Size mat­ters

Re­search shows how wrens adapt to their lo­cal cli­mate

BBC Wildlife Magazine - - Contents -

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.

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