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. www.jstor.org/stable/43419959.
[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.


Sunday, 4 September 2016

Parent-offspring recognition in European ospreys

4th September 2016

'How sharper than a serpent's tooth it is to have a thankless child' 
                                                                             [King Lear, I, iv]

Over the last couple of weeks I've been inundated by literally two e-mails, both asking much the same question:- 

Do parent ospreys recognise their returning offspring from previous seasons?”

There is no simple answer to this seemingly-straightforward issue. It seems clear that many species of birds can recognize each other as individuals, once they become acquainted. Of course, the term “kin-recognition” encompasses a whole raft of responses and behaviours, but in this article I am going to focus on the question asked, which concerns parent-offspring recognition.

This has been proven in birds that nest communally, and the selection pressures for this ability to evolve in communal nesting are fairly obvious. Mechanisms seem to include both vocal and visual recognition. Penguin adults and chicks can locate each other – apparently by voice – even in the noise and visual confusion of a busy colony. Beecher (1988)[1] showed that bank swallows are able to do much the same thing. Birds of prey, in contrast, tend to nest by territory – but Alcaide et al (2007)[2] demonstrated that these species (ospreys included) also carry the MHC gene sequence, which had previously been associated with kin-recognition ability (and lots of other things) in mammals.

Ospreys, at best, nest semi-communally and their migratory lifestyle complicates the issue still further. Their young leave the nest at the end of summer, and undergo a complete feather-replacement moult into adult plumage before returning north to their natal regions. This means that a returning youngster will be completely different in appearance, calling into question the whole principle of visual identification by the parent.

All this stuff isn't really getting us anywhere...

UV on the nest
(detail from original
watercolour by
K Davies)
To find out more, I turned to one of the most closely-observed young ospreys on the planet. Yes, you've guessed it – it's UV again.

UV is the trifecta for this job: he has a high-visibility leg ring for ease of spotting, he also has a satellite tracker for detailed following, and his parents' nest at Kielder Forest Park is monitored by video recording cameras, (courtesy of Forestry Commission England.) On 1st July 2016, he landed on that nest while his mother was looking after this year's brood of his siblings. Such an event – a year-two returnee alighting on the natal nest and interacting directly with a parent – is sufficiently rare that filmed recordings of it are almost non-existent.

But we got one.

Video (c) 2016 Forestry Commission England
Used by permission
Short though it is, this sequence repays careful study. We know from tracking data that UV approached the general nest area from the south-west, and that he altered course directly towards it when about 1000m up-range. The nest female sees him approach and goes onto the nest to defend her brood. The chicks - of which there are four - cannot yet fly at this date, so they crouch down in response to the female's evident alarm-calls.

UV makes several passes over and round the nest, before finally landing on it. He alights right on the edge, prepared to take the air again at a moment's notice. His mother mantles and calls in protest at his temerity, and within seconds she has chased him away.

Is there any evidence of “recognition” here? The answer has to be “no”... ALL the birds involved behave and react in exactly the same way as if the intruder had been an unrelated individual.

Expert opinion is still divided on the initial question. Some hold strictly to the view that there is no parent-offspring recognition in European ospreys. Others prefer the view that such recognition “ought” to have evolved, and that there is no evidence to confirm its absence. A few (this author included) suspect that the parent birds cannot recognise a returning youngster, but that the youngster has some instinctive awareness of the kinship.

 But all are agreed on ONE thing: it doesn't make a blind bit of difference who is right! The returnee is no more welcome at a nest site that any other osprey would be, and the breeding adults would always react in exactly the same way, offspring or not.

So is that the end of the story? Not quite.

UV nabs a free meal at Nest 2
(Forestry Commission / J Dailey
For the remainder of summer 2016, we continued to follow UV's activities. He remained interested in Kielder Forest, and intruded at all the osprey nests there on several different occasions. All the nests, that is, except N1a... He only made one more return visit to his parent's nest and did not land on it again. For most of the time, he maintained a discreet distance from it while moving around the Park.

 Does this mean that UV is somehow aware that the parental nest is not a proper place for him, whereas all the others are fair game for investigation? I am not sure. Only the birds know these things for certain.

And they're still not telling.

[1] “Kin recognition in birds”: Beecher, M.D. Behav Genet (1988) 18: 465. doi:10.1007/BF01065515 

[2] “Characterization, Polymorphism, and Evolution of MHC Class II B Genes in Birds of Prey”: Alcaide, M., Edwards, S.V. & Negro, J.J. J Mol Evol (2007) 65: 541. doi:10.1007/s00239-007-9033-9

Saturday, 13 August 2016



When you have eliminated the impossible, whatever remains, however improbable, must be the truth”
        Arthur Conan Doyle The Sign Of The Four (1890)

Strange things have been occurring at several of the monitored osprey sites, this season. Unaccountable, worrying things that seem - on the face of it - to be so far removed from the usual that they defy explanation.

17.7.2016 Fledgling Z0 “Ceri"
falls from a perch at MWT Cors
Dyfi and sustains serious injuries
(Infra-red illumination)

[Montgomeryshire WT]
Fledgling birds have been falling from their perches. Youngsters have appeared to have difficulty in walking, holding their legs or talons at strange angles. Some have flown and failed to return; others have gone off their food, seemingly ill, only to recover later. One chick (at MWT Cors Dyfi) has been so badly hurt that she went into circulatory shock and died. And on social media and the forums, everyone has been asking:-

What the hell is going on? 

These are wild creatures and every moment of their lives is at hazard. Bad things can and do happen to individuals, whether they are under observation or not. It's Nature. Flying is an instinctive ability, but the judgement of speed, distance and timing are not: these are acquired skills and they have to be learned and practised. Young inexperienced birds sometimes have accidents, and the outcome depends very much on chance and circumstances. Usually, they get away with it unharmed – but not always. 
We on the other hand are not wild birds. We are human beings and one of our less-rational habits is to search for patterns in isolated events, where often none exist. Psychologists even have a term for it: “apophenia”. Scientists are trained to recognise this trait in their own day-to day work, and to eliminate it in favour of objectivity and rigour.

There's an app for that...

Statistical mathematics include a set of tools that can tell us whether the frequency of an event (or series of events) is higher, lower, or the same as might be expected to occur by chance. Numbers were crunched and the answer came back: it was higher. Not by much, but detectable.

30.7.2016 Osprey W7 sustains
apparent grazing impact injury.
Close-up of bruising and
swollen leg joint.
[BGGW Glaslyn Wildlife]

This begged the next question: could there be some underlying factor that was affecting some of these birds but not others, even though the incidents were happening in widely-separated nest locations and at different times? Powerful minds (much more powerful than mine) applied themselves to the problem. The 'usual suspects' were rounded up: provisioning rates, environmental pollutants, fungal toxins... one by one, they were eliminated from the inquiry. Exotic diseases were invoked: could it be bird flu? Avian malaria? Trichomonosis? Any of these can affect lower body function but they have other symptoms, too, none of which seemed to be present.

The leg rings themselves were checked on, and checked on again. But all were the same rings as had been used in previous years, made from the same materials and fitted by the same teams. It wasn't the rings.

Other theories were suggested, from the mildly unlikely to the just-plain-bonkers. Recessive gene mutations? Nope. Calcium deficiency causing paralysis? Can't be. Short term variations in the Earth's magnetic field? Gimme a break.

Perhaps there IS no answer. Alternatively (and it's just another theory) the answer might have been staring us in the face, all the time...

Kielder osprey Y0 fledged on 15th July, out of
view from the nest cameras. She did not return
to the nest after fledging. Current whereabouts
unknown. [Img: Paul McMichael]
I was chatting with young Damien the other day. Damien is one of the guys on the local red squirrel conservation survey, and he gets the job of installing the higher-level trail cameras and bait trays. Part biology student and part gibbon - with vocalizations to match – Damien is not a happy arborealist these days.

Duuude... real gnarly climbing up there... like, slippy and slidey, y'know... maaad dangerous...”  

And this is a bloke who will happily hang from a branch by the crook of one elbow while thumbing tweets into his mobile with the other hand. The boy's prehensile. If Damien thinks that working up in the trees has suddenly become “mad dangerous”, then something is amiss.

My UK readers will have noticed that, in 2016, the notorious British Summer has been even less summery than usual. Rainfall - markedly in the west and north - has produced totals for June that were 30% higher than the moving average. The figures for July are not likely to be much better. Cloud cover has been over 90% for almost 90% of the days. These unsettled conditions have featured blustery winds, variable in direction and gust strength. In technical terms, it's been rubbish.

What effect might such unseasonably poor weather over such an extended period have on the environment?

Wet and humid conditions are ideal for growing the algae and moss that has
been troubling our Damien. Branches of deadwood trees (exactly the ones that are favoured by ospreys for perching) never get the chance to dry out, making them slippery and even liable to breakages. Cloud cover makes for poorer visibility, more difficult judgement of distance, while gusting winds are the last thing that any fledgling bird needs to encounter on its first experimental flights.

Any one of these factors could cause a problem on its own – but put them all together and they stack: generating a persistent and cumulative rise in the accident rate that we start to notice.

And so we mustn't give in to that seditious inclination of ours: that everything that happens must have a single identifiable cause and agent, so that we can hold someone or something “responsible” before dashing off in pursuit of the next conspiracy. I find no mysterious or even sinister forces at work here. There's an underlying rough-and-tumble to any wild fledging season - and the rougher it gets, the more tumbles it produces. In certain isolated years, all the adverse factors combine in one relentless stream of bad luck and unhappy accidents.

I blame the weather – but the weather didn't mean it. All these incidents are still just Nature, doing that stuff She does.


The views expressed in this article are those of the author himself, and are not intended to be representative of any other organization or group.

Monday, 25 April 2016


UV and the passage of the Sahara

So we think that UV has commenced his return migration. His first challenge is to traverse the western desert – a 2000 km flight to reach the straits of Gibraltar. We tend to think of this as a hazardous route, and for us it would be. But for birds – not so much...

22nd April 2016: UV sets out from Senegal, catching a few thermals on the way.  (Data: Forestry Commission England) 

There are few sizeable predators, and even fewer that would be capable of taking on an osprey. There are almost no invisible overhead cables to trap the inattentive; no mad fat bastards with guns to shoot at them; no speeding vehicles to collide with them. The skies are clear and visibility is good. It's just a matter of flying.

Of course there are no fish and precious little water in a desert – but an adult osprey in the early stages of migration does not require these things. Before leaving Senegal, UV will have bulked up, laying down fat reserves as fuel for the coming journey. And as he flies northwards, his body deploys Evolution's secret weapon for long-distance migration... “ß-oxidation.”

This is part of a three-stage process whereby lipids (stored fat) are converted into energy to power UV's flight muscles. And as a by-product, metabolic water is released at the cellular level. The numbers seem like a conjuring trick: 110g of water is produced for each 100g of fat consumed! (Okay, some other stuff is going on here as well, but it's still a remarkable adaptation.)

This biological miracle is the key to long-distance bird migration: the longer and harder you fly, the more water you get!


Thursday, 10 March 2016

Evidence for communal roosting of wintering European osprey Pandion haliaetus

The breeding habits of the European osprey have been well documented over the years but, until comparatively recently, much less was known about their lifestyle at winter quarters.

UV on 26th Feb 2016 (Vic Paine)
 A combination of satellite tracking, high-visibility rings, and diligent fieldwork from many sources has increased our understanding of this seasonal behaviour, but much remains to be discovered.

In the last ten years, bird tracking devices have used on-board GPS sensors in preference to the much-less accurate doppler VHF system. This has allowed us to observe specific movements during both day and night, and some patterns have started to emerge...

It had always been thought that ospreys were mainly sedentary during the winter season, keeping to a small activity area and seldom if ever leaving it until the time for spring migration. But the higher levels of detail now available has shown that this simplistic picture is not quite right. Many ospreys - and especially those that forage along exposed beaches and river systems - leave their hunting areas for the evening and roost overnight at favourite spots further inland, often in the same tree each night. Only where there are coastal or riparian “forests” (such as mature mangrove) do they tend to remain in place.

The reasons for this behaviour are not difficult to deduce: beaches may be warm and sunny during the day but chillier at night. Vegetation tends to be low and sparse, so predation risk would be higher. Evenings along river margins are when biting insects become most active.

Many tracked ospreys, including “Ceulan” (Blue 3C in 2012) have returned tracking info that demonstrates this transference to off-area roosting. As I studied their various data files over the years, I kept asking myself the same question: why is this bird flying five, ten, even twenty kilometres to THIS roosting tree? It must have flown past hundreds of other suitable trees to get there – so what's so special about this particular roost?

When Joanna Dailey, Pip Rowe and Vic Paine visited NW Senegal [1] in February 2016, I encouraged them to pay special attention to the roost site(s) of osprey Blue UV – a 2014 osprey from Kielder Water in Northumberland. This bird was carrying a “second generation” GSM tracking device, so we knew exactly where he was sleeping and were getting regular updates each day on his diurnal movements. (I was under no illusions about what I was asking of them: night falls very quickly at latitude 16ºN, and the west African bush is no place for newly-arrived white folks to go wandering around in the dark.)

Nonetheless, the group DID manage to observe UV at his habitual roost site – and he wasn't the only osprey using it! On successive mornings, they were able to see several birds leave the area and fly westwards towards the fish-rich Langue de Barbarie coastal strip. It was exactly what I had hoped they would find.
UV leaves his favourite baobab tree in the early morning (Vic Paine)
Later, Dirk Vanhoecke (director of the Tougoupeul Project)[2], confirmed in conversation with JD that his study group regularly see numbers of ospreys - six or seven, typically - roosting together in the wooded hinterland areas of NW Senegal. In this specific case, UV appeared to be totally relaxed in the company of other ospreys: was seen preening at perch and flying with them in the early mornings. The roost was also occupied by other species, including red-billed hornbills T. erythrorhynchus kempii.

Although large birds of prey do not form daytime flocks, communal or semi-communal winter roosting has been observed in species such as Bonelli's Eagle[3] and Montague's Harrier. In recent years, migratory lesser kestrel roosts of over to 20,000 individuals have been found in west Africa [4]. The survival benefits of communal roosting suggest that ospreys should have evolved similar behaviour and, as their overall population numbers slowly recover, we are beginning to see evidence that this is so. 

An osprey flies out over the Langue de Barbarie, Feb 2016 (Joanna Dailey)
 My thanks to Vic Paine and JD for permission to use original photographs in this article.

[Links and sources...]

[2] Project Tougoupeul : http://www.tougoupeul.be/

[3] Communal Roosting in Young Bonelli's Eagles (Aquila fasciata); Marcos
Moleón, Jesús Bautista, and Agustín Madero (in Spanish)
: http://dx.doi.org/10.3356/JRR-11-30.1

Saturday, 31 October 2015

An Osprey For All Seasons – Blue 7H

30th October 2015

Bad luck! If I’d any good luck to spare he could have some. I wish we could all have good luck, all the time! I wish we had wings!” [Robert Bolt: A Man For all Seasons, II,i.]

We call it the “activity counter” - and in the world of GPS-based wildlife tracking, it can be a dreadful, portentous, doom-laden thing.
The activity counter is a column of numbers in a data file. When a bird or animal is being tracked, sensors in the PTT unit record any movements between sample points, and increment this number at each occurrence. An activity count that has NOT updated within a significant span of time, brings on a feeling that is all too-familiar to wildlife researchers:-

Oh, no – not again...” 

Whether tracked or not, the population survival statistics for sub-adult overwintering ospreys are – as with the young of many other predator species – brutal and uncompromising. Fewer than one in four ever return to their natal regions in northern Europe to breed. It's unclear if this ratio is getting worse, but it certainly isn't getting any better. Many adverse factors seem to be at work, and the renowned conservationist Roy Dennis has estimated that some 60% of losses are due to “human activities” - either directly (illegal shooting and trapping, vehicle collisions, wire strikes, and so on) or indirectly, through loss and conversion of habitat and potential nest sites. Ospreys and other raptors that survive to breeding maturity have one thing in common: 

They all had good luck, ALL of the time.

7H was fledged at Kielder in 2014.  She became special to us in the way that these things always happen: though interest and familiarity. But 7H was not a pet - she was and remained a wild, free creature, doing exactly what her nature had inclined her to do. In following her adventures and those of her kin from Kielder Forest, we were able to learn more about this nature – and at a level of detail that may not have been achieved before.

We observed her migration with bated breath, over land and sea. We watched as she caught rides on passing ships (occasionally in the wrong direction.) We saw her learn to catch fish, and how she became gradually more proficient at this.

7H was part of my study into osprey flight strategies, proving that even very young birds of this species have an innate ability to exploit the winds and thermal air currents, and to cover vast distances with the minimum expenditure of energy. She contributed to new understanding of how first-winter ospreys settle into a feeding territory, and interact with others of their kind. She even taught us how fast she could fly, and how far... 

On her initial migration from Northumberland to Morocco, 7H traversed 5188 km. This might sound like a whole lot of flying to us, but her activity in the year AFTER arrival puts this in a startlingly different perceptive:
In the ensuing twelve months, 7H's total distance flown was an additional 9500 km – all carried out within about 40km of her “home” roost locations. Clearly, our traditional view that wintering ospreys “are mostly sedentary” does not reflect how active they are on a day-to-day basis.

 Because we were “with” 7H through the full cycle of seasons, we are able to see how those seasonal changes affected her foraging behaviour. We had confirmation that ospreys will seek for, and use, different hunting areas – even when there are more than adequate supplies in just one of those areas. And in particular, a detailed analysis of 7H's favoured perches and roost sites revealed that – to an osprey – seclusion and the absence of human disturbance are much more important than scenic views or epic landscapes.

So, yes – we have learned a great deal from Blue 7H and this knowledge is important for future conservation efforts. But 7H herself would not have cared one jot for that. Instead, she spent 399 days in Africa living the life that she was born into, free and wild. For those 399 days it had appeared that she was doing everything right: she had chosen a country where wildlife protection law is regarded more seriously than it is in some less-civilized parts of the world – such as Ireland, or Scotland.

Personally, I choose not to grieve for the passing of 7H, but just to be thankful for these many insights into her unique way of life. It has been a rare privilege and one that I will not forget any time soon.



Joanna has written a valedictory article about 7H on the Kielder Ospreys blog, HERE
Data provided courtesy of Forestry Commission England