A loud 'zit-zit!' call grabbed my attention in the garden yesterday, sharper and more penetrating than most of the birdsounds I hear. It came from a bird that typically needs its voice to be heard above the noise of rushing and gushing waterfalls and weirs, hence the cut-through nature of the sound - it was a Grey Wagtail.

In typical bounding flight, it looped down to the pond edge, for the draw of the waterside is inescapable for this bird. If it hasn't got its toes damp, it's not happy.

On landing, it immediately began to pump its long, long tail with great zest, more than living up to its 'wagtail' name.

There's a couple of things that deserve immediately interrogation here. The first is why this bird should be called the 'Grey Wagtail' when the most obvious plumage feature is its sunshine-yellow rear end. Yes, it is grey above, but surely this should be the 'Grey-and-Yellow Wagtail'? And when I say 'grey above', actually you can see - even in my grainy photos in the half-light - that it is a grey with strong hints of steel blue and moss green. This is a fantastic looking bird.

The second point of interest is this tail-wagging. Why? Why should a bird pump its tail so vigorously? Well, actually no-one knows for sure. Young wagtails do it as well as adults, so it is not a sexual signal (although the males do make full use of their tail during courtship). Theories have been put forward that it helps them flush the insect prey they rely on, or that it in some way makes them less conspicous against the surging waters of the streams they like to inhabit. Or is it a signal to predators that the bird is alert and ready to flee if needed, so don't bother trying to catch it?

What is clear is that the wagging is often more intense when they feel threatened or are a bit unsure, such as straight after landing, or before taking off, so it is certainly not exclusively a feeding device. It often looks rather like a comfort behabiour - "I'm a bit anxious but with a bit of wild-wagging I feel ok". But I'm convinced it does perform some kind of message for other wagtails, too. When the Grey Wagtail lands, its first pump is so strong that it flashes its white outer tail feathers.

The next time you see a wagtail, whether a Grey or the more familiar Pied, see if you can work out what it is that makes it tick, literally.

So how do you get a Grey Wagtail in your garden? Running water is the sure-fire method of luring one down if one is in the neighbourhood, so a mini-cascade in a pond or sometimes even a solar fountain will do the trick. But, if you want it to stay awhile, they then need the chance to skip along a bare, gravelly pond margin where it can hunt for flies and drowned insect flotsam. Now there's a fantastic bird to aim for in January's Big Garden Birdwatch!