Michael McCarthy: A wave of migrants flocking to our shores

Nature Notebook

"Oh to be in England," wrote Robert Browning, "Now that April's there", and it's certainly true that the month which begins tomorrow is exhilarating, in that it probably has more happening, in terms of the natural world, than any other four-week period in the year.

Tot it up. Buds are bursting. Leaves are opening. Blossom is blooming. Flowers are flourishing. Butterflies are emerging. Trout are rising. Birds are singing. Birds are nesting. And indeed, birds are arriving. The arrival of the migrant birds that winter in Africa and spend the summer breeding in Britain is perhaps the most exhilarating April event of all, as this month to come holds the peak of it.

Four well-loved species in particular – the willow warbler, the swallow, the cuckoo and the nightingale – will pour into Britain in the next three weeks or so (although it should be said at once, in rapidly declining numbers).

Even as you read this, millions of them are heading northwards towards us; it has been estimated that (before the declines began) as many as four million willow warblers alone landed in Britain every spring – four million pairs of tiny whirring wings crossing the Sahara, the Mediterranean, Spain and France and finally the Channel before fanning out across the land to begin their silvery descending songs.

The sheer scale of this phenomenon has always been hard to comprehend, but a few years ago our principal bird research organisation, the British Trust for Ornithology (BTO), found a way of doing it. The BTO has a network of several thousand observers who send in, online, records of birds they have seen, and these are represented on maps as red dots.

As the migrants pour in, the red dots start to cover Britain as if the country were suffering from chickenpox, and you can watch this happening each week on the animated maps of the BirdTrack section of the BTO website. Go to www.bto.org/birdtrack, click on "View Maps and Results", click on "Britain and Ireland", click on "animated maps", select a species, such as willow warbler, and select a time period, say February to April.

You can see the spring happening nationwide, live on your screen. Robert Browning would have loved it.

Clock a cuckoo

If you look around the BirdTrack site you will see that the willow warblers are just starting to build up their numbers, although their warbler cousins the chiffchaffs are now streaming into Britain. Swallows are just trickling in, but over the next fortnight that trickle will become a flood. There hasn't been a single nightingale yet; there have been just a couple of early cuckoos; but they're on their way, and after about 15 April both will be cascading into our countryside.