Why are Willow Warblers decreasing in the south, but not the north?

Willow Warbler, photograph by Chris Knights
Populations of many species of migratory bird are declining in Britain. However, the picture is not equally gloomy across the country. Many species are doing much better in northern Britain than they are in the south. Recent research, led by Cat Morrison at the University of East Anglia in collaboration with BTO staff, has used BTO data to understand why this difference occurs.

Among the species faring better in the north is the Willow Warbler. This tiny inter-continental traveller used to be one of our commonest species, but it is now very scarce indeed in some places in the south-east of England. This work combined data from several BTO schemes to better understand the demographic causes of these patterns. By constructing an integrated population model (IPM) for each region, the authors untangled the different effects that productivity (from nest record visits) and survival (from CES captures) have on the number of breeding birds (from the BBS counts). This work shows that while changes in the number of breeding birds are primarily affected by the survival between years, the difference in the overall population trend between the two regions arises as a consequence of differences in productivity.

Between 1994 and 2012, annual survival and productivity rates ranged over similar levels in the two regions, but years of good productivity (i.e. lots of chicks fledged) were rarer in the south, where the population is declining. In particular, years of good productivity never coincided with years when the survival rate was also high. In contrast, population growth in the north was fuelled by several years in which good productivity coincided with high survival rates.

To assess the importance of this difference we modelled what the population changes might have been in the south using a realistic range of productivity values (including those achieved by birds in northern Britain). This showed that, with productivity similar to their northern cousins, populations in the south would have recovered.

Consequently, actions to improve productivity on breeding grounds, for example improving the size and quality of available habitat, especially in areas (such as southern Britain), where there are currently population declines, are likely to be a more fruitful and achievable means of reversing migrant declines than actions to improve survival on the breeding, passage or African wintering grounds.

Related publications:

Morrison, C.A., Robinson, R.A., Butler, S.J., Clark, J.A. & Gill, J.A. 2016
Demographic drivers of decline and recovery in an Afro-Palaearctic migratory bird population Proceedings of the Royal Society B.
Link to publication (DOI: 10.1098/rspb.2016.1387). Read the abstract

Notes

The Breeding Bird Survey is organised by the British Trust for Ornithology (BTO) and jointly funded by the BTO, the Joint Nature Conservation Committee (JNCC, on behalf of Natural England, Scottish Natural Heritage, Natural Resources Wales and the Northern Ireland Environmental Agency), and the Royal Society for the Protection of Birds. The Ringing and Nest Record Schemes are funded by a partnership of the BTO and the JNCC on behalf of the statutory nature conservations bodies (Natural England, Natural Resources Wales, Scottish Natural Heritage and the Department of the Environment Northern Ireland). Ringing is also funded by The National Parks and Wildlife Service (Ireland) and the ringers themselves. This work was only possible due to many thousands of hours of field work by dedicated volunteers and we are extremely grateful for their efforts. This study was funded by NERC Grant no. NE/L007665/1.

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