Bird ringing provides clues to migration

David King

Ranger, Rye Harbour Nature Reserve

Bird ringing is the process of catching wild birds and putting uniquely numbered rings on them. When they are caught, or found again, anywhere in the world, we are able to gain a lot of knowledge from the tiny little ring around their leg. When ringing started, over a 100 years ago, we were most interested in bird movements and how long they live. But it's now providing an insight into global warming and the effect it's having on changing some migration routes. With the complete computerisation of all the ringing records, researchers are now able to analyse the data in many different ways, like working out why certain species are declining so rapidly.

As a ringer, it's always exciting to catch someone else's bird (control) or have one of your own turn up elsewhere (recovery), especially when these occur abroad. Last year I had a number of Warblers that involved foreign movements. I catch reasonable numbers of Reed and Sedge Warbler at Rye Harbour and although they occupy similar habitats here, they have different wintering grounds and migration routes.

Sedge Warblers winter south of the Sahara but leave Britain and make a short hop to Northern France, where they feed up and probably do a big hop south. I do have quite a number of Sedge Warblers that make this short trip to France to feed up.

Reed Warblers winter on the west coast of Africa and I haven't had any birds recovered aboard until this year. Last summer we caught a Portuguese Reed Warbler that was ringed in August 2023 (on its way south). This bird was caught in breeding condition at Rye. It appears that all British Reed Warblers stop in Portugal to feed up before continuing south.

Willow Warblers and Chiffchaffs are both small green Warblers that look very similar, but again, they both have different migration strategies. Willow Warblers winter in broadly the same area as Sedge Warblers, south of the Sahara. However they are increasingly breeding further north, they are almost completely absent as a breeding bird in southern England and I think their numbers are decreasing alarmingly in East Anglia - no doubt driven by climate change.

Chiffchaffs are short distance migrants wintering around the Mediterranean Sea, but also in this country, particularly near sewage farms! It's warmer and there are plentiful insects there even in winter. Although there is a considerable overlap in their breeding ranges, Willow Warblers tend to breed further north than Chiffchaffs.

At the end of July last year I caught a Chiffchaff wearing a Norwegian ring. It was an adult that was undergoing full wing moult - like many birds in Northern Europe, they moult after breeding, on or close to their breeding territory. I was rather surprised to find out that this bird was caught in the latter half of April last year in the southern half of Norway. On that date it should have been on its first breeding attempt of the year. Firstly, their aren't many Chiffchaffs breeding in Norway as it's right on the northern edge of its range. Secondly, why did it move so far across the North Sea to get here to moult? I would have expected it to moult in Norway or possibly Denmark. One possible theory is it was a failed breeding bird and it went wandering around in the summer until it found its way to Britain and Rye Harbour Nature Reserve.