Heralds of spring

Spring arrives at different times for different birds. Ravens and Grey Herons start to breed as early as mid-February, while Swifts and Spotted Flycatchers don't arrive in the UK until early May. And that traditional herald of spring – the Cuckoo – doesn't generally make its appearance and start singing until the middle of April.

But a handful of birds, and one insect, are reliable indicators that the winter is behind us. Brimstone butterflies start to appear as soon as we have a couple of bright, war, days in March, and by that time the first Little Ringed Plovers have arrived on gravel pits, the first Wheatears are strutting around sheep pastures, and the first Sand Martins have appeared at your local reservoir. The latter are small, brown, seemingly frail birds that look like they would struggle to fly in the merest breeze, and yet they will have flown here from sub-Saharan Africa.

And then there's the Chiffchaff. Take a walk through Corby's woods today, and you'll hear, if not see, this unobtrusive warbler, endlessly repeating the song that gives it its name. "Chiff-chaff, chiff-chaff", with a lilting, sing-song intonation. Some of these, too, may just have arrived from southern Europe or Africa, but more and more are starting to eke out a winter existence in the UK, often by hanging around insect-rich sites such as sewage farms and chicken sheds. If you're not looking for them, they can go unnoticed, until they emerge to serenade the new season.

They're a reminder not only that spring is here, but that migration is a very complicated business indeed. Go out and hear one, and you'll know that nature's calendar has turned another page.