Little Owls are in decline in the UK, but are hard to monitor, making it difficult to establish this species' conservation and management needs. Newly-published research by the BTO demonstrates how playback could be an effective tool for helping to detect and monitor this species. A non-native species, the Little Owl was introduced to the UK and first bred here in the late-1800s. Although it spread rapidly across lowland agricultural habitat, the population has declined fast in recent years, in a similar trajectory to that found in parts of its native range elsewhere in Europe. However, because this species is largely nocturnal and found at a low population density, it is hard to construct reliable population estimates, and the most recent number of 5,700 pairs is derived from work published in 2008. As a declining predatory species that largely occupies agricultural areas and has a varied diet, a better understanding of the Little Owl population and the drivers of its change is valuable irrespective of this species’ native status, so new and improved techniques monitoring Little Owl numbers are desirable. New BTO research, led by Gary Clewley, has tested the use of playback to monitor Little Owl numbers. Here, the response of local birds to recordings of Little Owl calls is assessed. This method was tested in a well-monitored Little Owl population in the Lincolnshire Fens, on whose habitat 12 squares of 2 km by 2 km were surveyed on four occasions between sunset and midnight in March and April 2015. The results showed that playback was a very effective means of detecting Little Owls. Little Owls were found in 100% of squares where individuals were known to be breeding after only two visits. This method could be scaled up for use in wider ecological research or indeed a national survey of Little Owls, the need for which BTO is currently considering. Either way, the application of playback in Little Owl monitoring could prove valuable in decisions about how to manage and conserve this species.

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. 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.