New research from BTO implicates rising temperatures in the steep decline of one of spring's most-loved songsters, Willow Warbler.
The study, published in the journal Ibis, shows that Willow Warblers are doing better in Scotland, where temperatures are cooler. Evidence is building to suggest the population in southern Britain is a casualty of recent anthropogenic climate change.
BTO researchers used volunteer bird counts and habitat data from the BTO/JNCC/RSPB Breeding Bird Survey to separate the impacts of climate change and habitat change on Willow Warbler and Common Chiffchaff populations. The analysis revealed that Willow Warbler numbers across the UK fell by 41% between 1994 and 2018, while those of the closely related Common Chiffchaff grew by 133%. The picture was very different in Scotland, where temperatures are cooler than the UK average. North of the border, Willow Warblers increased by 77% and Common Chiffchaffs by 244%.
Willow Warbler is declining across much of England (Grzegorz Burkowski).
During the period studied, the mean breeding season temperature was 12.7°C in England and 10.2°C in Scotland, close to the optimum breeding temperatures for Common Chiffchaff (13.5°C) and Willow Warbler (11°C), respectively. However, Met Office data show that UK temperatures in the last 30 years have been about 1°C warmer than those during the three preceding decades. Changes like these can have impacts including producing a mismatch between young birds' food requirements and insect abundance, an overall reduction in food abundance and shifts in habitat suitability.
The BBS habitat data showed that woodland and scrub, the preferred habitat for Willow Warblers and Common Chiffchaffs, increased across the UK, with more habitat improvements in Scotland. However, this study showed that the increases in Scotland were more likely to be linked to climate change than habitat change.
Its UK decline means Willow Warbler has been on the Birds of Conservation Concern Amber List since 2002. With global temperatures forecast to keep rising, studies like this demonstrate the importance of long-term monitoring to ensure conservation measures are targeted effectively. More woodland and scrub, for example, would be more likely to be taken up by breeding Willow Warblers if it were concentrated in cooler areas of the UK.
Blaise Martay, BTO lead author on the paper, said: "We've discovered that these two superficially similar warblers have quite different temperature requirements during the breeding season. Climate change means Willow Warblers are now faring worse in warmer parts of the country, something that has implications for future conservation measures, such as habitat creation. The connection we've uncovered between temperature and breeding success suggests that conservation interventions for this species should be targeted in cooler parts of the UK.
"I'd like to thank all the volunteers who take part in the BTO/JNCC/RSPB Breeding Bird Survey. Their records allow scientists like me to undertake analyses like this one and help us understand the pressures birds face as a result of climate change."
Martay, B, Pearce-Higgins, J W, Harris, S J, & Gillings, S. 2022. Breeding ground temperature rises, more than habitat change, are associated with spatially variable population trends in two species of migratory bird. Ibis. DOI: https://doi.org/10.1111/ibi.13101