Inside a wooden hide at the edge of a Perthshire loch, there is a flurry of excitement and a crackling of waterproof clothing. Binoculars are raised and whispered instructions exchanged. But hopes quickly fade as the alarm proves a false one. The bird that has swooped into sight is not the one they'd been waiting for.

"She is taking a defensive stand; it's not Him," said seasonal ranger Anna Cheshier. "Look, she's seeing him off. It's just an interloper trying his luck."

There is a palpable feeling of disappointment in the hide, where half a dozen people sit glued to the goings-on on a platform of sticks less than 200 yards away, 60 feet up a Scots pine tree. Inside the nearby visitor centre, many more are watching the action in real time on two large HD television screens. More than 100,000 people have already viewed the webcam.

The object of all the attention is Lady, the osprey, who stands in her giant nest and looks out to the blue skies. Having confounded the experts by not only living to the age of 26, against the eight years' lifespan the bird was thought to have, but also by producing 48 fledglings, she is now waiting for Him – a 10-year-old male with whom she mated last year.

He is due to land any day after a 3,000-mile migration back from west Africa. Ospreys mate for life so, if he has survived, he should be on his way. But if he doesn't get here within the next few days, Lady is likely to presume him dead and move on to another male. In her lifetime, she has already outlived two mates.

"The interest is huge," said Cheshier, 25, from the Scottish Wildlife Trust's Loch of the Lowes nature reservation outside Dunkeld, an hour's drive north of Edinburgh. "Lady is a star attraction and also very important. She has been coming back to her nest here for 19 years, but last year she was very ill and we all thought she was going to die, so no one imagined she'd be back this year."

Lady survived her near-death illness and arrived back from her African winter late last Monday night. She is not chipped or ringed, so it wasn't until later, when the cameras got a look in her eyes, that the rangers were sure the remarkable raptor had returned.

"She has a unique defect in the iris of her right eye – it looks like a lightning bolt," said Cheshier. "It was amazing to see her come back; she is bucking every trend, rewriting the books."

Since her return, Lady has been helping herself to the loch's supplies of perch and trout, even visiting the nearby Tay to catch herself a salmon, tidying up the nest, and waiting. Meanwhile, she is being closely watched by experts and fans. On the branches around her are positioned discreet cameras trained on the nest, one for day and one for night, and two microphones that pick up every ruffle of her feathers and her occasional piercing hawk cry. Live pictures are being eagerly watched around the world. Last year 33,000 people viewed the webcam online, but this year 120,000 have viewed the Lady of the Loch.

"We will have the computer on all day in the background, just having a look every now and again," said Jenny Hillier, up from Southampton with her husband, Pete, on a short break to see the bird. "We followed her on the webcam last year and the year before, but assumed she'd be dead. It's amazing she's back."

Pete Hillier has been writing about their trip on a wildlife blog to envious bird lovers around the country. "It's quite something to see her – I think it's the age of her, and the fact you can see her so close up here, that makes her so special," he said.

Colin and Dorothy Wilson from Dunfermline, Fife, are rooting for Lady, taking a detour from their spring break to make a pilgrimage to the nest. "We were here last June to see her and then we heard she hadn't been so well, so we were astonished that she was back, and we had to come. It makes such a difference to be able to see wildlife like this," said Dorothy.

Two other diehard osprey fans, Alan Barraclough, 77, and Hazel Studham, 74, have come up from Cumbria to see Lady. "She's a very special bird; we didn't think she'd make it through the winter. I hope her beau turns up," said Studham.

Smaller than an eagle, larger than a hawk, the osprey disappeared as a species from the UK in 1916, when the last pair was killed by egg and bird hunters such as Victorian collector William Dunbar, who guiltily wrote to a friend that their obsessions "had finally done for the osprey". Even when they returned in the 1950s to recolonise old haunts, their small numbers remained under threat, especially from postwar pesticides such as DDT.

But now the osprey's tenacity gives real encouragment to environmentalists. Roy Dennis, a conservation veteran and honorary director of the Highland Foundation for Wildlife, said Lady's return was an astonishing feat. "It's a real emblem, the osprey. People can see it [while they are] having a picnic on the side of a loch and you'll see one dive in, so it's very visible, distinct and identifiable, unlike a lot of birds.

"It's a great ambassador. But the reason osprey came back is that the habitat and the food supply are still here. It's the persecution of the species, the shooting, that has stopped. With some of our other birds, it will be harder as their habitat is going. If Scotland isn't becoming entirely the nature reserve of the UK, then it's certainly its lungs – the successes with sea and white-tailed eagles, red grouse are great, but we need to do more for conservation, encouraged by these successes."

But as Dunkeld's aged raptor enchants wildlife lovers around the world, Dennis thinks Lady may have a wait ahead of her. "I was out checking on osprey nests near me today and of 12 only two birds had returned. The weather hasn't been so good and the closest of the tracked males is still in Spain, so it's early days," he said. "It could be another three or four days."

Find the webcam at swt.org.uk/wildlife/webcams/loch-of-lowes2/