Unusual Winter Warblers?

During these cold winter days it is nice to think back to the summer full of the song of the Whitethroat, Willow Warbler and the reeling Grasshopper Warbler. Whilst most warblers migrate from Britain & Ireland to enjoy the warmth of their African wintering grounds, some migrate from Europe to spend the winter here. Blackcaps have been commented on before, here, here, and here, but they are not the only warbler species seen here in winter.

There are usually quite a few Blackcaps and Chiffchaffs seen in the country at this time of year, with the odd Yellow-browed Warbler present in the south. However, last Friday (30/01/2014) a Reed Warbler was caught at Chew Valley Ringing Station, North Somerset! This bird should have been in Sub-Saharan Africa at this time of year. There is one previous electronic record of a Reed Warbler being ringed in January (a bird ringed in Hertfordshire in 1981) whilst there are two records of birds ringed in December (2000 and 2003).

The graph below shows the average number of ringed Reed Warbler, Chiffchaff and Blackcap over the last five years. Reed Warbler just makes an appearance in November but is swamped by the over wintering Chiffchaff and Blackcap. Birdtrack data show a similar pattern in the seasonal abundance of these wintering warblers.

There are many parameters that govern the over-winter survival of these birds and there are clear differences between years. Temperature can greatly affect food resources, especially for insectivorous species. The graph below shows the number of Chiffchaff ringed in winter and you can see the figure for the winter of 2010/2011 (which was very cold) drops substantially.

Numbers ringed during November appear to have increased recently, but this is not reflected in late winter catches. This suggests that more birds are arriving in the UK, but either dying, moving on southwards or dispersing into areas away from ringing hotspots like sewage works that are already at carrying capacity. More systematic winter ringing across a wider range of sites is required to identify which of these mechanisms is responsible.