Even in a place so packed full of natural migratory wonder as The Straits of Gibraltar, there are few sights as wow-inducing as a flock of hundreds – if not thousands – of migrating White Storks. As they move along the coast in huge, glittering black and white columns, tracing the patterns of the thermals they ride, they really are the epitome of visible migration!
These flamboyant voyagers capture the imagination and curiosity, and have been inspiring research into migration for hundreds of years. Back in 1822, a White Stork turned up in the German village of Klütz with what was clearly an exotic spear lodged through its neck. It turned out to be from central Africa. At a time when it was still commonly believed that Barn Swallows spent the winter hibernating in the bottom of muddy ponds, this Pfeilstorch – ‘Arrow Stork’ – opened our eyes to the possibility of incredible avian journeys, and migration science was born!
Their great size, conspicuous presence and predictable return to nest sites makes White Storks fantastic candidates for study, and extensive ringing (banding) programmes were already underway as early as 1906. From then until the onset of the Second World War, about 100,000 mainly juvenile birds were ringed, resulting in over 2,000 long-distance recoveries of birds reported between 1908 and 1954. To this day, this wealth of information is arguably the foundation of what we know about where they travel and the routes they take.
White Storks breed extensively across Europe. Almost like a watershed, there is a line that runs right through the middle of Germany, along which the westward-flyers separate from the eastward-flyers. The eastern route leads over the Balkans, the Gulf of Iskenderun, the countries of the Middle East over to East, Central and South Africa. The White Storks we see here, crossing over The Straits in such spectacular fashion have come from west of that line, their migration leading them down through France and the Iberian Peninsula to concentrate at this point.
For a bird with such a large wing span, flapping flight uses around 23 times more energy than gliding. Since there are no thermals over the sea, they are driven to seek out the very shortest distance between land masses. That means that, in an average autumn season, 150,000 White Storks of all ages – pretty much the entire western European population – are pushed towards this one point, looking for their moment to cross the 14 km (9 miles) of sea to Africa. This gives rise to the spectacle of these huge gatherings, spiralling upwards on rising warm air until they emerge up to 1500 m above the ground and then gliding out into the blue.
From here they continue their journey south into Morocco, across the Sahara and down to their wintering grounds in central Africa. In fact, having cheered them on in The Straits, we often get to see them again when we travel to The Gambia each December!
In the 90’s came satellite-tagging technology, a new way of gathering information about birds that was set to change the way we understand so many things. Technology tends to be larger when new and unrefined – some of the earliest tags were the size of a brick! Since a tag is required to be below 3% of a bird’s bodyweight to avoid hindering it, these gigantic nomads with their fascinating journeys presented the ideal species to take this new toy out for a spin!
This opened up the opportunity to study a whole new world of detail not just about migratory routes, but about migratory behaviour. White Storks usually migrate in mixed groups of both adults and younger birds. A number of studies have followed the fortunes of young Storks making their first migration without adults to follow, in order to look at the innate-ness – or not – of the journey plan.
Although their in-born sense of direction takes them vaguely in a south-westerly direction, if displaced by weather conditions they are unable to orientate themselves with any precision and many never fully migrate. This is very different to small passerines, which migrate more-or-less alone, often by night, following an inherited map and with no guidance from adults.
The high importance of this social inheritance makes a great deal of sense. As a day-flying, soaring bird, the efficiency of their route is heavily reliant on thermals generated by local topography. They follow adults to learn an exact route – a kind of thermal highway – which on future travels they will be able to recognise visually and be sure of the optimum journey.
Tags also give us more information on the temporal nature of migration in these birds. It turns out they treat it rather like a nine-to-five, flying for around 8-10 hours every day when the air is warmest, before resting until the following morning. They barely take a day off, covering the 4,000 km (2,700 mile) journey from northern breeding grounds to sub-saharan Africa in two to three weeks. Rather than feeding up before migration like some birds, Storks evidently snack en route only to meet their immediate needs, and lose weight on the journey. Presumably when you’re reliant on literally being lighter than air, every invertebrate over-indulgence counts!
As satellite tags become lighter, cheaper and more precise, the insights they give us become ever-more fascinating. In 2018 a project set out to explore how White Storks navigate thermals as a group by analysing individual high-resolution GPS trajectories of individual Storks during circling events.
A thermal is a complex, drifting, constantly changing column of air. To thermal efficiently, birds need to adjust their flight speed and circling radius to find, and remain close to, the centre of the thermal where updraft is highest. Thanks to the precision of the data obtained from the tags, we are able to see that Storks navigate the thermal based not only on their own perception of the airflow in their immediate surroundings, but also on a complex series of social interactions, reacting to the movement changes of Storks within their nearby subgroup, as well as the leaders of the group at the highest vertical point in the thermal.
How amazing to think that each Stork is effectively acting as an individual sensor, such that the whole flock becomes a distributed sensory array. In this way, they explore and gather information on the thermal as a group, effectively mapping its structure and enabling them to use the optimal airflow within it.
From solving ancient mysteries to changing our perception of collective movement, to simply turning a good day into an amazing one, these really are inspirational birds. And they are pouring over our heads at the moment here in The Straits! We’re thrilled to be assisting as always our conservation partners, Fundación Migres, with the annual autumn migration count – to date over 38,000 of them have made the crossing, and we look forward to many more inspirational moments in coming weeks!