Icons of Migration – How White Storks Have Inspired the Study of Migration for Centuries

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!

116720399_956813574746126_5220556821838261274_o
White Storks heading to Africa – July 2020 © Inglorious Bustards

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.

white stork mig figure
Collective flight trajectories and synchronization in circling direction. (a),(b) Flock trajectories of migrating storks showing different flight types (circling and gliding). Grey arrows show flight direction. Tracks are colour-coded based on horizontal path curvature. The thermal is drifting with the wind resulting in distorted trajectories even if the bird flies in a perfect circle relative to the moving air (c) Curvature against time for each individual. Arrows at the top depict time delays between switches relative to first individual. (d) Horizontal trajectories of birds that switched their circling direction.

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!

Get some migration inspiration in your life!  Get in touch for day trips or bespoke tours this autumn, or give yourself a trip to look forward to, taking in Migration and Cetaceans in 2021 or beyond…

117111322_958331314594352_4427656211608511783_o
The group reach optimal height and reach for Africa © Inglorious Bustards

 

 

 

 

A tale of two Eagles

A young Italian called Michele caused a frisson of excitement here in The Straits last month, turning up safe and well after being missing in Africa since mid-November! Not long after his reappearance he was on his adventures again, awaiting fine weather on the northern shores of Morocco before jetting across the Straits and spending the night just outside Sevilla.

DSC06487
Short-toed Eagle © Inglorious Bustards

The plucky individual is a young Short-toed Eagle, satellite-tagged in 2019 as part of a collaboration between MEDRAPTORS, the Vertebrate Zoology Group at the University of Alicante (Spain), and tag providers Tecnosmart. He is a second-generation project participant – his father Egidio was one of seven young eagles tagged between 2010 and 2013, who reached adulthood and bred successfully in 2019 while still transmitting data.

This father-son team continue to shed light on the surprising and convoluted migration of Short-toed Eagles from Italy. The two-kilo eagles wear their tiny GPS tags on their backs, and the fascinating story they tell has confirmed what many suspected. Instead of beginning the journey to their wintering grounds in sub-Saharan Africa by heading directly south to Tunisia, they start with a 1600-mile flight north and west, right up through the Italian Peninsula and across southern France, before crossing the Pyrenees and migrating down through the length of the Iberian Peninsula, more than doubling their journey distance to Africa´s shores.

The reason? it´s all about wings and Straits.

Screen Shot 2020-05-02 at 12.50.52
Michele’s journey – 2019 autumn migration in red and 2020 spring migration in green
Screen Shot 2020-05-02 at 12.51.08
Egidio’s migrations – autumn in red, spring in yellow, 2020 spring in green

The long, elegant wings of this beautiful raptor mean it is perfectly adapted for soaring on air thermals, and it can do so for many hours, expending very little energy. But where there is no rising air – such as over expanses of water – the bird must beat those long wings to stay airborne, and becomes quickly exhausted.

Leaving the south-western point of Sicily, where Italy is at its closest to Africa, the bird would have to fly almost 100 miles to the next bit of land – an impossibility for such a long-winged raptor. So Italian Short-toed Eagles have learned to go directly against their strong instinct to head south, forcing themselves to fly north in order to eventually arrive in southern Spain. Here the mere 8.5-mile sea crossing at The Straits of Gibraltar presents much less of a challenge to a healthy, experienced bird choosing the right weather conditions.

While southerly migratory behaviour is innate, the study shows that this northerly trek must be learnt. First-year birds follow an adult who has already learnt the circuitous route to avoid the sea. Around 20% of first-year birds get it wrong and follow their hearts south, ending up wintering on Sicily because there is no more “south” to fly.

Michele was named after the late Michele Panuccio, the raptor researcher who studied the species for several years and promoted the tagging of Egidio’s first son, who was tagged a few weeks after he passed away. He left the nest site at the end of September, several days before his father, presumably following another adult bird. No doubt aided by years of experience, Egidio left late but made the journey in considerably less time. By the end of October, he had already arrived in Mauritania while Michele was still taking it easy in Extremadura!

When the lazy adolescent did finally make the crossing, the signal from his tracker was soon lost as he moved out of areas with the mobile coverage necessary to follow him. For the research team, there was nothing more to do except wait and hope that he had found his way to the wintering grounds safely. So when his tracker was spotted on the move again earlier this month, it must have been a tremendous relief!

His northward crossing of the Straits was unusual behaviour for such a young bird, as non-breeders would normally stay in Africa for another year, “finding themselves” until they come of age. Interestingly though, back in the day, dad Egidio was also a precocious exception to this rule – he too returned to the breeding grounds as a one-year-old.

This year, seven-year-old Egidio was no less keen to breed, and started his migration back to Italy as early as mid-February, arriving there by 12 March! In early March we spent some absolutely joyous days here in the Straits of Gibraltar, watching raptors arrive across the narrowest stretch of sea between Europe and Africa. We clapped them all as they arrived on our shores! We´d love to think that Egidio was one of them…

With thanks to Ugo Mellone of MEDRAPTORS and Universidad de Alicante for kindly giving permission for citation and reproduction of maps and data.

REFERENCES

Agostini N., Baghino L., Coleiro C., Corbi F. & Premuda G. 2002. Circuitous autumn migration in the Short-toed Eagle (Circaetus gallicus). Journal of Raptor Research 36: 111-114

Mellone U., Limiñana, R., Mallìa E. & Urios V. 2011. Extremely detoured migration in an inexperienced bird: interplay of transport costs and social interactions. Journal of Avian Biology 42: 468-472

Mellone U., Lucia G., Mallìa E., Urios V., 2016. Individual variation in orientation promotes a 3000-km latitudinal change in wintering grounds in a long-distance migratory raptor. Ibis DOI: 10.1111/ibi.12401

Panuccio, M., Agostini, N. & Premuda, G. 2012. Ecological barriers promote risk minimization and social learning in migrating short-toed snake eagles. Ethology Ecology and Evolution 24: 74-80.