Monday, 30 March 2009

Has the cold weather hit our Wrens?

There is a chill to these bright mornings, the clear skies overnight allowing the mercury to dip below zero and delivering a brushstroke of silver frost to the short heathland turf. Even so, such bright mornings bring with them the hope of spring, lifting my spirits, and it feels good to be out at such an hour. The change of season appears to be having a similar effect on many of the woodland creatures, including the birds, and the ranks of conifers echo with bird song. The Robins have been singing right through the bleak days of winter but now their wistful song has a more strident tone. Blackbirds too, are delivering a purposeful chorus and many pairs will have already started to build nests or lay eggs.

There are other singers; the thrice-repeated call of the Song Thrush strongly sung, the ‘teacher-teacher’ of the Great Tit and, here and there, the less often heard songs of Siskin and Crossbill. What is missing, however, is one particular component of this early season orchestra – that delivered by one of our smallest songsters. In recent years, this part of the forest has been alive with the metallic ringing trills of the Wren. This species has fared well over recent years because of a run of mild winters and I fear that this year’s cold snap may have hit the Wren hard.

The Wren is one of our most numerous breeding birds but its numbers can fluctuate quite dramatically from one year to the next if the winter weather goes against it. The severe winter of 1962/63, for instance, resulted in an 80% decline in the breeding population nationally the following year. Another study, carried out in a Nottinghamshire woodland, saw virtually the entire adult breeding population lost after the very cold winter of 1985/86. In both cases, it took several years for the breeding population to recover to former levels.

While the most recent winter may not have been as severe or long-lasting as these, it might have been sufficient to reduce the number of Wren territories locally. One other characteristic impact of a cold winter on the Wren population is a noticeable change in habitat preferences. In the year that follows a cold winter, breeding Wrens are often absent from formerly favoured hedgerow and garden habitats. Those territories that remain occupied tend to be in woodland or alongside riparian habitats, suggesting that woodland is a high quality habitat, while hedgerows and gardens are less suitable and the last to be filled with breeding birds. We will have to see what long-term monitoring programmes, such as the BTO’s Breeding Bird Survey, reveal later in the year. Have our Wrens been hit by the cold?

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