BRACE yourself, it’s science lesson time and the subject this month is convergent evolution. This posh-sounding term describes a trend in nature where completely unrelated organisms end up looking or behaving in the same because they have similar life-styles. For instance, sharks are fish and dolphins are mammals but they have similar shaped bodies because they both have to swim fast to catch fish in the sea. One did not evolve into the other – their separate evolutions came from different directions and converged at the same outcome.

What got me thinking about this phenomenon recently was observing some birdwatchers in a hide at a local nature reserve who had their binoculars trained, not on the distant ducks floating on the lake but on thistles growing on a bank in the foreground.

"Two peacocks, a small tortoiseshell and a red admiral," one said to the other - they were watching butterflies.

It’s no real surprise that some birdwatchers are also butterfly spotters, because in many ways these two groups of animals are similar. They have both evolved colourful display plumages, the males often vigorously defend feeding and breeding territories from each other and, crucially, they can both fly.

The most startling look-alikes in the airborne stakes are hummingbirds and hummingbird hawk-moths, the latter not quite butterflies, I admit. These two species drink nectar from flowers and they do it using the exact same technique, by hovering in mid-air in front of the bloom and inserting a long, thin tube - the bird's beak and the moth's proboscis. Being a similar size they are often mistaken for each other.

The shared behaviour that I find the most astonishing, though, is that of seasonal, long-distance migrations. As farmers and gardeners in the south of England know only too well, cabbage white butterflies cross the English Channel in vast numbers during some summers, and cause havoc in arable fields - some even make it up to Ryedale and boost our local brassica crop pest numbers.

Even more impressive is the journey of a butterfly related to the famous red admiral. Painted lady butterflies have an incredible life cycle which happens to some degree every year. It all starts in January in the Atlas Mountains of North Africa, the central part of their range.

The first generation of adults emerge and migrate north to Europe where they breed to produce the next generation within two months, which again head north. The third generation are the ones that cross the Channel to us, mate, then find thistle plants to lay their eggs on.

Some years relatively few painted ladies make it to Britain, but 2009 was a boom year and this year promises to be similar. There has been a flood of hundreds of thousands of them heading towards Britain because beneficial rains in North Africa last winter started the whole process off so well.

What do they look like and where can I see them, you may be asking? Well, everyone is familiar with red admirals; imagine one of them in washed-out, pastel colours, the same pattern but in grey and salmon pink instead of black and red. These are in fact closely related both being in the Vanessa genus; the red admiral is Vanessa atalanta, and the painted lady Vanessa cardui.

Where to see them is not so straightforward as they could turn up anywhere, but thistle is a favourite food flower for the adults and caterpillars, so that's a good bet. Also, like many butterflies, they can’t resist buddleia bushes, so check among the tortoiseshells and peacocks on the purple flower spikes, because there may well be a painted lady or two among them. If all this gets you as excited as me and you do see some, you can report your sightings online and be a part of a Butterfly Conservation Society survey. See butterfly-conservation.org