Return from the Sahel
BY: Oliver Fox, West Oxfordshire Farmland Bird Project
It has been five and half months since the last drop of rain fell, the grassy plains are tinder dry and almost all the herbaceous plants have shrivelled and died through lack of water. Most of the trees scattered around this landscape have lost their leaves and stand lifeless under the baking hot sun. It’s here in the Sahel in early spring that millions of migratory birds must battle to find enough food so that they can sustain themselves on their epic journey back to Europe. The Sahel is a biogeographical region that lies between the inhospitable Sahara Desert to the north and the richer Guinea-Sudanian savanna further to the south. It stretches from the Atlantic shores of Senegal and Mauritania in the west to the Red Sea in Eritrea and Sudan. Characterised by infrequent rainfall and harsh, arid conditions, the Sahel is home to many species which are specially adapted to surviving in the strongly seasonal climate.
In West Africa, the heavy summer rains between July and October turn the arid Sahelian savanna into a lush green landscape. The new growth of grass is fundamental for the Fulani pastoralists who inhabit much of the Sahel, moving their herds of cattle, goats and sheep around the region to make use of the seasonal grasslands. This flush of new growth is also vital for both the local wildlife and for returning migrant birds who arrive in the Sahel from breeding grounds in Europe between September and November. After their long trans-Saharan flight, the abundance of insect food in the revitalised Sahel is often critical to the survival of these migratory bird species.
Lesser Whitethroat
Many of these new arrivals in the Sahel will be birds we are familiar with from the British countryside – a large proportion of the Blackcaps and Chiffchaffs breeding in our local woodlands and gardens spend the winter months south of the Sahara. Lots of other species which we are used to encountering in the breeding season all make the long-distance journey to West Africa each autumn and rely of the bountiful resources of the Sahel when they arrive there. Whitethroats and Lesser Whitethroats, common birds of our farmland hedgerows, both spread into sub-Saharan Africa, Whitethroats in the western Sahel of Senegal and Mali and Lesser Whitethroats in the eastern Sahel of Sudan and Ethiopia. Garden Warblers and Willow Warblers, which breed in wetland and scrubland areas in northern Europe, embark on longer journeys into West Africa but stop in the Sahel to refuel on their way south. Redstarts, Tree Pipits and Pied Flycatchers, which have nested in the oak woodlands of the western British Isles, also make the incredible migratory flight south into West Africa. These species are joined by a host of other birds, including Melodious Warblers from France, Olivaceous, Great Reed and Subalpine Warblers from Spain and Nightingales and Reed Warblers from across Europe, all escaping the cold and wet northern winter. In all, it has been estimated that over 2 billion birds make this migratory journey southwards into Africa in autumn, many stopping in the Sahel and seasonal wetlands that appear in the rainy season.
While this region provides an incredible lifeline to migratory birds in autumn, the same cannot be said in early spring as the birds move northwards back to breeding grounds in Europe. The lack of rainfall means that the arid region these birds must traverse is much wider now – the Sahara contains few stopover sites where birds can rest or refuel. Stopping in the desert is not an option and means almost certain death for any exhausted migrant. Only those birds which put on the necessary fat reserves for a non-stop flight will make it back. Research on these migratory species in winter is still incomplete but we know that, for example, Reed Warblers that usually weigh 10g can increase their body weight by 50% as they accumulate energy stores for the journey back. Similarly, Nightingales increase their mass from 18g to 28g in the few weeks before flying north.
To enable them to store enough energy, many migrant species switch to eating berries in early spring, allowing them to fatten up for their long-distance flights. In the arid Sahel another lifeline is provided by the Acacia trees which burst into flower in the dry season, attracting a host of pollenating insects which have co-evolved to make the most of this unseasonal flowering. While much of the Sahelian zone is dried to a crisp, the abundant flowering Acacia trees can be buzzing with life and provide another key resource for migrant warblers on the move.
From these arid, seemingly inhospitable, lands most of the spring migrants arrive back in the United Kingdom in April and May. Blackcaps and Chiffchaffs leave their wintering grounds earlier and so arrive first, most of the other warblers and chats from West Africa appear in late-April and early-May, while the Spotted Flycatchers, which may have travelled as far as South Africa during the winter, do not arrive back on our shores until late-May. As soon as they appear at their preferred breeding sites, often faithfully returning to the same location as they bred last year, they begin setting up a territory or finding a mate. Landscapes which have been relatively silent over the cold winter months now burst into a cacophony of spring birdsong.
Common Redstart
At Thyme, the wet meadows along the clear-flowing River Leach provide a perfect destination for a host of these returning migrants, with many returning to same section of hedgerow each season. The careful, light-touch management of the hedges, scrub and grassland making up the meadows allows a matrix of different vegetation densities to form, all of which allow each returning migrant bird to find its preferred niche in the rich landscape. An abundance of insect-life in the spring and early summer bursts out, giving the ideal conditions for these warblers to raise their young. In the best years, some can even have two broods of offspring before they need to leave on their return journeys in autumn.
We have already hinted at some of the challenges these birds face during their lives, not just on wintering areas in West Africa or their breeding grounds in the United Kingdom but at stopover sites on route too. Wild areas which support enough invertebrate prey for these birds are becoming more difficult to find – wetlands and wastelands are being developed, bulldozed and drained right across the flyway. Traditional refuelling stops are missing or badly deteriorated so that the birds have to fly further to find the resources they need to make the longer desert or sea crossings.
Climatic change is producing increasingly erratic weather patterns, with extremes of temperature and wind speed becoming much harder to predict. Traversing these long migrations routes is fraught with dangers for birds that only weigh a few grammes. Mortality due to inclement weather or mis-timed departures is unfortunately becoming more common. Higher temperatures in the Sahel and parts of Europe also limits the amount of plant and insect food available and makes putting on fat reserves much harder. Taken together, the compound influences of habitat loss, unpredictable weather and a drier climate means that populations of migratory species are hit harder over their lifetimes and numbers are declining faster than resident birds. Similarly, conservation of species which spend months of the year away in far-flung places is more difficult to coordinate and manage. Creation of new reedbed habitat in the breeding areas of species such as Reed Warblers is undermined by widespread loss of the dry scrub in its West African wintering grounds. Likewise, at many locations in the British Isles damage to the woodland understorey is caused by an overabundance of native and non-native deer. This has a catastrophic influence on the availability of suitable breeding sites for Nightingales and Wood Warblers, whose numbers have crashed across the country. For other species, the critical issue is a mismatch in the timing of their arrival back in Europe. A warming climate has meant that the emergence of moth and butterfly caterpillars is happening earlier and earlier each spring and many birds are not able to match their breeding season with this rapid advance, leading to less successful reproduction rates.
Each migratory bird species carves out its own unique niche in the ecosystem, relying on a fine balance of food, shelter and nesting habitat to survive. This makes them exceptionally vulnerable to influences from outside. Local reserves for nature, such as the Thyme water meadows, offer valuable sanctuary for migratory birds to make home. At the height of springtime, once these long-distance travellers have returned to Southrop, the raucous dawn chorus can be mesmerising and well worth a visit for anyone who is amazed by the wonders of migration.
West Oxfordshire Farmland Bird Project is run by a small team of volunteers to promote conservation, research and engagement with several local farmers and landowners. The project aims to help turn around the worrying declines in farmland bird populations seen in the UK over the last 50 years, through the promotion of wildlife-friendly management practices, environmental stewardship options and awareness of challenges faced by farmland birds throughout their life cycles. Over the last five years the project has worked on a large number of farms on the conservation of several key species, including Corn Buntings, Yellowhammers, Reed Buntings, Tree Sparrows and Barn Owls.
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