Waltham Brooks, the Sussex Wildlife Trust reserve. Photograph: Rob Yarham

The still pool reflects the blue sky. The kingfisher sits in the low willow branch. It flicks its tail up and down, up and down, like a switch, while it looks down, transfixed by something in the water below. It suddenly blurs into movement, there’s a splash, and the colourful missile returns to its perch with a tiny silver fish in its bill. It bashes the minnow on the branch twice, and swallows it.

The bird bobs its squat body up and down again. Then it launches, low across the water, its wings whirring, the light catching its shimmering blue back. It whistles briefly three times as it rises up, over the flood bank – a territorial warning to any other kingfishers that may be nearby to get out of its way – and then flies away, out of sight, downriver.

Walking past the hawthorns and bramble bushes in the marsh, I hear soft “seep” calls, and so I wait, scanning the leaves and branches. Five small brown birds, then a sixth, flutter from one tree to the next so much like leaves blown on a breeze that, for just a moment, I think that’s exactly what they are. But then they chase each other back towards me.

I watch patiently until I can see them clearly, feeding on the insects that are still plentiful in this marshy habitat. They’re chiffchaffs – small warblers that you’d expect to have flown on south by now. Until the 1960s, there had been only 50 records of chiffchaff in winter in Sussex, but many more seem to stay now. Will these birds choose to remain or, now it has turned colder, and their food becomes more scarce, will they fly on to somewhere warmer?

As I turn back towards the river, I hear harsh cackling calls from above, and look up in time to see six fieldfares fly over, heading towards the woods. The large, grey and brown thrushes, which have flown in from Scandinavia, where they breed, are the first I’ve seen here this winter. They probably arrived with the colder air from the north and east.