The first time I hear a peacock scream it scares the bejesus out of me. I’m sussing out my tree tent – cleverly strung between four sweet chestnuts a few feet off the ground – and the sound stops me in my tracks. The bird struts past, glorious tail ablaze, in pursuit of the less colourful (and seemingly unimpressed) peahen before embarking on an elaborate twerking ritual.

Close encounters with nature are part of any experience on Brownsea Island, the largest isle in Dorset’s Poole harbour and the second biggest natural harbour in the world after Sydney. As I sit back and soak up the view across to the Purbeck Hills, I spy white bunny tails disappear into bushes; oystercatchers flap above the sea and I lock eyes with a sika deer grazing nearby before she darts gracefully away.

Ten minutes by ferry from the mainland, National Trust-owned Brownsea is an Enid Blyton hideaway – it inspired the Famous Five’s adventures on Whispering Island. But it’s perhaps best known as the birthplace of the Scout movement: in 1907, Lord (then plain Major) Baden-Powell brought a group of 20 boys here to take part in an experimental camp, living close to nature and practising practical skills he had learned in the army during the Boer War. It launched a global movement, and now groups from 75 countries visit the island each year.

Until recently only Scouts, Guides and other private groups could camp here – but a new “eco-camping” option now welcomes the general public on certain dates, with numbers capped at 30 mid-week or 150 at weekends. There are tents or hammocks for hire (including three tree tents, hung by the organisers), or visitors can bring their own, while gas cooking stations and all utensils are provided. Hot water for showers is heated by biomass, using wood from the island, and there’s a sheltered communal dining area.

I arrive on the last ferry from Sandbanks. As the daytrippers head home, I can’t help feeling a little smug. The campsite’s on the south shore, a 20-minute walk from the dock, and in late May I’m the only person staying.

I wander into the forest and it’s not long until I spot a red squirrel, and then another – Brownsea is home to one of England’s last colonies. I’ve been given a star chart and as darkness falls I study the constellations before climbing into my tent. It’s a bit like sleeping on a trampoline, but there are no issues with hard ground or deflating mattresses and I wake around dawn to the sound of a woodpecker tapping overhead.

Despite its size – just one and a half miles long and three-quarters of a mile wide – Brownsea has wide-ranging habitat, from heathland to sheltered lagoons and woodland with more than 100 tree species. For kids, it’s an adventure playground: there are nature trails, crayon rubbings and tree climbing routes, and a natural play area. Regular ranger-led safaris hunt for wildlife and, over the summer, special family adventure weekends run too, with campfires and activities from archery to canoeing.

I’m not always a fan of organised tours, but there are two free introductory guided walks daily, and volunteer Clive brings the island’s history to life so vividly I’m soon wondering why there hasn’t been a BBC drama about this place.

So many colourful characters have shaped the island. Henry VIII built the castle in the east (now leased by the John Lewis partnership as a hotel for its staff). I love the tale of Colonel William Petrie Waugh and his wife Mary, who bought the island in the 1850s thinking they’d found a source of high-quality clay and built a pottery, village and church, but fled to Spain bankrupt when the material proved substandard. There was bon vivant tobacco baron Charles van Raalte, who used it for holidays and insisted all employees played an instrument, and whose widow turned the whole place into a daffodil farm. In the 1920s came reclusive Mary Bonham-Christie, who banished all inhabitants and let nature take over. During the second world war the island served as a decoy for Poole, with pyrotechnics tricking Nazi bombers into targeting Brownsea instead.

The north of the island, run by the Dorset Wildlife Trust, is a very different landscape again with lagoons and lakes. There are five hides for watching the prolific birdlife, and I sit for ages spotting common and sandwich terns, listening to their cries, before heading to the island’s one cafe for a cup of tea.

It’s late afternoon when I walk back to camp, again passing daytrippers on their way home. In the height of summer 1,500 people visit, but with so many different areas to explore the island can swallow them up, so it rarely feels crowded.

My wildlife spotting isn’t done yet. Those on adventure weekends can hire bat detectors and, as dusk falls, I take one and creep quietly through the woods. Beeps warn me that I’m getting close and I look up to see tiny, perfect bat-shaped silhouettes whizzing and swirling in the sky.

The next morning on a seashore ramble, led by ranger and marine biologist Miranda, we lift rocks and seaweed to find bugs, anemones and crabs. We learn about the sealife in Poole harbour, and how the deer can swim to other islands. Indeed, the water looks clear and clean and there are several beaches giving easy access to it – but it’s still chilly so I stay shore-bound.

You don’t have to camp to stay overnight on Brownsea. The National Trust has two cottages on the east coast near the castle while South Shore Lodge, a lovely Victorian gamekeeper’s house with a private garden leading to the sea and dormitory accommodation for 24, has just opened for public bookings for the first time. But camping feels like the best way to enjoy the island in its rawest form – and now you don’t need to know your “dyb dyb dyb” from your “dob dob dob” to do so.
• Camping was provided by the National Trust (nationaltrust.org.uk). The campsite is open from 1 April-30 Sept. Camping costs £22 a night adult, £11 child (5-17), under-5s free including communal cooking facilities; see website for available dates. Three-person pre-pitched tree tents (which come with an additional tent for kit storage) are £30 extra

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