Causes of change

There is little evidence explaining either the demographic or ecological drivers of the decline in this species and the causes are largely unknown.

Further information on causes of change

Nest failures now seem more likely to occur at the chick stage, although nest record samples are small. There has been no real change in the number of fledglings per breeding attempt, with a very shallow increase in productivity until the mid-1990s being followed by a similar decrease.

Bibby (1989) postulated that soils, climate, competition or predator numbers have probably had an effect on Wood Warbler numbers but provided no evidence in support. Smart et al. (2007) state that the loss of oak trees, the decrease in canopy cover, and the large increases in understorey cover could have been particularly detrimental for Wood Warbler, but again, direct evidence to validate this is largely lacking. Mallord et al. (2016) found no evidence that changes in woodland structure affected populations in their study areas in the west of the UK. Bellamy et al. (2017) found that nest predation was slightly lower in areas with more medium-height vegetation (e.g. brambles) but concluded that the effect of vegetation management would be small, apart from in extreme cases where very low cover exists. Smart et al. (2007) and Amar et al. (2006) did find that Wood Warblers have tended to decrease more in woods with fewer dead limbs on trees and at sites surrounded by more woodland, which suggests that changes in dead wood could be important or that dead limbs could be a surrogate for other changes in habitat, although Smart et al. (2007) found an overall increase in the amount of dead wood, which should have been beneficial for this species. In another Welsh study, Mallord et al. (2012b) found that Wood Warblers were associated with a number of structural features of the study woods, which could relate to their past management; they suggest that management should aim to restore habitat quality through introducing a moderate grazing regime.

Studies in Poland, where an average of over 70% of nests were lost and predators were responsible for over 80% of the losses, have reported that varying predation rates were a main factor responsible for variation in production between years and habitats (Wesolowski 1985). Wesolowski & Maziarz (2009) provided further evidence relating to this, finding that both Wood Warbler numbers and ratios of their change were significantly negatively correlated with rodent numbers. However, predators in Poland were mainly medium-sized carnivores (Maziarz et al. 2019) hence the observed correlation with rodent numbers may relate to changes in predator behaviour when rodent numbers were high.

Note that the findings from Poland cannot necessarily be linked to UK trends as different predators have been observed here. In Wales, nest predators during 2009-11 were mainly avian and rates of predation did not appear to have changed since 1982-84 (Mallord et al. 2012a). Subsequent analysis, looking at nests from the New Forest (2011-13) and Dartmoor (2012-13) alongside those from Wales also identified avian predators as the most important, but also found that higher predation rates had occurred in the New Forest where the predators were more diverse (Bellamy et al. 2017).

Mismatch between timing of breeding and the seasonal peaks of caterpillar abundance is potentially not a serious problem for Wood Warblers, because of their ability to feed their young successfully on flying insects and spiders (Mallord et al. 2017).

This species is a long-distance migrant and therefore changes outside the breeding grounds cannot be ruled out. A wintering study identified some possible resilience to forest loss as wintering birds can still use degraded habitats, but only where a substantial number of trees remain (Mallord et al. 2018). Data suggest that loss of tree cover in the wintering range may not be a key driver of the historical declines (Buchanan et al. 2019). However continued loss of trees in the future is likely to affect Wood Warblers in winter (Mallord et al. 2018).

Information about conservation actions

Habitat management to provide and maintain good quality woodland is likely to be important to conserve this declining species, with Mallord et al. (2012b) suggesting that habitat should be restored by implementing a moderate grazing regime. Wood Warblers require large areas of broad-leaved woodland and territories are associated with habitat variables including higher canopy height and intermediate levels of canopy cover, slope steepness and field level vegetation (Mallord et al. 2012b). Grass and sedge tussocks were important for nest survival in a study in Switzerland, which suggested stands with little or no shrub layer should be preferred to encourage ground level vegetation (Grendelmeier et al. 2015), although a UK study found predation rates decreased slightly when there was more medium height vegetation (Bellamy et al. 2018). Nests are located on steeper slopes (Bellamy et al. 2018); hence broad-leaved woodlands on slopes should be prioritised for conservation (Huber et al. 2017).