Following Wood Warblers in the UK and Africa

Guest blog by John Mallord, Senior Conservation Scientist at the RSPB

It is obvious to everyone who takes a walk in the countryside that summer is here: swallows hawking for insects, the melodious songs of willow warblers and blackcaps, the unmistakeable cuckoo proclaiming its name. Migrants have arrived. Their arrival is an annual highlight for birders, but over the last few decades, it has become apparent that fewer migrants have been returning to their breeding grounds. Here at the RSPB, we have recently embarked on an ambitious new programme of conservation action for migrant birds – a critical part of which is to understand why populations of so many of our migrants, especially those that travel all the way to sub-Saharan Africa and back, have been in decline.

A gap in our knowledge of so many of our migrants is what happens to them after they have left the breeding grounds, which can be for up to two-thirds of the year. Our work, therefore, will focus on what is important to these species on their wintering grounds and stopover sites, the latter being sites where birds may rest and feed en route to their final wintering destination.

One of the first species we are focussing on is the wood warbler. Once widespread in the UK, the wood warbler is now mainly confined as a breeding bird to the upland oakwoods in the west of the country.

Due to their complex annual cycles, discovering the drivers of migrant species’ declines is far from straightforward, and there is still so much we have to learn about wood warblers outside of the breeding season. Not least of which is where birds that breed in the UK actually spend the winter, or the migration routes that they take. There have been no sub-Saharan recoveries of birds originally ringed in the UK, and although there has been a recent revolution in migration studies through the development of long-range tracking technology, the devices are still too heavy for us to attach to wood warblers. We may have to wait a few more years for the further miniaturisation of this technology before we can answer this most basic of questions.

We have already spent five years studying the breeding ecology of wood warblers in these oakwoods, both in mid-Wales and on Dartmoor in Devon. Our work in Wales benefitted from a similar study that took place in the same woods in the 1980s, before the onset of the species’ decline, allowing us to compare the two periods to see whether circumstances have changed to an extent that can explain the warbler’s predicament. However, we found no evidence to suggest that any of the potential causes – changes in habitat quality, increases in nest predation, a reduction in the abundance of the birds’ invertebrate food or climate change (i.e. a warming climate leading to asynchrony between the birds’ breeding season and the peak abundance of one of their main food sources, caterpillars) – was driving the observed population declines.

So off to Africa we went, working in collaboration with local Partner organisations from Birdlife International. Firstly, we have spent 3 winter seasons (October – March) studying the ecology of wood warblers at a site in southern Ghana –a site where we have found wood warblers ‘wintering’ for  at least four months of the non-breeding period (in fact, at least as long as they are in the woods of Wales or Devon!). Secondly, we have also been studying the species in the grounds of a monastery just outside Ouagadougou, the capital of Burkina Faso. Here we find wood warblers staging in wooded habitats for  4-6 weeks in October-November on their way south.

The importance of these two sites to individual birds was highlighted this winter when we resighted three birds (identified by their coloured rings on their legs) that we had originally ringed the previous winter, illustrating a site-faithfulness outside the breeding season previously unrecorded in wood warblers. Working at these two sites has allowed us the opportunity to radio-track birds to see how they use their African landscape. Thankful that radio tags are a much lighter weight than longer-range tracking devices, the aim here is to identify which species of tree they favour. The results are fascinating, although not unexpected. Wood warblers do seem to target particular tree species, but these species are not the same in Ghana and Burkina Faso. There are a number of reasons why this could be the case, and better understanding these relationships will be the focus of future research.

During broader-scale surveys of southern Ghana, and contrary to all our expectations, we have found relatively few wood warblers in denser areas of forest. Instead, we recorded that birds are more likely to occur along forest edges and in clearings, in secondary woodland and even farmland as long as there are a number of trees present – one of our future tasks will be to try to work out what that ‘number’ is.   

As for the future, we will start to follow them at other points along their migration route, especially at the key times just prior to crossing the Sahara, when they will need to build up their energy reserves to complete this arduous journey. Even the location of these sites is not well known, so we have been encouraged by recent news from our local colleagues in the far north of Burkina Faso that they have been locating good numbers of northbound wood warblers. And as the spring continues, we have a small team in Devon awaiting the birds’ arrival, looking out for returning colour-ringed birds, as we try to link up the entire migratory flyway of this special little bird.