The ospreys are back for the fifth consecutive year. Having flown over 3,000 miles from West Africa, the couple were reunited on Thursday 27 March when the hen arrived a few hours after the cock.

Spring is in the air and there is no time to waste. About once an hour the couple are mating and will continue until the first egg arrives, usually 12 days later.

During this time the hen needs to regenerate her reproductive organs which were subsumed into her body last autumn prior to flying off for the winter - no point carrying excess baggage across continents. The biology sounds like something out of science fiction.

In between mating, essential DIY repairs to the nest are undertaken and frequent sorties to catch fish. The female has been catching her own fish throughout our winter, but as soon as the first egg appears, the male will need to fish for the whole family catching as many as eight a day as the chicks prepare to leave the nest.

The ideal habitat for ospreys includes conifers for nesting and good fishing, which makes the upper reaches of the Glaslyn estuary a great location.

If the fish stay more than three feet below the surface they are safe beyond the reach of the osprey as it plunges vertically into the water. Sea trout, rainbow trout and brown trout are in season now and, as the weather warms, the grey mullet will come inshore and onto the menu.

The nest is huge, about six feet in diameter and big enough to accommodate four at a time. With chicks reaching adult size and a five foot wing span before leaving the nest, space is at a premium and can get a bit cramped.

British breeding ospreys were finally exterminated in 1916, not returning until 1954. Since then the numbers of breeding pairs have risen steadily to two hundred, mainly in Scotland. Will any of the Glaslyn chicks return?

Six have successfully fledged since 2005 and with a one-in-three likelihood of survival to the first birthday, there should be two out there somewhere. The most likely age to return to Europe is three years, so will 2008 mark the first Welsh osprey?

Returning is one thing, but finding a mate is another. It is likely the youngsters will return to the Glaslyn, but unless their arrival coincides with a suitable partner they will keep flying north until they find one. These are the tricky, delicate years in trying to re-establish the species, until the colony gains critical mass its long term future will not be assured.

In the meantime one of the greatest threats to the species are ruthless egg collectors, so the nest is guarded round the clock until the eggs are hatched.

The RSPB hide, about three miles upstream of Porthmadog, is a fantastic place. There is always something to see.

Cameras above the nest and from the adjacent tree produce live images displayed on a series of four large screens. If the birds are not on the nest, highlights of recent footage will be shown, a birdwatcher's 'match of the day'. This year a microphone by the nest will relay the sounds of chicks screaming for their supper.

Within the viewing hide itself are four top-of-the-range telescopes trained on the nest. More importantly there are lots of friendly RSPB staff, mainly volunteers, always on hand and willing to answer just about any question. "No sir, that's not the osprey, that's the camera!"

The setting is a treat, the river in the foreground, trees and the nest in the middle distance, with Snowdon in the background. On the day the ospreys returned the mountains were covered in snow. Blue sky, white fluffy clouds, snow topped peaks reflected in the Glaslyn and performing ospreys - magical.

And if you like steam trains the Welsh Highland Railway will be steaming past when the line to Porthmadog is reopened next year. I understand the drivers have agreed not to blow their whistle when the birds are on the nest.

Huw Jenkins