Sunday, 11 August 2019

Foraging Study - 7L in Sussex

Here in the UK, recreational angling is big business. In England, angling was estimated to have put £STG 1.4bn into the local economy. (2018 figures)  As a proportion of regional GDP, the amounts for Scotland, Wales and Northern Ireland respectively are probably even higher than that. And as the man said: 1.4 billion here, 1.4 billion there – pretty soon you're talking real money. [1]

7L's parent, 37, bringing in a fish
Angling Clubs, and the vast majority of their members, share our love of the countryside and the wild creatures who live there. But predatory birds will still be predators and occasionally this can lead to some understandable friction between anglers and conservation groups. “We can't catch any fish in this lake because the ospreys (or cormorants, herons, grebes, etc) have eaten them all!” It seems like a fair complaint, but is it true?

Let's do some science...

When a young female osprey, Blue 7L (Kielder N2, 2017) made her first return migration this year (2019), she arrived back in Northumberland on the 25th May, took one look at our horrible spring weather, and headed south again the following day. Of course, as a non-breeding bird, 7L is free to wander wherever she chooses and – after roaming around the south-east of England for a while – she eventually settled in an area of Sussex; about 50km south of London.

We were following 7L's detailed movements using GPS tracking. And what she did in the month that followed provided us with some very significant information.

But first, a question: why was 7L down in the Weald of Sussex at all? It's not a place where ospreys nest, although they are sometimes reported there on passage. The countryside is wooded and rural but with many large towns, and the busy area around Gatwick international airport is close by.

The answer is, of course, that she was there to eat. Having just completed a long and arduous migration (for the conditions in 2019 were far from helpful to northbound migrant birds), 7L's first priority would be to feed up and recover physical condition. Perhaps the absence of other ospreys competing for resources was a plus point for her. This part of Sussex features a couple of significant river systems, reservoirs large and small, and - most importantly for our study – there are many small lakes and ponds, often on private land and most (if not all) containing fish.

Sussex may not be popular with ospreys but it's well supplied with angling clubs and venues for them. 7L soon found one of the best of these – a series of stocked fishponds with a large wooded area close at hand for roosting. It is identified as “Site 13” in our study.

This single location could have supplied all the food that a lone female osprey could possibly need. All she had to do was stay there and collect a free meal as and when necessary. But THAT isn't what happened...

Over the next 30 days, 7L explored the whole area, seeking out every body of water - large or (preferably) small, she could locate.  Following her GPS path, we had to locate them too – no simple task as several were not even marked on our larger-scale maps!  Over the following weeks she visited these places multiple times, flying a total of 486 km during the period.

Using the QGIS 3.8 geographical information system, I set up an analysis of these movements.  The system selected a number of location “visits” according to set criteria which, after some experimentation, seemed to match the behaviour of a foraging bird. (It was not sufficient for 7L to have just flown over a given lake on her way to somewhere else.)  You can find a detailed description of how this was done in the technical paper linked at the end of this article. [2]

In all, 18 locations were selected and the total number of visits recorded for each. Although “Site 13” remained her favourite and she often returned there, it was certainly NOT her only resource for food.  With a maximum distance between sites of some 20km, we conclude that these are not random movements, but rather that they demonstrate a structured (and probably highly-optimized) foraging strategy.

Examination of  inter-site flight paths supports something that we have always known about ospreys – they remember every single foraging location and, once fixed in the context of local landmarks, can fly back there by direct point-to-point routing – even in poor weather conditions with restricted visibility.

We found that 7L showed a clear preference for smaller lakes and ponds.   By contrast, the largest body of water in the area, Weir Wood Reservoir (a notable coarse angling venue), does not appear in our data table at all.  She overflew it once, did not stop there, and never visited it again.


Evolutionary Theory suggests that species such as ospreys should avoid exploiting a single food source, IF more than one such source is available. Computer modelling of predator-prey simulations  also indicate that this ought to be true.  BUT... obtaining confirmation of that behaviour in the wild is much more difficult than running a simulation.  There are many extra variables – and the notorious English weather is only one of them.

During her sojourn in Sussex, and thanks to high-resolution GPS tracking, 7L enabled us to observe  her foraging behaviour in detail, and the results speak for themselves.  Although some sites were favoured above others, ALL of them contributed to her overall food supply requirement and we suggest that the hypothesis is thereby proved...

Ospreys do not over-exploit a single fishing location if other resources are available. In fact, they adopt an innate strategy which avoids doing this.

Note on redactions

Ospreys are a protected species under current UK environmental legislation.[3]  They are sensitive to disturbance at all stages of their life cycle – including “well intentioned” disturbances by hobbyist photographers or over-enthusiastic birders. In cognizance of that, we implement a policy of withholding certain information in documents such as this.

Since 7L is still active, and it is possible (even likely) that she may return to the study area in future seasons, we have removed specific references to named locations and held back or modified maps that may describe movements and roosting / foraging sites in too much detail.  We hope that the reader will appreciate the necessity for this.

P. McMichael
J. Dailey
8th August 2019

Data and facilities from Forestry England and Kielder Water & Forest Park Development Trust 
Additional mapping from OpenStreetmap.Org
Additional computing facilities from Sun Microsystems and Hewlett Packard Inc.

Sunday, 23 June 2019

Off The Grid... Aln/7L in Winter

7L on her nest at Kielder

We had last heard from Aln (Blue 7L) on the 7th of November 2017, when her unusual, weather-dominated migration route through Holland and Germany via the Swiss border with France and through eastern Spain, appeared to have ended in the foothills of Morocco's Atlas mountains.

For days we waited, and then weeks. Hope faded – other sub-adult ospreys had vanished from the tracking charts at this point in their journey. But then...

In late December of that year, we received a fragment of data, indicating that Aln was alive and wintering somewhere in northern Mauritania. This “somewhere” was obviously out of range from the GSM cell towers that carry our communications, and only when she wandered away from it could we expect to hear more news.

(Click any image for larger)
So we waited. Eventually, in April 2019, Aln embarked on her return migration. As her transmitter struggled to upload the mass of data it had accumulated during the previous 18 months we began to fill in the picture of where Aln had been, and what she had been doing. We don't have all of it, and probably never will, but here – in the context of one selected day – is part of Aln's story...

24th Jun 2018, 04:03 UTC Aln (Blue 7L) is an early riser. On her first migration, back in Europe, this was an inclination. Here, in midsummer of the Sahara Desert, it is a survival strategy...

Stars still litter the sky but there is light enough to fly. Sunrise may be be a couple of hours away but already the eastern horizon is a narrow glaring band, like molten silver. Aln's favoured roost sites are among the sand dunes of the desert interior. At night, these locations are away from the chilly onshore breezes of the coastal margin, and the scattered rocks store warmth from the previous day. But she cannot be here when the sun rises and temperatures start to soar. She heads west towards the nearest of several tidal lagoons, four kilometres away.

This is the Banc d'Arguin National Park in Mauritania. It is a World Heritage site and protected under the Ramsar treaty as a location of international importance for biodiversity. By any standards, Aln has chosen a remote and isolated location: there are no people here, no roads, no fishing villages – even the Park staff, based at the southern part of the reserve, face an arduous cross-country expedition of more than 300 km just to get here. It's an ephemeral terrain of tidal channels, quicksands and ever-shifting dunes – because this “land” really belongs to the Sea.

06:27 – 07:20 UTC Aln arrives at the lagoon and waits there. The tide - now rising - has not yet reached this far inland and the shallow water carries a thin film of dust that has fallen during the night. The margins of the pool show a white crust of evaporated salt. Although there are small fish here, it would be hard for her to see them.

It is the exact hour of sunrise. Although Aln is not conscious of it, the sun's appearance feeds important information to the autonomous centres of her brain: her internal “clock” registers that this daybreak is slightly later than the one before, and the one before that. Her innate navigational sense is also recalibrated by the time difference, confirming that she knows exactly where she is.

08:39 UTC Aln flies west again another 6.5 km, crossing the southern lagoon to a rocky promontory on the coast. From this vantage point she views the area, taking in the activity of other birds and assessing the state of the tide. It is still too early for fishing, as seawater floods into the lagoons through a wide breach in the dune margin. After a few minutes she takes wing again, moving north along the shoreline...

Other birds... There are many thousands here, in dozens of different species – but far fewer than when Aln first arrived. The Gulf of Arguin is a unique and vital location for migrating wildlife - one of the most productive marine environments anywhere on the planet.

Cold-water currents carry organic nutrients up and over the edge of the African continental shelf. Meanwhile, dust and sand blows onto the water surface from the desert interior, bearing minerals such as phosphates and nitrates. Fuelled by sunlight and fertilized from above AND below, life explodes into action.

Microscopic plants - the phytoplankton – multiply at such a rate that their biomass is visible from space. They form the basis of a complex food web that attracts all manner of marine creatures. Some - like the rare and endangered Atlantic right whales - travel thousands of kilometres to join the feast.

09:07 UTC  Aln arrives beside the wide and shallow flood channel that feeds the northern lagoon, It is one of her routine fishing spots, but she does not hunt as ospreys do in other places, on the wing. There is no need. Instead, she stands on the side of a half-submerged sandbar and watches for the fish as they struggle past in a few inches of water. She is joined by a host of other piscivorous birds – all taking advantage of this unusual situation...

The fish have been betrayed. Normally a rising tide would provide deepening water to protect them from the predators along the channel, but here this seems to be happening far too slowly. While the tide attempts to flood the land, the evaporative power of the desert sun is lifting eighteen metric tons of water out of the lagoon EVERY MINUTE.

13:59 UTC   Her hunger satisfied for the moment, Aln moves off to the beach where it should be cooler. She perches on some sand bars along the foreshore but it is still too hot for comfort. After a while, she takes off and flies out to sea.

We have observed other tracked ospreys doing the same thing. Flying is an extra way for large birds to regulate the their body temperature. Veins and arteries under the skin of their chests and flanks can be dilated, allowing the slipstream to cool their blood directly.

15:05 UTC   Aln has flown south 2.5km, arriving back at the cape where she was this morning. Tracking data shows that it is one of her favourite places – a rocky promontory that juts out into the sea. Here, in a comfortable sea breeze, she can chose from many safe perches that overlook the gulf. In front of her, migrating dolphins race past, leaping and spinning clear of the water. 16 different species of dolphin and porpoise have been identified at Arguin – a record for the west African coast.

17.28 UTC    “What do wintering ospreys do all day?” From tracking and many field observations, the answer seems to be – as little as possible for as long as possible. Aln has remained at her vantage point for the last two hours. This is not idleness – it is an exercise in inactivity and the object is to conserve as much energy as possible.

We have analysed Aln/7L's flying speeds over the available recorded period and found that they are much lower than those typically seen on migration. It appears that wintering ospreys are in no hurry to do anything, and this fits in with the whole pattern of instinctive energy management.

19:40 UTC   Aln has arrived back at her roost site just as the sun is setting. With minor variations, this will be her daily routine for another ten months, until the time comes for her to return northwards. By that time, she will have finished growing her new set of adult feathers and will look very different in appearance.

She has made her wintering location work for her. To our human eyes, the Arguin coast of Mauritania might look like a harsh and desolate landscape – but it seems to be a good place for ospreys.

Study period: 1-5-2018 to 19-4-2019 (354 days) Data points recorded : 175,451

Saturday, 24 February 2018

Catching the Worm

Migration Forecasts 2018, #1

Yep – it's that time of year again....

The end of February is the period when some early birds are already feeling the urge to leave behind the lazy sunshine of west Africa and head for the wet and still-wintry north lands where they will breed. 

A timely start to migration is important for all birds, and adult ospreys in particular. Even though parts of northern Europe may still be blanketed by snow, they can reach staging areas in Spain, Portugal or France where food is available while they wait for Spring to take a firmer hold.

Atlantic low-pressure weather system (c) McMichael 2018
(Click for full-size)
This time last year, these early migrators had a “goldilocks” window to assist them – and this season, the exact same condition has arisen again. This MODIS satellite picture shows a large circulating low-pressure system in the eastern Atlantic. Winds on its south-eastern quadrant are swinging round to create the long-distance coastal flyway up through Mauritania and Morocco, while still leaving open the Gibraltar straits, where wind speeds will be light.

Running the forecast models a bit further on reveals that a second Atlantic low is developing behind the first, and the models suggest that it will be on the same track. Unlike 2017, this means that the migration “window” could remain open for much longer – possibly out to the 5th March and maybe even beyond that.

GFS model forecast for 25 Feb 2018.        (Click for full-size)

It will be interesting to see what kind of reports come in from the key watch points over the next ten days or so.

Kielder Forest Partnership

Saturday, 8 July 2017

Instrumental Solo

Why don't ospreys migrate together as a family unit?” It has almost assumed the status of Frequently Asked Question, and the answer is simple...
It's because they're ospreys...

Some other species of birds, for example geese and wild swans, do exactly this: they travel from their breeding sites in family groups, with the adults escorting their offspring to the wintering grounds, hundreds or even thousands of miles distant. Without this guidance, the fledglings would never know where to go. But in evolutionary terms, the logistics of this have a very specific result: in a given population and after only a few generations, all the birds end up wintering in the same place! For grazing birds, this is a perfect survival strategy. They assemble in a huge flock, food is not a problem, (grass doesn't run away or hide) and there is safety in numbers with many eyes watching for danger.

But for ospreys and other species that hunt a limited resource, this system won't do at all. They are apex predators, not flocking geese...

A thousand ospreys arriving on a single African lake would exploit the local fish supply at an unsustainable rate. To solve this problem, evolution has arranged matters so that young ospreys have to find their own way in Autumn. They are born with all the navigational instincts needed to do this, but built into those same instincts is sufficient positional uncertainty to ensure that the young birds are dispersed over a wide wintering area. 

This random end-point factor (referred to by scientists as a “stochastic element”) is one of the hidden secrets of osprey migration and is instrumental in forming many aspects of their post-juvenile behaviour.. It has evolved because a general dispersal in winter quarters gives the incoming youngsters a slightly better chance of survival as individuals. 

But where does this "randomness" in the selection of a final destination come from?  That's a whole other story and really needs an article all of its own.

Watch this space... ☺

Thursday, 25 May 2017

Wooden Headphones

... reckless behaviour affecting the conservation of a Schedule 1 protected species

During the Pacific campaigns of World War II, native peoples on many small islands (now generally called Melanesia) came into contact with the products of Japanese, and then Western, technological culture for the first time. Aircraft and ships brought stockpiles of military support supplies – tinned food, construction material, tents and clothing - to the islands, which the inhabitants referred to as “Cargo” - the pidgin word for trade goods. Then, as mysteriously as they had arrived, the service personnel accompanying this logistic effort vanished again, as the fortune of war ebbed and flowed across the region.

The islanders weren't stupid. They knew the value of the goods, but their own culture had no terms of reference to explain how or why all this stuff had been produced and delivered. The Ancestral Spirits had obviously meant the Cargo for them – and if some had arrived, there must be more on the way...

They had observed the baffling rituals performed by the visitors, rituals which preceded the arrival of Cargo. Soldiers marched up and down on the beaches. Others lit fires in straight parallel lines, and then spoke while wearing curious headdresses connected to grey boxes. The islanders reasoned, logically, that if they performed these same rituals correctly, more Cargo would be sent to them.  THAT was how the world of the Spirits had always operated, was it not? 

So they marched up and down. They built “control towers” from bamboo, and carved wooden headphones to use in them. They lit the landing fires, and waited... But, far away, WWII had ended and the aeroplanes never returned. These strange versions of religion persisted in the South Pacific for many years, and anthropologists coined a general term for them..
“Cargo Cults”

In certain areas of Wales, and also in the Scottish borders, some misguided people have been running around the countryside, putting up platform poles for “osprey nests”. The rationale behind this seems plain enough: in the past, ospreys have nested on artificial platforms, therefore many more platforms means many more ospreys will come. Simple, innit?

Except that it's not that simple at all.

We now know that population, dispersal, and nesting dynamics in a recovering osprey population are complicated and have a pattern of development that must be taken into account by any responsible conservation plan. Low population density in a given area means that male birds defend very large nesting territories. Young birds returning to these areas disperse widely, often having to cover hundreds of kilometres in their search for a mate and a nest.

As (and if) the recovery proceeds normally, defended territories become gradually smaller, pre-nuptial dispersal covers shorter ranges, and more local nest sites are taken up. BUT it can take several generations of birds for these changes to happen, and ill-considered attempts to manipulate them cause problems: polygynous nesting being only one of these.

Unplanned platform deployment isn't conservation at all – it's Cargo Cult Ecology.

These hobbyist platform-builders do not understand the underlying processes in a recovering osprey population. But unlike the rather smarter south-sea islanders, they seem to have little or no interest in finding out the real facts, preferring just to perform their Ritual of Poles repeatedly in the superstitious hope that ospreys will magically appear.

The only things missing are the wooden headphones.

Source material:

Cargo Cult: A Melanesian Type-response to Culture Contact” T. Schwartz, UCSD, 1968
A Review of Thirty-five Years of Osprey (Pandion haliaetus) Nesting Data in Rhode Island” E.S. Walsh, University of Rhode Island, (2013)
The demography of a newly established Osprey Pandion haliaetus population in France” Wahl, Barbraud (2016) doi: 10.1111/ibi.12114
Distribution pattern of an expanding Osprey (Pandion haliaetus) population in a changing environment” Bai, ML., Schmidt, D., Gottschalk, E. et al. J Ornithol (2009) 150: 255. doi:10.1007/s10336-008-0345-3
Density dependence in a recovering osprey population: demographic and behavioural processes” Bretagbolle V, Mougeot F, Thibault J-C, (2008) doi:10.1111/j.1365-2656.2008.01418.x

Tuesday, 24 January 2017


Judging the onset of migration

“The time is out of joint—O cursèd spite,
That ever I was born to set it right!
Nay, come, let's go together. “
                                                               Hamlet I,v

As we approach the end of January, it might seem as if the time of Spring migration is still a long way off – but in fact, things are already changing for our birds down in Africa...

Long-distance migration is no trivial matter. A bird cannot just one day decide “It's probably time that I was heading back to my summer nest. Better get moving.” Before THAT happens, birds must make physical preparations and some of these preparations can take many weeks. In some species, feathers must have been moulted and new ones grown out. And in almost all species, substantial reserves of body fat have to be laid down as fuel for the coming journey.

And then there's the matter of timing.
"Sunpath" diagrams for 55ºN (left) and 15ºN (right) Click for larger
It's been known for centuries that migratory birds use changes in the length of day (“photoperiod”) as one primary factor in knowing when to migrate. The biological mechanisms of how this works are complicated, and links to further reading are given below. We now know that animals have internal “clocks” that keep track of time over various periods, from daily rhythms to the cycle of the whole year. These timers are moderated by external factors including photoperiod, changes in temperature, and even (in some cases) location-specific alterations in the flux of Earth's magnetic field.
5F in the Gambia [C. Wood]
In west Africa, the ospreys are already measuring changes in the day length and the accuracy with which they have to do this is rather surprising.

Here in northern Europe, even we humans can notice that we have now passed the Winter solstice and the days are growing longer again. 

At a latitude of 54.5ºN (where this author lives) each day is three minutes longer than the previous one (19th Jan 2017) and the rate of this change is increasing by about three to four seconds every day.

But nearer the equator - where our birds are still consulting their ease in the Senegal sunshine - the changes in day length are much less obvious. At 15ºN, each day is only 30 seconds longer, and rate increase is less than one second per day.
This rate-of-change variable seems to be very important to migratory birds. After all, some of them (swallows, for example) are wintering south of the equator – where the days are still getting shorter.

These changes are tracked by photoreceptors deep in a part of the brain called the hypothalamus. The exact details of what happens during this process are not fully understood, but it appears that some kind of phase-comparison is being made between the amount of light being detected, and the “position” of the bird's internal circadian clock. We suspect this because if either mechanism is interfered with, the bird will no longer be able to evaluate photoperiod changes correctly.
Unringed osprey, Gambia [C.Wood]

But, however it all works, the fact is that the faraway birds are already making their preparations to return for the coming summer.

I'm ready. Are you?

(Thanks to Chris Wood for the use of original photographs)

[1] Evans, P. R. “Timing Mechanisms and the Physiology of Bird Migration.” Science Progress (1933- ), vol. 58, no. 230, 1970, pp. 263–275.
[2] Ubuka T, Bentley GE, Tsutsui K. Neuroendocrine regulation of gonadotropin secretion in seasonally breeding birds. Frontiers in Neuroscience. 2013;7:38. doi:10.3389/fnins.2013.00038.

Thursday, 13 October 2016

Cast a Giant Shadow

UV started his second annual migration on the morning of the 10th of September 2016. It would be pleasant to recount that his departure was accompanied by cheering crowds of well-wishers lining the route – perhaps even that the traditional northern colliery band played a stirring march, while fluttered cambric handkerchiefs and a silent tear from the more demonstrative hearts sped him on his way.

Of course, it wasn't like that at all.

Be they ever-so-famous in Internet Land, individual ospreys perform this last act of the summer season unobserved, for the most part. It marks the point where they transition back to the solitary existence of winter, and only those who are directly observed by scope or camera - or have a tracking unit – will have the actual start of their migration recorded as such.

Perhaps it is as well that the trombones didn't turn out, because UV's commencement was an inauspicious one.

For days, a succession of southerly and south-westerly gales had lashed across the west of Great Britain. After the misery of a damp summer almost devoid of sun, it was looking like one of the most turbulent Septembers on record. But by the morning of the 10th, the wind seemed to have changed direction for the better and many birds began to move southwards towards the coast. The adventure was under way. UV was off to Africa at last.

He got as far as Gloucestershire.

Another series of weather fronts raced in, bringing poor visibility and yet more adverse winds. From Kent to Cornwall, thousands of migrant birds were already waiting for a chance to cross into France. An inexperienced juvenile osprey might have joined their ranks but UV is no longer a raw recruit. He paused his migration at the River Severn and went fishing instead. The weather finally relented on the morning of the 16th, enabling our boy to make his move.

And move, he certainly did.

In the following nine hours he covered 373 km. Giving Weymouth no more than a passing glance (as you would) he launched himself out over the Channel. Off the island of Alderney, UV's (wind assisted) flying speeds were some of the highest ever recorded by us for a migrating osprey, peaking at 118 kph.

The accompanying maps show what happened next: UV 'cut the corner' through Brittany and by afternoon the next day was out over the sea off Saint-Nazaire.

Adult ospreys have the same journey to undertake as juveniles of their kind, but they have one major advantage: they know where they are going and - perhaps even more importantly - they know how far they have to go. A migrating adult has the option of breaking the overall route into manageable sections, according to the prevailing conditions and how much each bird has in the way of energy reserves. This strategy is known as “staged migration”.

UV was aiming to stage at his favourite group of irrigation reservoirs – the barragens of south-west Portugal, and he wasted little time in reaching them by the 21st.

He may have got an unpleasant surprise there. Diligent research by Joanna Dailey found that serious drought conditions have been affecting this region, and we believe that many of the dams had unusually low water levels this year. This may have made fishing more difficult for UV and we noted several unusual local movements at high altitudes, where he seemed to be surveying the area and – perhaps – keeping an eye on distant weather conditions with a view to moving on. (We have withheld exact details of UV's foraging locations in Portugal for protection reasons.)

Whatever the fishing situation might have been, it persuaded UV that two weeks in Portugal was enough. On 4th October he was off again, following closely the same route that he had taken in 2014, and this time his navigation was precise. After a 980+ km over-sea flight, his landfall in Morocco was only 14 km away from the previous one.

On that occasion, UV had staged at the remote Gulf of Cintra in Western Sahara. So... would it be Cintra again, this year? It would not. That weather-enforced break on the River Severn had shifted UV's “refuelling” schedule just enough. There were no more helpful tailwinds in the desert but, employing the energy saving 'soar-and-glide' technique on plentiful thermals, UV easily covered the remaining distance to northern Senegal.
UV Migration Map 2016
(2014 route in green)
On the evening of the 11th, as the shadows lengthened on the Langue de Barbarie, UV slept in the very same roost tree that he had used in February of this year.

Because that's what they do.

All of this information comes from lines and dots and plots on a computer screen, the shadow-tracks of the processed data. But Blue UV is no shadow. For all our marvellous technology, we never forget that he is a living creature. As we track their movements, so UV and his kin track our understanding, waiting to see how we apply it to the benefit of all their species.

And that's one hell of a responsibility.