by Mike Dilger
It was the pioneering individual ‘OAD’ that first revealed that the Western Sahara, in north-west Africa, may well figure prominently as the first landfall for British Nightingales freshly arrived from southern Europe. Bordered by the Atlantic Ocean to the west, the territory of the Western Sahara consists mostly of desert flatlands, making the Cuckoo habitat in southern Chad and Nigeria seem positively lush in comparison. In fact with no permanent streams, Western Sahara encompasses some of the most arid and inhospitable terrain on the planet. The sparse greenery that is present seems to be largely confined to the Atlantic coastal desert habitat, a narrow strip of land fringing most of Western Sahara’s 1,110km coastline which continues south all the way into Mauritania. Despite the climate being extremely hot and arid here, with only low amounts of episodic rainfall, the mists blown in from the Atlantic help maintain a range of succulent shrubs and arid-adapted plants along this ‘green corridor’, and also permit the growth of lichens both on the bare ground and on the vascular plants themselves. Classified as part of the Sahara Regional Transition Zone, this ecoregion is far more vegetated than most of the Sahara Desert and also forms part of the East Atlantic Flyway migration route, a corridor enabling a whole range of bird species to travel between Europe and Africa.
In contrast to migratory species like the Bewick’s Swan, Nightingale and Cuckoo, which tend to travel along a prescribed migration route, the movement of Puffins after the breeding season is considered more of a dispersal, as they radiate out to sea. Until recently it was thought that the Puffin populations from east and west Britain were spatially segregated, both during and after the breeding season. However, thanks to the work of Mike Harris’s team with geolocators on Puffins from the Isle of May, off Scotland’s east coast, this is now thought not to be the case. Three-quarters of the birds followed from this colony were seen to carry out a major excursion from the North Sea into the Atlantic at the end of the breeding season, leading to the conclusion that there must after all be some mixing of Puffins from eastern and western breeding colonies during the winter. Recent research also indicates that the rich fishing waters around the Faroe Islands may well be an important overwintering location for many British-breeding Puffins. Consisting of 18 major islands and situated approximately halfway between Norway and Iceland, this remote archipelago lies some 320km north-northwest of Great Britain. Despite its diminutive size, the rugged rocky islands and the rich surrounding seas are able to hold over half a million pairs of breeding Puffins each summer. Mixing with their Faroese cousins, British-breeding Puffins will probably be spread out, possibly in small, scattered groups as they divide their time between fishing and roosting.
In contrast to a possibly large number of our breeding Puffins feeding just south of the Arctic Circle, the Swallows which summered in Britain will be streaming towards the equator by late September. Wanting to avoid a long sea crossing, many Swallows will opt for entering Africa from southern Europe via the Strait of Gibraltar. At just 14km in width, this narrow crossing, which also connects the Atlantic Ocean to the Mediterranean Sea, is not just an incredibly important shipping lane with huge historical significance, but is also arguably the most important bird migration route between Africa and Europe. Used by hundreds of thousands of kites, storks and buzzards each spring and autumn, the narrow strait minimises the distance that these large soaring birds need to spend above the sea. Unlike land, water is incapable of producing thermals, so large birds will need to wait for sufficient lift to get them high enough before resorting to the far more energetic technique of actively flapping across the strait until they can once again access the thermals over dry land. While Swallows will have more than enough energy to keep them flapping the whole way, even across the wider sections, the lack of food available for them to forage over the sea may mean that crossing at the narrowest point will dip least into their limited reserves.
Upon reaching North Africa there may then be a brief opportunity for the Swallows to feed in the fertile crescent of land bathed by the Mediterranean Sea before pushing on inland, over the Atlas Mountains and into the Sahara Desert. Taking on 1,500km of the world’s greatest desert on a broad front, mostly west of the Greenwich meridian, will once again provide very few opportunities to feed, as they cross the desert interiors of Morocco and Algeria, before then taking on the equally arid countries of Mauritania and Mali further south. Most small migrants are thought to use an intermittent migration strategy which involves resting in the desert during the daytime, migrating at night and then refuelling at any oases encountered along the way. Swallows are, however, thought to continue their day-flying strategy, while taking advantage of any feeding opportunities that may prevail along the way. One such oasis that a number of trans-Saharan migrants will pass through can be found at Ouadâne, situated on the southern edge of the Adrar Plateau. A World Heritage Site, the old town is situated close to an oasis surrounded by Date Palms and Sorghum, which will undoubtedly provide very welcome feeding opportunities for any desperately hungry and thirsty Swallows which may not have fed since leaving North Africa.
October
October is a month of huge change. As the vibrant green hues which dominated much of the countryside all summer rapidly retreat from view, the russets and reds that characterise this most autumnal of months will suddenly catch the eye. For birds, the breeding season will have been reduced to little more than a distant memory, which will differ markedly from some of our best known mammals, as Grey Seals and Red Deer move centre stage in the mating game. This is also a frenetic month for migration, as a huge variety of birds hone in on their winter destinations. Already one step ahead of an array of winter visitors heading to Britain, many of our resident birds, with their moults now completed, will be using this key month to re-establish territories for the colder and shorter days ahead.
Early October
When the seminal Migration Atlas was published in 2002, the whereabouts of British Cuckoos in winter was still considered largely a mystery. Up to this date only a single record of a British Cuckoo had ever been recovered south of the Sahara, belonging to a bird ringed as a nestling in Berkshire in 1928 and then subsequently shot in Cameroon in January 1930. It was the ornithological pioneer Reginald Moreau who speculated in 1972 that British-breeding Cuckoos must winter in Africa south of the equator. Recent ground-breaking work by the BTO tracking Cuckoos by satellite has not only confirmed firstly that Moreau was indeed correct, but furthermore transformed our knowledge of the movement of Cuckoos throughout the year. By additionally opening up this information to the public, thousands of interested bird enthusiasts have been able to follow the precise route taken by Chris and his band of brothers both online and in real time.
One of the main revelations when tracking the Cuckoos was the fact that the migrating birds opted for one of two routes, but still with only one final destination. Depending on whether the Cuckoos take the ‘Iberian route’ or ‘Italian route’ into Africa will then dictate where they rest up after crossing the Sahara. For those birds taking the more westerly route, Nigeria is seemingly most favoured as a staging point, while Chad was the country of choice for Cuckoos flying into Africa from further east. Despite occupying different countries, both populations of Cuckoos will still have been resting up in locations dominated by savanna. With this entire region’s marked wet season running between April and October, the Cuckoos must have fed sufficiently well in order to now be fit enough to undertake the next leg of their mission, as they head off for the Congo. In the four years Chris the Cuckoo was followed by satellite, 2011 could be considered a typical year. After a very settled couple of months in Chad, on 6 October Chris then suddenly upped sticks to fly almost entirely across the Central African Republic, before then pitching down 790km further south, and a distance of just 8km from the border with the country of the Congo. Poised on the edge of the Congo Basin, and just to the north of the main rainforest block, Chris’s movements in that year seems to have followed a similar pattern
to that of many of the other tracked Cuckoos in most years, whereby on arrival at the portal of Africa’s greatest forest, the birds initially seem reluctant to rush in headlong, but instead prefer to proceed at a more leisurely pace.
For those Swallows that managed to successfully tame the Sahara Desert, there will be no resting on their laurels of a job well done, as they continue to push across into the sub-Saharan Sahelian belt, in the full knowledge they still have an awfully long way to go before finally reaching their wintering grounds. Moving through Mauritania and Mali, the precise route the Swallows take is unclear, but with recoveries of 35 British-ringed birds, representing a substantial proportion of all the British Swallows recovered south of the Sahara (but still north of their final destination) it seems that southern Nigeria figures prominently on the route. With most of these records coming from just north of the Gulf of Guinea in southern Nigeria, it would suggest that the Swallows, upon leaving the Sahel behind, will suddenly take a more easterly direction. Travelling quickly through Côte d’Ivoire, Ghana, Togo and Benin, the Swallows will then enter Nigeria as they converge on the region unattractively nicknamed the ‘armpit of Africa’. As per their northward journey in spring, the Swallows are most likely to be migrating in small, loose flocks, consisting of a few tens of birds feeding on the move, and are probably only likely to gather in any large numbers at key roosting sites.
By early October most of the adult Swallows will also have begun their annual moult, which due to the birds’ need to fly efficiently throughout the entire journey, will be a long and drawn-out process taking potentially up to half the year. Starting with the body and wings, the first feathers to be replaced will come from the back and rump, as their glossy blue-black upperparts suddenly become far duller and flecked with white. Only when halfway through the protracted wing moult will the tail feathers finally begin to be replaced. The timing of the adults’ moult also differs slightly to that of the juveniles, which usually wait until they have arrived in their winter quarters before instigating the change that will ultimately see them transformed into adults.
As both the Swallows and Cuckoos make a beeline for the Congo Basin, the Nightingales will have already taken their foot well off the accelerator pedal following their initial leap across the Mediterranean. Believed to be slowly working their way south along the Atlantic coastal desert of West Africa in early October, the Nightingales will be taking advantage of this gentle rate of progress as an opportunity to feed on the way. The few human inhabitants living in this region are likely to be either nomadic herdsmen or fishermen, so that despite much of this coastal vegetation having been heavily overgrazed, the habitat should still be sufficiently intact for the Nightingales to remain true to their skulking character as they forage for invertebrates on the move.
While Estonia and the Gulf of Finland are likely to hold a high proportion of the European Bewick’s Swans at this time, other individuals will have travelled even further south to take advantage of a variety of inland and coastal sites in Lithuania, with the Nemunas Delta being considered the most important site. Protected under the Ramsar Convention, this globally important wetland on the Baltic coast is considered by BirdLife to be the most critical bird area in the whole of the country - representing a crucial layover for millions of migratory birds each year. Formed as the River Neman reaches the Baltic Sea, the delta consists of a maze of river branches and canals which criss-cross to form polders and wetlands, in turn creating the perfect habitat for the swans to rest and feed after their long journey. Having completed their wing moult well before they left their breeding grounds, the adults should by now also have replaced their tail feathers. The body plumage, however, will continue to be replaced throughout the entire winter, and in all probability will not be completed until the birds are preparing to leave their wintering grounds in February of the following year.
As the Bewick’s Swans recover condition before undertaking the last leg to Britain, the sheer number of Waxwings in southern Scandinavia during an invasion year will cause the entire berry crop to be rapidly stripped. The scarcity of food will then give the hungry birds little option other than to either make the short trip across to Denmark through the Strait of Øresund or make the longer sea crossing over the North Sea to take advantage of the plentiful berry supplies on offer in Britain. With autumn now proceeding at a brisk pace, the Waxwings may not be the only continental European breeding birds being pushed towards Britain.
The probable autumnal migration routes of Waxwings between their breeding grounds in northern Europe and their wintering grounds in Britain during an invasion year.
Unlike the largely sedentary British subspecies, migration amongst continental Blue Tits is much more commonplace, particularly in those years when a series of successful breeding seasons can combine to impose such a strain on the amount of food that a substantial number will have little choice other than to inundate coastal areas of Britain. Identifiable with a keen eye, the continental Blue Tits tend to be slightly larger and brighter than their British counterparts and while it’s little more than guesswork as to how many of these birds arrive each year, very occasionally the influx can be huge. The celebrated ornithologist Stanley Cramp collated data from the autumn of 1957 when hundreds of Blue Tits were recorded at a whole host of migration watch points along easterly and southerly coasts. With reports of an influx of 447 Blue Tits at Sandwich Bay in Kent and 460 recorded at Portland in Dorset in early October, Cramp thought that these birds then moved inland to spend the winter in Britain. The departure of these continental birds is believed to have occurred gradually, as they returned overseas any time from January the following year, right the way through to March or April.
Also continuing to arrive in good numbers in order to bask in our maritime climate, as continental Lapwings begin rubbing shoulders with resident birds, this should push the total population currently in Britain towards an annual high. Certainly the resident Lapwing which bred here should by now be completing the moult that will see them through winter and beyond, ultimately making both sexes look much more similar until a partial pre-breeding moult early in spring once again differentiates the sexes. The young Lapwings that fledged in the summer, by contrast, will be only halfway through a partial moult not likely to be finished until December, and so can still easily be picked out from the flocks by their shorter crests, scalloped backs and wings and incomplete breast-bands. Like their parents, these immature birds will also undergo a partial breeding moult in the New Year, but will not be finally indistinguishable from their parents until the completion of their first full moult, which will be carried out after their initial breeding attempt.
Needing to complete a full moult themselves, it’s likely that the adult Kingfishers will not have replaced all their feathers until November at the earliest. As the old primaries are often not shed until the new ones are sufficiently well grown, the replacement of flight feathers will certainly take far longer than that of the body moult which should already have been completed. When the juvenile moult is carried out will depend on which brood the young Kingfishers were reared from. Confining their first moult to all the feathers apart from those responsible for flight, juveniles from earlier broods may have started growing new feathers by July, while birds fledging from much later broods will probably not be able to start their post-juvenile moult until at least December.
With juvenile Kingfishers leaving the nest during a period spanning more than four months, their moulting process will be a much more staggered affair compared to that of Peregrine youngsters which, irrespective of the location of the nest, are likely to fledge within the month of June. Looking decidedly streaky and brown throughout their first winter, it will not be until they are 18 months to two years of age that the young Peregrines will finally begin to resemble their parents.
As the juveniles make do with their first set of feathers for the time being, the adult Peregrines by contrast should be reaching the final stages of a complete moult th
at may have begun as early as April or even March. Needing to remain highly effective in the air at all times, the female is thought to begin the all-important wing moult after laying her third egg, while the male will often delay until the young begin hatching. The flight feathers are often replaced carefully in sequence, starting with the fourth of the ten primaries. The replacement then slowly radiates from this point both towards the wing tip and the body. While the body feathers are moulted at the same time as those belonging to the wings, the tail feathers are started later, but should still have been completely replaced by the time the outermost primaries reach their full extent. During this time, the gaps created by feathers being actively replaced will make the adults slightly less efficient at both flying and catching prey. In cases where this severely hampers their ability to catch food, the moult can sometimes be suspended to a time when prey is more plentiful.
As the temperature drops and the nights lengthen, a large part of the Puffins’ day is thought to consist of finding enough food to eat as they prepare to change their primaries, an action that will render the Puffins flightless for a short, sharp period, contrasting with the ‘slow but sure’ technique as exemplified by birds such as Kingfishers and Peregrines.
