Mexico. I'd been looking forward to introducing Stephen to the largest animal on the planet and it more than lived up to expectations. Stephen was so taken by the experience that he said it was one of the best days of his life. It's hard to describe what it's like being next to an animal almost as long as a Boeing 737, but suffice to say it is one of the greatest wildlife experiences anyone could ever hope to have.

The whole point of the series, of course, was to retrace the steps I'd taken with Douglas Adams twenty years earlier - and the final programme was the only time we didn't actually do that. We should have gone to China to look for the Yangtze river dolphin, but we had to cancel our plans when this troubled freshwater dolphin was officially declared extinct before we could get there. It was this, more than anything else, that brought home to me what had been happening in the twenty-year gap.

Last Chance to See was a series about endangered species and conservation, of course, but six hours of doom and gloom television ranting about all the terrible things we are doing to the planet, just wouldn't have worked. Few people would have watched and many of those would have switched off halfway through. But our hope was that, by mixing extremes such as these - the silliness of Sirocco mating with my head, the sheer joy of encountering a blue whale and the shock and sadness of the Yangtze river dolphin disappearing before our very eyes - we could make a series about conservation without anyone really noticing that it was a series about conservation. 

The Yangtze river basin, for example is home to an astonishing 10 per cent of the entire human population and its increasingly endangered wildlife has been fighting a battle against the odds for years.

Garamba National Park - home of the northern white rhino, which featured in last week's programme - isn't much better, slap-bang in the middle of a war zone.

I don't think I'd choose Madagascar, either: this week's endangered species, the aye-aye, faces an inordinate number of threats from habitat destruction and civil unrest to an unrequited prejudice against anything that looks like an alien. I was really shocked by the damage to the rainforest there over the past 20 years.

I suppose the safest place in the world depends on what sort of endangered animal you happen to be. If you're a kakapo, one of those trusting parrots we meet in the fifth programme, then Codfish Island off the southern tip of New Zealand is a fairly safe bet. Every single bird is cared for 24/7 by a veritable army of volunteers.

If you happen to be an otter, on the other hand, the UK would be better: decades of intensive conservation work here have enabled otters to make a dramatic comeback.

There are official ways of measuring these things. The Environmental Performance Index, for example, quantifies the environmental performance of every country's policies. Switzerland is currently top of the list, but I lived just outside Geneva in the 1980s and, to be blunt, wouldn't necessarily choose to do so again.

So if I had to pick just one country that really, truly, honestly cares about conservation, I think I'd go for Costa Rica. This tiny nation is fifth on the Environmental Performance Index but is home to roughly five per cent of the world's biodiversity. It's by no means perfect, of course, but it does have one of the best conservation records in the world: more than a quarter of the country is under some form of official protection and sustainable development is actually being incorporated into national policy.

As a bonus, Costa Rica has just been rated number one in the 2009 Happy Planet Index (the UK came 74th and the States 114th). Whoever said that conservation is not in tune with our own survival?

It was originally called Congo Free State, then Belgian Congo, then Congo-Léopoldville, and then Zaire. Now it is called the Democratic Republic of Congo, or just the DRC for short.

Expatriates living in this humanitarian disaster zone say the acronym stands for 'Danger: Rebels Coming'. It's scarily apt, under the circumstances. The Country Formerly Known as Zaire lies in the heart of war-torn Africa. It has been fighting a series of complex, many-sided wars for umpteen years. Millions of people have died and many more have been displaced from their homes.

Garamba itself - where Douglas and I saw no fewer than eight of the 22 northern white rhinos that survived at the time - is now surrounded by armed refugees, guerrillas, rebels, Congolese army troops and horse-riding poachers. Stephen and I responded to a question about Zaire that was sent into the website.

The poor rhinos couldn't have picked a worse place to live, and I suppose the result was inevitable. So, unfortunately, Stephen and I were too late to see them - they are now believed to be extinct in the wild. Read more about the Northern White Rhino in the animals section.

Conservation is challenging at the best of times, but protecting wildlife in a war zone is another matter altogether. However, it's not impossible, as we discovered when we went to see mountain gorillas on the same shoot. They live in the border zone between Uganda, Rwanda and the DRC and, like the rhinos, have been caught up in a vortex of human conflict and misery on and off for years. Yet, despite some serious setbacks, the remnant population of about 700-750 clings on for dear life - and may even be increasing. The reason is simple: tourism.

Now, I realise that the business of gorilla tourism is a vexed one (some people worry that it might cause disturbance or expose the animals to deadly human diseases) but there's no denying that it makes gorillas worth a lot more alive than dead. By generating hundreds of millions of dollars a year, even in times of trouble, it's the one thing that has so far guaranteed their survival.

One species disappears, another hangs on for dear life - it's rather typical of the mixed bag of good and bad news we encountered during our five months of filming.

It didn't even cross my mind at the time, but he was a little apprehensive too. We did have at least one thing in common. Our Amazon adventure had been occupying our thoughts a great deal over the previous year or so, and we were both thrilled and eager to get started.

We went to the Amazon to look for the Amazonian manatee.

Despite weeks of trying, Douglas and I had failed to see one on our original expedition twenty years ago. Actually, that's not strictly true. Our jungle guide saw a manatee disappear beneath the surface of a remote tributary of the Rio Negro. I saw the ripples after it had disappeared and Douglas nearly saw the ripples. (You can listen to the 1989 radio programme for more on this!) Stephen and I were determined to do better. But we failed too. We were all outfoxed by one of the slowest creatures on Earth.

Perhaps it wasn't surprising. No one knows how many Amazonian manatees are left, but here's a hint at their current status: in the 1980s, there were reports of as many as 1,000 manatees huddled together in a single river or lake; nowadays, a gathering of half a dozen is considered a lot.

But we did see a lot of other wildlife along the way, of course, and one of the highlights for me was a day we spent swimming with pink river dolphins. These are the kind of animals young children paint at school. A particularly naïve teacher might relegate the exuberant splashes of dazzling pink, the chubby cheeks, the long beak crammed with crushing teeth and the gargantuan designer flippers to a wild and fertile young imagination. But pink river dolphins are real - and they really are as pink as an embarrassed teenager's blush.

I also discovered the science of dendronautics - the navigation and study of the jungle canopy. If you fancy a career change, with more day-to-day risks than coal mining, deep-sea diving or flying with the Red Arrows, this is the job for you. And it's perfect for dinner party conversation because, joy of joys, you would be allowed to call yourself a dendronaut!

But I think what stands out most in my mind from this trip is the Amazon rainforest itself. It's hard to grasp the sheer scale and splendour of the largest non-stop expanse of pure, unremitting nature on earth. Larger than the whole of western Europe and heaving with wildlife, it is utterly mind-boggling.

It's also disappearing fast. You couldn't dream up a bigger list of more damaging activities if you tried: cattle ranching, land clearance for soya-bean plantations, small-scale subsistence agriculture, logging, and a mixed bag of commercial agriculture, mining, urbanisation and dam construction... just for starters.

What's really frightening is that nothing much seems to have changed since deforestation first hit the headlines several decades ago. Have all those years of campaigning, fund-raising, pleading, cajoling and cautioning by so many individuals and conservation groups made the slightest difference? I suppose the positive response would be to say that it must, surely, have slowed things down. But clearly it hasn't slowed things down anywhere near enough.

I hope you enjoy the first programme in the series. It starts on Sunday 6 September, 8pm on BBC Two.