Under the BTO's Internet bird migration study, which we have featured several times in the past few weeks, this is the Week of the Swallow and according to their experts, the first waves of this wonderful visitor should have been arriving on the UK's southern coasts over Easter weekend.

Well I spotted the first two arriving on Good Friday - over the River Lune in Cumbria, which is just about as far as you can get from the south coast in England.

They came swooping down the river, gave a few victory rolls, and then immediately started hunting for food, no doubt much needed after their heroic flight from Africa.

And that's when I began to worry. For I had spent the whole sunny morning casting a fly for trout and not only had I failed to see a fish move but, alas, I had counted just two olives hatching on the water in some four hours.

The olive, I should explain to the uninitiated, is the generic name for a large family of winged insects which spend most of their lives on the river bottom until, one fine day, they float to the surface, hatch, mate and die - but not before the females have laid their eggs back into the water.

Olives used to arrive well before the swallows, in March or even late February on a warmish day, and were the staple diet of trout recovering from rigours of the winter breeding season. A hatch would bring trout in their hundreds slashing at their floating prey - and 20 years ago, I had to buy a special spray to remove the corpses crushed into my windscreen during the drive to the river.

Later in the year, they were the staple diet of birds feeding their young. Not just swallows and sand martins but dippers, the odd gull, and yellow wagtails, which would chase them across the water tumbling and wheeling like clowns tripping over their baggy pants.

But that was then. Last weekend, I spotted olives at the rate of one every two hours - and not a rising trout at all. And those two poor starving swallows: how were they going to replace the fat they burned in those thousands of miles from Africa?

My chagrin grew on Easter Day, when the Sunday Times carried a long, sad article on the decline of the River Bourne in Hampshire, once one of the finest chalk trout streams in Britain (and perhaps the world).

There, it seems, fly life has all but disappeared. And, the most worrying facet of all, no one seems to know why.

There could be hundreds of reasons: nitrate pollution, as we discussed two weeks ago; increased erosion of river beds caused by more flooding; acid rain and many, many more. But as far as I know, no one is studying this problem, which has the potential for devastating long-term damage to our wildlife.

We have, and I am happy about this, pressure groups which protect threatened species like the otter, the crested newt, all manner of birds, even the humble hedgehog. But no one seems to care a jot about our insects.

If the olives and their myriad millions of cousins in the insect family are dying out, the bottom of our wild food chain is being ripped away. Without a plentiful supply of insects, my first two swallows of summer will have difficulty rearing healthy young strong enough to make the arduous return flight to Africa to give us pleasure next year.

Isn't it about time that English Nature or similar quasi-government bodies started taking this threat very seriously indeed? It might be too late already to avoid much of the danger - but we might be just in time to prevent disaster.