Northern Wrens weather the winter better than southerners

from Alwyn Jackson

New BTO research reveals that one of our most widespread songbirds – the Wren – varies in its resilience to winter weather, depending on where in Britain it lives. Scottish Wrens are larger than those living in southern Britain, and are more resilient to hard winter frosts.

Wrens are amongst the UK’s smallest songbirds, and their populations can decline following periods of cold winter weather, due to the cold itself and difficulties in finding sufficient insect prey. New BTO research, in collaboration with the University of East Anglia, shows that Wrens inhabiting regions where winters are more severe have adapted to this. The study used information on Wren populations that had been collected by volunteers participating in the Breeding Bird Survey to show that Wren populations were susceptible to severe winter weather, measured in terms of the number of days with a ground frost. However, northern populations were found to be resilient to winters with up to 70% more frost days than southern populations, suggesting a degree of local adaptation. This work indicates that each Wren population is closely adapted to its local climate; there was a close correlation between the historic regional climate and the degree to which the population was resilient to severe winters. Using information collected by bird ringers, the study also found that Wren body mass was approximately 5% lower in the warmest (south-west) than in the coldest (east Scotland) region. Large individuals are likely to be favoured in colder regions due to the thermal advantage of larger  size and their ability to store more body fat, matching the pattern seen more widely across other species – a pattern known as Bergmann’s rule. The findings of this study have particular relevance to our understanding of how birds and other species respond to climate change. Although this work shows that Wren populations may adapt to at least some change in temperature, they are short-lived and therefore probably more adaptable than most other bird species. Ultimately, the ability of species to cope with climate change will depend upon whether the future rate of warming exceeds their ability to adapt.

This work has been published as:

Morrison, C.A., Robinson, R.A., & Pearce-Higgins, J.W. 2016 Winter wren populations show adaptation to local climate Royal Society of Open Science.

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