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. 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.

Most studies of evolutionary responses to climate change have focused on phenological responses to warming, and provide only weak evidence for evolutionary adaptation. This could be because phenological changes are more weakly linked to fitness than more direct mechanisms of climate change impacts, such as selective mortality during extreme weather events which have immediate fitness consequences for the individuals involved. Studies examining these other mechanisms may be more likely to show evidence for evolutionary adaptation. To test this, we quantify regional population responses of a small resident passerine (winter wren Troglodytes troglodytes) to a measure of winter severity (number of frost days). Annual population growth rate was consistently negatively correlated with this measure, but the point at which different populations achieved stability (λ = 1) varied across regions and was closely correlated with the historic average number of frost days, providing strong evidence for local adaptation. Despite this, regional variation in abundance remained negatively related to the regional mean number of winter frost days, potentially as a result of a time-lag in the rate of evolutionary response to climate change. As expected from Bergmann's rule, individual wrens were heavier in colder regions, suggesting that local adaptation may be mediated through body size. However, there was no evidence for selective mortality of small individuals in cold years, with annual variation in mean body size uncorrelated with the number of winter frost days, so the extent to which local adaptation occurs through changes in body size, or another mechanism remains uncertain.