Research shows how wrens adapt to their local climate
When temperatures drop, every long midwinter night poses a serious challenge for a small bird. The smaller you are, the faster you lose body heat, the narrower the margin between survival and succumbing to the cold. So the stakes are high for the tiny wren, which weighs around 9–10g – fractionally more than a new pound coin. Even if it fluffs out its plumage and joins one of the communal winter roosts for which this species is known, the mouse-like bird will by dawn have lost nearly all of the weight it put on during the previous day’s foraging.
Fascinating research published in 2016 by the British Trust for Ornithology (BTO) and University of East Anglia showed that the impact of winter weather has led to physical divergence between wrens in the north and south of Britain. Wrens checked by bird ringers in the country’s mildest region, south-west England, were found to be on average five per cent lighter than those in north-east Scotland, our coldest, most frost-prone area, where extra size offers an evolutionary advantage. It may not sound like much, but for our third-smallest bird (after the goldcrest and firecrest) it could just make the difference between life and death.
AT UP TO 16 HOURS LONG, JANUARY NIGHTS ARE AN ENDURANCE TEST FOR THE PLUCKY, PINT-SIZED WREN.”
GET INVOLVED Take part in the RSPB’s Big Garden Birdwatch on 27–29 January 2018.