Causes of change

The causes of decline are uncertain. Decreased breeding success is likely to be an important driver of the decline in the south-east, and the differing trends across the UK suggest that climate change (or possibly habitat changes occurring over wide areas) could be a factor behind the changes. However, problems on migration or in winter have not been completely ruled out.

Further information on causes of change

Willow warbler is among a suite of species that winter in the humid zone of West Africa and correspondingly are showing the strongest population declines among our migrant species (Ockendon et al. 2012, 2014). Pressures on migration and in the winter are likely to be affecting the population, as is a reduction in habitat quality on the breeding grounds (Fuller et al. 2005). A study based on BBS results from 1995 to 2006 found a negative correlation between the abundance of deer and Willow Warbler, with the species declining the most where deer population increase had been greatest, although the size of the impact was relatively small with modelling suggesting that deer could have caused a decline of around 4% over this period (Newson et al. 2012).

The decline in the south in the early 1990s has previously been linked to reduced adult survival (Peach et al. 1995a). However, more recent analysis of annual population changes and winter survival estimates across western Europe shows only a weak relationship between survival and population change, suggesting than long-term population change may be mostly driven by reduced productivity or juvenile survival ( Johnston et al. 2016). This is supported by CES results: the recent population decline is associated with a decline in productivity as measured by CES and with a substantial increase in nest failure rates. There is also a small but significant decrease in the number of fledglings per breeding attempt. Average laying dates have shifted earlier by a week, perhaps in response to recent climatic warming (Crick & Sparks 1999). In the southeast, the seasonal decline in productivity has strengthened and, despite the advance in timing of breeding, overall productivity has declined, whereas overall productivity has been stable in the northwest (Morrison et al. 2015). Although annual productivity rates and survival are variable across the UK, regional integrated population models showed that high annual productivity during 1994-2012 sometimes coincided with high survival in the north-west of Britain, leading to population growth, but high productivity is rarer in the south-east and never coincided with high survival (Morrison et al. 2016c).

There is also evidence that sex ratios vary across Britain and have become male-biased in many areas of low abundance such as south-east England, which may affect local productivity (Morrison et al. 2016b).

Information about conservation actions

The decline of this species in the UK has been driven by rapid declines in the south and east of England which contrasts with increases in Scotland and Northern Ireland. Conservation actions to reverse the declines therefore need to focus on identifying and addressing the problems in the south-east whilst maintaining conditions further north and west.

If these problems are caused by issues occurring during the breeding season rather than at other times, they could be linked to climate change or possibly to widespread habitat changes; therefore conservation actions in the south-east should focus on creating and maintaining good quality breeding habitat for Willow Warblers in order to help mitigate some of the effects of climate change (Mallord et al. 2016c).

Restoring or creating forests can help provide such habitat. Research suggests that Willow Warblers prefer woods with mean vegetation heights (3.7-5.3 m) and that these need to be relatively large (>0.5 ha) (Bellamy et al. 2009). The same study found that woodland that was 6-11 m high appeared less suitable and therefore rotational woodland management techniques which ensure that early successional patches continue to be available are most likely to retain Willow Warblers. Control of deer abundance within managed woodlands could possibly also benefit Willow Warblers (Newson et al. 2012; see Causes of Change section, above).

In addition to the woodland management activities described (and policies to encourage such activities), decisions about infrastructure projects can potentially have effects on Willow Warblers which extend some distance beyond the boundary of the project itself. A Dutch study found lower densities breeding in areas with apparently suitable habitat within 200 m of highways, and breeding productivity was also lower in those areas, which were occupied later in the season (Reijnen & Foppen 1994).