I’m sure most of you have noticed more Wrens in your garden this year than for quite a while, perhaps not an unexpected result of several winters without long frosty spells. To confirm this observation, some new BTO research has revealed that Wrens vary in their resilience to winter weather, depending on where in Britain they live. Maybe not too surprisingly they have found that Scottish Wrens are larger than those living in southern Britain, and as a result are more able to survive hard winter frosts.
Perhaps the most extreme UK example is the St Kilda Wren, which is a sub-species of our inland Wren and has only been found on Hirta, Dun, Soay and Boreray islands which lie some 40 miles west of the Outer Hebrides. It is both larger and has a more heavily barred appearance than its mainland cousin.
Altogether there are only a few hundred pairs, making it a great rarity and specimens of the adult birds and their eggs used to be highly prized. The St Kildans themselves even used to gather eggs to sell to collectors. Nowadays they are, of course, fully protected.
Wrens are amongst the UK’s smallest songbirds, and their populations can decline following periods of cold winter weather, due both to the cold itself and to difficulties in finding sufficient insect prey. This new research, in collaboration with the University of East Anglia, shows that Wrens living in regions where winters are more severe have adapted to improve their chances of surviving.
The study used information collected by volunteers participating in the Breeding Bird Survey to show how susceptible Wren populations were to severe winter weather; measured in terms of the number of days with a ground frost. Northern populations were found to be able to cope with winters having up to 70% more frosty days than their southern cousins, showing quite a degree of local adaptation. The results demonstrate how closely adapted Wren populations are to their local climate.
Using information collected by bird ringers, the study also found that Wren body mass was approximately 5% higher in the coldest (eastern Scotland) region than in the warmest (south-western) region. Larger individuals are likely to be favoured in colder parts due to the thermal advantage of larger size and their ability to store more body fat, something that has also been observed in other species of both birds and animals.
The findings of this study add to our understanding of how birds and other species respond to climate change. The work done shows that Wren populations should be able adapt to at least some change in average temperature, and indeed, being short-lived birds they can probably adapt more rapidly than most other species. Ultimately, the ability of any species to cope with climate change will depend upon whether the future rate of warming exceeds their ability to adapt.
Yet again we see that nature can adapt, it always has, but the worry is that the rate of change predicted is so rapid that many, especially longer lived species, simply will not be able to adapt fast enough.
If you find the lives of our garden birds to be of interest, and would like to join in and count the feathered occupants of your garden, please contact me or visit the BTO Garden BirdWatch website (www.bto.org/gbw).
Mike Gray (firstname.lastname@example.org)