Blog post by Paul Bellamy, Senior Conservation Scientist, RSPB Centre for Conservation Science.
As part of an RSPB research programme investigating the causes of wood warbler declines, 434 nests were monitored in mid-Wales, Devon and the New Forest from 2009-2013.
One of the main causes of nest failure was predation and a recent paper uses this data set to look at whether vegetation structure around the nest has an influence on predation rates, and whether habitat management has the potential to reduce predation.
A wood warbler bringing food to its nest
The problem
In the UK, wood warblers breed mostly in our western woods, but spend the winter in sub-Saharan Africa. The numbers breeding in the UK have more than halved since 1995, leading to the start of ongoing research in 2009 that has involved the RSPB, Natural England, British Trust for Ornithology and amateur ornithologists.
This research has looked for possible causes in the breeding season, locating wintering areas and winter habitat use.
Wood warblers breed in mature woods often on valley slopes. Breeding woods are typically oak or mixed woodland with very little shrub layer. They build their nests on the ground usually under a vegetation tussock or similar feature.
A wood warbler nest
Monitoring nests
The birds often call when close to the nest and by watching birds from a distance nests can be located. Nest are then visited every three to four days to monitor their fate. A sample were also monitored by nest cameras to identify the main nest predators.
At the end of the breeding season detailed vegetation measurements were taken from the area around each nest including density and height of different categories of ground cover, density of shrub and canopy layers.
What is predating the nests?
Just over 40% of the nests monitored were predated - which is in the normal range for small songbirds.
A paper published in 2012 shows no difference in predation rates in the welsh study area since the 1980s. So, it seems unlikely that increased predation rates on their own would have caused declines.
Those monitored by camera showed that this was mostly due to avian predators including jays, buzzards and sparrowhawks, but included a range of mammals and even adders.
Researcher recording vegetation in Devon as part of the wood warbler project.
The results
Nest predation was found to be less likely in areas with greater cover of vegetation associated with nest concealment, but was not affected by density of the shrub or canopy layers. Predated nests were in areas with less cover of medium height ground vegetation (10-30cm) made up of herbs and bramble, but had a greater cover of moss.
The location of nests within a territory was also related to the structure of the ground cover, nesting in areas with more bramble cover and moderate cover of tall vegetation (30-50 cm).
However location of the nest was also associated with taller trees and moderate density of understorey and on steeper slopes, so nest site selection seems to be affected by other factors in addition to nest concealment.
However, despite measurable nest site selection for concealing vegetation and lower predation rates in areas with greater cover, vegetation structure does not give a good prediction of whether a nest will be predated or not. Most of the variability in nest predation was unexplained with vegetation only explaining 12% of the variation in predation probability.
Jay predating wood warbler nest
Application to conservation
It doesn’t appear that managing woodland ground vegetation will be a widely applicable management tool to boost breeding productivity for wood warblers. However, when we looked at nest predation rates for the different study areas, we found that the New Forest suffered much greater predation (59% of nests predated) than the other two (36% and 39%).
This study area had different ground vegetation, which was more grassy and had lower cover of bramble and herbs than the other two. It also had a wider range of predators recorded taking nest contents.
So in some situations where there is high predation pressure and poor structure of the ground vegetation habitat management may reduce nest predation. As with all migrant species where we are unsure where the pressures on population are acting it is worth maintaining habitat in good condition to maximise breeding productivity.
Read the full paper
This work was jointly funded by Natural England and RSPB through the Action for Birds in England programme.