Extreme migratory connectivity and mirroring of non-breeding grounds conditions in a severely declining breeding population of an Afro-Palearctic migratory bird

This study used a combination of geolocator and GPS tracking devices, fitted to Nightingales at both their UK breeding grounds and their non-breeding grounds in West Africa, to provide data on their birds’ movements between 2009 and 2024. The authors also analysed ringing data collected between 2000 and 2022, and used data from other research studies which tagged Nightingales breeding in different parts of Europe.

The results showed that Nightingales breeding in the UK overwinter in a very restricted area of West Africa, centred on The Gambia. This migratory movement between the species’ breeding range in eastern and southern England, and their non-breeding quarters is an example of extreme migratory connectivity; Nightingales breeding in other parts of Europe overwinter in a much larger area of West Africa, and do not therefore show this degree of connectivity. The study found very little mixing on the non-breeding grounds between UK-breeding Nightingales and those breeding elsewhere, so any issues affecting the UK Nightingales’ overwintering location primarily feed into UK breeding population declines, but not population trends in other parts of Europe. Moreover, the small non-breeding range of UK Nightingales makes this population more vulnerable to negative changes in this region than their European-breeding counterparts, which are more buffered from such changes by their larger non-breeding range, not all of which will be affected to the same extent.

Nightingale numbers in the UK are falling. The population declined by 90% between 1967 and 2022, and the breeding range also contracted. Previous BTO research has linked this decline to habitat changes during the breeding season, including the destruction of understorey nesting habitat by deer browsing. However, it was also suggested that wider scale factors such as impacts on the non-breeding grounds were likely to be contributing to the UK decline. This study reveals that the suitability of UK Nightingales’ overwintering grounds is lower than that of populations breeding in other parts of Europe. This might be associated with climate change, including drought, but also human-mediated habitat degradation, such as overgrazing and firewood collection, and may also have deteriorated earlier in the period of UK population decline.

This research provides evidence of unusually high connectivity and suggests that such patterns may become more frequently detected as high resolution tracking devices become more miniaturised, allowing them to be used instead of geolocators on small-bodied species. It also provides further support for the idea that high-connectivity populations of long-distance migratory species may be more vulnerable to climate change, as well as to any habitat loss that occurs within their non-breeding range, than species that show lower connectivity. Finally, it also helps us to understand the conditions under which such impacts may be detectable, namely when there is limited population mixing on a species’ non-breeding grounds.

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