Breeding ground temperature rises, more than habitat change, are associated with spatially variable population trends in two species of migratory bird

Willow Warbler. Allan Drewitt / BTO

Author(s): Martay, B., Pearce-Higgings, J.W., Harris, S.J. & Gillings, S.

Published: July 2022  

Journal: Ibis

Digital Identifier No. (DOI): 10.1111/ibi.13101

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BTO research has examined the effects of climate change and habitat loss on the population trends of Willow Warbler and Chiffchaff. These closely related songbirds, tricky to distinguish by eye, share breeding grounds across the UK but migrate to different wintering grounds. While Chiffchaffs mainly migrate to south-west Europe and north-west Africa, with a small number remaining in the UK, Willow Warblers head across the Sahara to the humid zone in central Africa. 


Habitat loss and climate change are key drivers of global biodiversity declines but their relative importance is rarely examined. We attempted to attribute spatially divergent population trends of two Afro-Palearctic migrant warbler species, Willow Warbler Phylloscopus trochilus and Common Chiffchaff Phylloscopus collybita, to changes in breeding grounds climate or habitat. 

We used bird counts from over 4000 sites across the United Kingdom between 1994 to 2017, monitored as part of the BTO/JNCC/RSPB Breeding Bird Survey. We modelled Willow Warbler and Common Chiffchaff population size and growth in relation to habitat, climate and weather. We then used the abundance model coefficients and observed environmental changes to determine the extent to which spatially varying population trends in England and Scotland were consistent with attribution to climate and habitat changes.

Both species’ population size and growth correlated with habitat, climate and weather on their breeding grounds. Changes in habitat, in particular woodland expansion, could be linked to small population increases for both species in England and Scotland. Both species' populations correlated more strongly with climate than weather, and both had an optimum breeding season temperature: 11°C for Willow Warbler and around 13.5°C for Common Chiffchaff (with marginally different predictions from population size and growth models). Breeding ground temperature increases, therefore, had the potential to have caused some of the observed Willow Warbler declines in England (where the mean breeding season temperature was 12.7°C) and increases in Scotland (mean breeding season temperature was 10.2°C), and some of the differential rates of increase for Common Chiffchaff. However, much of the variation in species’ population abundance and trends were not well predicted by our models, and could be due to other factors, such as species interactions, habitat and climate change in their wintering grounds and on migration.

This study provides evidence that the effect of climate change on a species may vary spatially and may switch from being beneficial to being detrimental if a temperature threshold is exceeded.

Despite their similarities in appearance and breeding habits, Breeding Bird Survey (BBS) results show very different population trends for Willow Warbler and Chiffchaff. In England especially, but also Wales and Northern Ireland, Willow Warbler numbers have declined rapidly in recent years, but they are increasing in Scotland. Chiffchaffs, on the other hand, are increasing across all of the UK, and far more rapidly in Scotland than elsewhere.

In this study, BTO scientists used BBS data to investigate how climate (measured using long-term averages in temperature and rainfall), weather (annual fluctuations in temperature and rainfall) and habitat features on these species’ breeding grounds influenced changes in Willow Warbler and Chiffchaff population demographics between 1994 and 2017.

The results showed that population size and growth for both species were associated with breeding ground temperatures, peaking at 11 °C for Willow Warblers and at 13.5°C for Chiffchaffs. These figures suggest that rising breeding season temperatures could have caused the divergent Willow Warbler population trends in England and Scotland over the study period: while the average breeding season temperature in England was 12.7°C and increasing, it was 10.2°C in Scotland, and predicted to rise towards 11°C in future. For Chiffchaff, the average temperatures in England and Scotland were below their optimum, but steadily approaching it, helping to explain the positive population trend across the UK, but particularly in Scotland. Of the other factors considered, the study found that woodland expansion was linked to small population increases in England and Scotland for both species.

The study adds to the existing evidence that Willow Warbler numbers in southern Britain are falling due to increasing temperatures associated with climate change. The optimum temperatures also suggest that, while Chiffchaffs might be benefiting from warming now, this could change if the average breeding season temperature reaches and exceeds 13.5°C. At this point, Chiffchaffs could join their Willow Warbler cousins in declining across the UK.

However, the analysis of climate, weather and breeding habitat did not explain all of the variation in Willow Warbler and Chiffchaff population trends. This suggests the birds are affected by conditions they experience outside the breeding season, on migration and at their wintering grounds. This highlights the importance of understanding the pressures faced by birds throughout their entire annual cycle, for effective targeting of conservation measures. 



The BTO/JNCC/RSPB Breeding Bird Survey is a partnership jointly funded by the BTO, RSPB and JNCC, with fieldwork conducted by volunteers. The research was funded by this BBS partnership. We would like to thank all the volunteers who participate in the Breeding Bird Survey. We would also like to thank Rob Robinson, Catriona Morrison and two anonymous reviewers for their comments on the manuscript.

Data availability statement

The data that support the findings of this study are available freely for research on request from the British Trust for Ornithology. The data are not publicly available as they are not free for commercial use.

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