Song of the Osprey

This year there's been a lot of interest in the various sounds and calls (“vocalizations”) that ospreys make, and in particular their responses to intruders. Take a look at this video with the sound turned well up...

  

 … it's from a camera at an osprey nest in Latvia. The audio quality isn't great but we can forgive that. (The ESTLAT Conservancy do a tremendous job with their monitoring, considering that the nests are among forests in the middle of nowhere.) Listening carefully, we can hear that there are two markedly different calls being given by the adult. Monty and Glesni use these same sounds and it might be interesting to figure out what they are “saying”...

The first type of call is a short, melodious chirp on a rising tone. This seems to be a territorial call and it appears to be specific to ospreys, directed at the incoming intruder as a kind of identifier. Although ospreys don't defend a feeding territory as such, many other bird species do, and they sing to define the range of the territory. A bird's territorial song – which we merely listen to and enjoy – is a very serious matter for it and its neighbours. Millions of years ago some ancient common ancestor of Monty and Glesni might have had similar behaviour, and it could well be that this little chirp is all that evolution has left to us of the osprey's song. It says: “I can see you. We are here, and this nest is occupied.”

The second call is very different: it is the true “alarm call”: a strident challenge and a warning. It says: “Go away! You are too close to my nest and we will chase you if you approach!”

Unlike the “song”, this call is not specific to other ospreys, but is employed generally. The alarm call does something else, too... Watch the behaviour of the nearest chick as the adult's “chirp” song changes to alarm-calling. The chick immediately lies down in the nest and keeps very still. This is an instinctive response to the alarm, technically known as “thanatosis” or playing dead. The young will remain in this posture until the alarm-calling stops – although it can be seen that they are not very good at pretending to be dead, as they keep raising their heads to see what is going on! Osprey chicks tend to do this, even though it rather spoils the effect...

 Chick "doing a Ceulan" and looking around during the alarm call phase.
(Click for larger)  Image: ESTLAT Conservancy

Nest intrusions are a part of life for paired ospreys during the breeding season – and few things are more often misinterpreted by casual observers. On blogs and social networking pages, we regularly see words like “attack” and “fighting off” being used, the users of them assuming that the intrusions are always being perpetrated with hostile intent.

It's much more complicated than that...

“A good place for ospreys is one that already has some ospreys in it.”

With the small (but gradually rising) population density here in the British Isles, it's natural to view the osprey as a solitary nester. But in other parts of the world, nest sites can be separated by only a few hundred of metres, with as many of them occupied as are available and/or can be supported by the local food supply. In this context it would not be wholly wrong to describe ospreys as being communal (or at least, semi-communal) nesters. And in birds, communal living inevitably means the evolution of a social hierarchy and the behaviours to moderate it.

There is plenty of evidence that this is happening among European ospreys, too. Young unpaired birds, both male and female, are fascinated by occupied nests of their own species. They feel compelled to investigate them and it's not just a matter of prospecting for sites: the very presence of a successful nest gives information about the availability of fish, construction material, local prevailing weather and much else besides.

Visiting other nests also enables a roving individual to gauge its own social status as it gets older and more experienced. (You know you're at the bottom of the heap when the local dominant breeding pair can't even be bothered to chase you away!) Although it's true that some “homeless” bonded pairs might attempt to oust an incumbent male from a nest he has just finished building, this doesn't often happen unless the overall pressure for nest sites is very great.

Blue 24(10) inspecting the nest at Cors Dyfi 26/6/13
Click for larger (c) DOP 2013

None of the foregoing means that intruders are WELCOME at an active nest – no matter how peacably-intentioned the visitor might be. The dramatic response of the incumbents demonstrates that - especially if they have eggs or chicks in the nest.

Increasing intruders

Recently, Dyfi Osprey Project revealed that they have seen more than 30 different individuals interacting at the Cors Dyfi nest site (to 2/6/13), compared with 25 by the same period in 2012. Does this mean that the UK population of ospreys has suddenly increased by 17%? Perhaps not...

A significant observation is how few of these visiting birds were ringed. Given the overall UK ringing rate, we might reasonably expect that 30-40% of them would have visible rings but in fact it was only a handful. It's possible that the appearance of these “extra” ospreys is a consequence of the unusual migration weather and the late spring, and that a proportion of them were in transit to other countries – most likely, Scandinavia and the Baltic.

REVIEW: Barr & Stroud "Sprite" 10x50 Monocular

Simple question: can you get decent birding optics for less than fifty quid?  Simple answer: no, of course you can't – the whole idea is ridiculous...

(Image courtesy of The Dreadnought Project wiki)

Back in the 1920's, when Britannia really DID rule the waves, her Majesty's navy went to one place and one place only for the supply of binoculars, telescopes, and optical rangefinders – Barr & Stroud Ltd of Glasgow. Equipped with these beautiful (and largely hand-made) instruments, British diplomatic policy consisted of loading up the main armament and letting Johnny Foreigner know who was boss. But times have changed. There are no battleships left to impose order on the poor benighted heathen, and there are no companies left like Barr & Stroud either. These days your MG car is made in China, your Webley & Scott shotgun is made in Turkey, and this new range of B&S “Sahara” optics originate in South Korea – via a new import firm and a smart bit of re-branding to cash in on the famous name.

I have many pairs of binoculars – far too many, if I am honest.  And I can't look through any of them any more.  Recent illness left me with a medical problem called “strabismus” which means that I get double vision when using them.  Although this doesn't affect my daily life to any great extent, it is a nuisance when out birding.   What I really need is HALF a pair of binoculars – so when I saw this 10x50 device being advertised, I decided to grab one and test it.  Unlike most monoculars on the market today, the B&S 10x50 “Sprite” is exactly that – whereas most of them are really cut-down cheap spotting scopes.  It is a roof prism design and features fully coated lenses, BAK4 prisms and an off-axis focusing system.  Supplied are a captive front lens cover (at last!), rip-stop nylon pouch, and a mini-tripod – of which more later.

A quick trip out to my local nature reserve on a dull and overcast day – perfect for optics testing.   First impressions are more than favourable: this glass is GOOD.  Images are bright and punchy, and the colour fidelity is well-nigh perfect.  Field of view is also excellent at 103m/1000 – as good as anything in its class.  The teeniest trace of chromatic aberration could be detected on brightly lit subjects but even this is a lot better than usual.   For a 10x50, edge distortion is minimal.  Up against my old Zeiss 10x50 porros – which cost ten times as much – the B&S Sprite matched them on optical performance in every department.  Of course there is no “stereo image” impression with a monocular but we pretty much lost that years ago, when roof prisms became the standard layout.

So far, so excellent.  But what really impressed me was the ergonomics

and handling - someone has really put some thought into the design, and it shows.  The frame is polymer, for a total weight of 420g.  This is covered with a rubberised armour giving good grip.  The tube is fully waterproofed and even nitrogen purged (something we just never see at this price point) so fogged-up lenses will not be a problem.  Overall, the exterior moulding fits my (rather small) hands well, with my index finger falling naturally onto a focus control which is both light, and as smooth as a double-glazing salesman's patter.

I really like this thing.  It's not perfect: compact dimensions and 10x50 usually mean critical eye relief, for the laws of Physics cannot be altered, but at 17.5mm this is better than average.  The eyepiece has a four-position click stop arrangement with positive detents.  With my mismatched eyes I would have preferred an extra two stops, but this would have increased the overall length.  For normal people – and spectacle wearers – it should be fine.  The other imperfection is the included table-top tripod.   It's cheap and rattly and very nasty, and will probably stay in the box.

I also did some quick experiments with “digiscoping”, for which the Barr & Stroud monocular is surprisingly well suited.  Even with adapters, proper digital SLRs don't like it: the vignetting upsets their fancy metering unless pulled back to full manual control.  Much better results came with a Samsung Galaxy S3 phone: its autofocus was quite happy to look through the Sprite and it figured out the correct exposure with no adjustments at all.   A couple of extremely boring test shots are included here.

Digiscoped shot with Galaxy S3. The houses facing are exactly 220m away

.

So what about the price?  Pacific-rim optics have come a long way in the last ten years and, in the medium price bracket, are now a match for anything made in Europe.   Last season, the same range of bins and monoculars was being brought in through Bresser in the USA and the equivalent model retailed at around £90.00.  The new importer has done some aggressive discounting and I bought this one (new and boxed) from Amazon for £49.00 including delivery, which is just a steal.   Even better, unlike some cheap instruments with their orange lenses and go-faster stripes, the “Sprite” 10x50s will not embarrass you down the bird hide: its black and green finish looks the business, and it still has that historic name on the side.  Truly “second kind of cool” and, if anyone asks – show them the receipt and watch their faces...

Barr & Stroud “Sprite” 10x50 Specification

Magnification: 10x

Objective: 50mm

Minimum focus distance: 2.3m

Field of View @1000m: 103m

Prism type: BAK-4

Lens Coating: Fully Multi Coated

Exit pupil: 5mm

Eye Relief: 17.5mm

Tripod bush: Yes

Waterproof: Yes (Fully immersion tested 1m for 60sec)

Nitrogen gas filled: Yes

Case supplied: Yes (Nylon with belt loop)

Dimensions: 165x88x60mm

Weight: 420g

[All images except #2  (c) Wildlifewriter 2013]

Great steaming piles of... bark?

Glesni chews some bark.
Image (c) Dyfi Osprey Project 2013

During this early part of their nesting season, the Dyfi ospreys – Monty and Glesni – have been bringing some strange items back to their nest.  As well as conventional materials, they have also been collecting dried cow pats, seaweed, and pieces of tree bark.  A lot of the bark appears to be silver birch, though other species have been sampled as well. Not only have they been collecting this stuff, but on occasion they have been eating it – and on camera, as well

As might be expected, this hasn't gone down very well with the legion of dedicated osprey fans.  They have been disapproving of the selected “nibbles”: ospreys eat fish and only fish, and birch bark and cow poo are not supposed to be on the menu.   So what's really going on...?

This is a kind of detective story.

Almost everything that animals do, they do for a good reason.  That reason might not be obvious to us, ensconced in our little anthropomorphic bubbles as we are, which is why we often misinterpret their actions.  But on other occasions, even a close and objective study of the behaviours cannot provide the correct answer: to get to THAT, we have to think outside the box – and even right outside the nest.  But first, let's define the problem. Fish, we are told, is an almost complete foodstuff in itself and ospreys shouldn't need anything else to sustain them.  That's what it says in all the books.  And here's the thing...

The books are quite right. They don't.

And yet is seems unlikely that Monty and Glesni are eating cow dung and chewing bark just because they like the taste.  I mean, would YOU? That means they must be getting something important from these things – something to do with nutrition or the maintenance of good health. And there, my dear Watson, is our first clue.   A second clue comes from something that cow pats and bark have in common, and that fish don't.   The cow dung consists of partly digested vegetable matter.  Once it has lain around for a while, and the urea and sulphide compounds have degraded or been leached out by the (very occasional) Welsh rain, it doesn't smell too niffy nor taste too acrid.   The birch bark includes an inner layer of woody cells (part of the cambium) which contains and transports many of the nutrients used by the tree during its own development.

Molecular structure of
unbound thiamine pyrophosphate
(c) Warwick University

There are some substances vital for life which animals (with a few rare exceptions) cannot synthesise for themselves.   One of these is vitamin B1, or thiamine.  Thiamine is important stuff: it is needed for the production of ATP, a molecule which supplies energy to the cells of the body, and also plays a role in the correct functioning of the heart, kidneys, and central nervous system.  Chronic B1 deficiency is a serious and potentially fatal condition in all animals, and in humans it has a disease name: Beriberi.

So perhaps the ospreys are getting this and other substances from direct ingestion of plant material, to supplement their typical diet.   If this is so...

Hold on just a cotton-picking minute there, Sherlock. Uncooked fish contains LOADS of B-complex vitamins. Everyone knows that!

And so they do. But some of them might contain other things as well. This is where we need to think outside the box (or nest) because the answer doesn't lie on the green pastures of Derwenlas, but out there in the grey waters of the Irish Sea...

Grey mullet

A favourite prey species of the Dyfi pair is the Atlantic grey mullet Mugil cephalus, which form more than 50% of their total catch.  Mullet are an ideal osprey food: rich in fats, protein and easy to catch because their habit is to feed at the top layer of the water-column.   There they graze on plankton – microscopic plants and animals – and small free-swimming crustaceans. It's been found [1] that mullet also contain quantities of enzymes called “thiaminases”.[2]  The thiaminase enzyme splits B1 molecules in situ, making them biologically inactive.  The amount of enzyme contained in mullet seems to vary seasonally and by location, and may depend on what the fish themselves have been eating. [3]

So this may be the answer to our little mystery: the wonderful mullet which ospreys adore also hides a sinister secret. By eating so much of it, the birds may actually be getting less available vitamin B1, because enzymes in the fish itself are blocking it. To redress their dietary balance, Monty and Glesni need to nibble at an alternative “top-up” source – one that is already part-processed (by tree or cow) but which doesn't contain any thiaminase. This would explain why other fish-eating birds, or other ospreys in different parts of the country, don't have to nibble on cow pats.

No shit, Sherlock?

None at all Watson. Pass me my violin, there's a good fellow....


Notes:
[1] Source: Cornell University Dept of Animal Sciences
[2] NEOPYRITHIAMINE AND THE THIAMINASE OF FISH TISSUES R Sealock & J White; J. Biol. Chem. 1949 181: 393-403.

[3] Or it is possible that this could be an evolving anti-predator adaptation. Other marine organisms use vitamin-busting enzymes as part of their chemical weaponry. Ferns - especially bracken - contain large amounts of thiaminase, which helps them to deter browsing herbivores.

The Puzzle of Portmore Lough

The view from the lower hide is oddly familiar, yet rather strange. We overlook a freshwater lake surrounded by low-lying wetlands of fen carr and unimproved grassland. Summer waterfowl chase each other around the shoreline, while nesting terns and gulls are busy on a couple of nearby nest rafts. The sky is big and open, though a few birch and alder trees stand to our left. It's sunny and getting rather hot out there. But where are we? It could be Cambridgeshire or somewhere else in East Anglia, or even northern Germany. But listen... from beyond the high ground on the far side of the lake, the sound of racing motorcycle engines drifts down from the roads where morning Qualifying is already under way. Behind the hide, a squadron of military transport helicopters is wheeling down from an air force station towards the army barracks about twelve miles to the south. Wherever we are, it's a long way from Norfolk...

 

This is the Portmore Lough nature reserve in Northern Ireland, and you can bring me here to die.

Gulls and terns at Portmore Lough
(c) Wildlifewriter 2013

On the first sunny weekend of the whole summer, more than 20,000 visitors have arrived at the Oxford Island centre down the road, and their car parks are already overflowing – yet here we have the place to ourselves. Like most fenlands it's remote and difficult to access, although RSPB who have the management of the reserve area, have done a lot of work on paths and boardwalks since last year. Wellington boots are no longer obligatory. This type of wetland habitat is rare in Ireland but, where it exists, Nature takes full advantage. As biodiversity hotspots go, the place is almost incandescent: 317 species of insects at the last count, including bembidion clarki and a colony of rhinoceros beetles found nowhere else. Tree sparrows Passer montanus are nesting in boxes by the upper boardwalk. On the water, common terns are still incubating: testament to the late spring and their difficult migration this year, but the black-headed gull chicks have hatched over the weekend. They stagger among the tern nests, trying to find their feet and avoid being pecked at the same time. The project to increase lapwing breeding is also paying off and numbers are already up on last year.

Anything could appear here – and usually does, if you time it right. In spring and autumn, the big-ticket items are passing through: ospreys are regular visitors, though none have stayed to nest as yet. It would be an ideal place for them, with perch and pike abundant in the water and minimal human activity. Short-eared owls feel secure enough to hunt in broad daylight and, after dark, rare Daubenton's bats are active along the gullies and hedgerows. By winter, internationally important numbers of wildfowl roost overnight on the lough, safe from ground predators and humans disturbance. Whooper swans and some Bewick's will be here, with gadwall and pochard occupying the cheap seats at the front.

Up at the observation platform, local birder Paul Toner is catching a few afternoon

Juv female marsh harrier, Portmore Lough, Jan 2012
(c) Ed O'Hara

rays and we stop to ask what the “craik” is: in January last year, Paul, Ed O'Hara and the local group recorded (and photographed) marsh harriers on the reserve – a good tick anywhere but big potatoes for Ireland where they are never regular. We talk about ospreys again, and the issues about getting planning permission for a nest platform. Bureaucracy will have its due course, and nothing will speed it up. A few other enthusiasts wander in and, as ever, the conversation turns to the mystery of the Lough itself: why is it here, and why is this place so different from the surrounding landscape?

No-one knows. The lake itself is almost circular, which is an odd thing for a natural formation. It is certainly not glacial in origin, as existing textbooks claim.

Google Earth 2013

Old maps and written descriptions from the sixteenth century make no mention of it, and recent core samples have confirmed that there was no large body of water present at that time, yet one seems have appeared - almost overnight - around the mid 1580's. It's even been suggested (quite seriously) that an airburst meteorite might have exploded there, excavating the circular depression in which Portmore now lies, but leaving no physical trace of what would surely have been a spectacular event at the time...

You'd think someone would have noticed.

RSPB Portmore Lough, Aghalee, County Antrim.
N 54.570768, W 6.299629
Open year round. Parking, wheelchair access.
Ph: 028 9049 1547

HUNKERING and BUNKERING

The unseen design elements of bird nests.

((c) Dyfi Osprey project 2013)

Many species of birds build nests which – to our eyes – seem unnecessarily vulnerable to the elements. Herons, most corvids, ospreys and many others choose to make their nests in exposed trees or other high places. Given the vagaries of typical British weather, we worry about them and their broods when the rain falls (as it usually does) and strong winds start to blow. However these nests – or at least the central “egg cup” area of them – could be better protected than you might think. But to understand why this might be, we have to look first at a seemingly unrelated subject: Golf.

All keen golfers will tell you that they love to play on the classic style of seaside course: the “links” - which take their name from the term for an area of sandy, infertile ground between farmland and the sea. Golf courses were first laid out in such places because the land is nutrient-poor and of little use for anything else. Links course are challenging to play on, and test the full skill set of golfers. We rejoice in the wild scenery, the undulating fairways and the tricky sloping greens, and we all agree that seaside courses would be perfect, except for one thing...

Rory McIlroy has a practice bunker

in his back garden at home

Pot bunkers.

We all hate them.

Unlike the wide shallow sand traps found on parkland courses, pot bunkers are small and deep, often with high revetted faces and a cup-shaped bowl of tight-packed sand at the bottom. It is all-too easy to get the ball in one, and difficult – sometimes impossible – to get it out again. Almost every amateur golfer believes that pot bunkers are put there to make the course harder, but that's not the real reason for their existence... 

It's often windy at the seaside, and strong winds would remove the material from a conventional sand hazard in a matter of days, meaning that the grounds staff would be forever having to replace it. The design of pot bunkers means that this doesn't happen: the deeply-shaped recess actually forms a high-pressure area above the sand surface, sheltering it from wind gusts.

Large bird nests have a similar layout to pot bunkers, and the same general aerodynamics. A wide raised rim surrounds a lower bowl-shaped interior, in which the eggs are laid. With this “design”, the weather may be as bad as it likes but the eggs – and later, nestlings – are protected from the worst of the wind, as is the bird which broods them. Rain gets in, of course, but rain itself is not the primary threat to eggs and chicks – as long as there is not too much. It is the WIND, with its effect of carrying away heat, which is the real problem.

So... the next time you watch a poor bird hunkering down in her nest while a howling gale blows around it, don't worry too much. Things may not be quite as bad as they look.

Seals and Eiders

There's no blog this weekend because my mother is ill in hospital and we have been rallying round, fetching stuff, doing good works and generally looking busy.

To fill the gap, here's a picture taken at Rathlin Island, Northern Ireland, during my last trip out there:  Seals and female eider with brood.  (There were actually 5 eiderlings, but they steadfastly refused to gather for a group photo.)

(Click for full-size)

"TRUE OR FALSE" OSPREY QUESTIONS (2)

(Originally at FB Osprey Webcam Group)

(c) There are no ospreys on the Indian sub-continent 

FALSE. Ospreys are still reported from India and Bangladesh, where they appear to be coastal, confined to specific areas and not common anywhere. One hot spot seems to be the Sundarbans wetland in Bengal, where ospreys are regularly spotted by visitors. The most recent sighting I could find was 2/2/2013 and this photo by Nazul Islam was taken in 2010.

Osprey at Sundarbans lagoon. (c) Nazul Islam 2010

The population appears to be migratory and there have been a few ring recoveries – one from a bird originally ringed in Norway. In their survey in the 1980's, Sakeur & Sakeur listed ospreys as one of 14 raptor species known to be in decline, citing habitat loss as the major factor in all cases.

Things must have been different in the recent past... A young Army officer named Elliot, on posting in the Punjab, complained to his diary of 1891 that there were no interesting or exotic birds on and around the lake outside his kitchen window, “...only the usual ospreys, everywhere to be seen.”

 

(d) GPS bird tracking devices are less accurate when it's raining.

Pretty accurate: Niacom active antenna with Garmin Q3600 using SBAS differential corrections for a combined error of less than 2 metres

FALSE. Occasionally, I am asked to explain to someone exactly how the Global Positioning Satellite System works. I try to avoid this, because usually the explanation is only a quarter way through before the “someone” has fallen into a deep and restful sleep. The GPSS is one of those things like Corporation Tax, or the Immaculate Assumption, or open heart surgery – if you needed to know how it works, you would already know exactly how it works.

The space-to-ground segment of GPS is a radio signal. In fact, two primary radio signals, known as the “L1 and L2 carriers.” When the system was being designed, the wavelength for this signal (20cm L-band) was carefully chosen because these frequencies are least likely to be affected by water vapour in the atmosphere. Clouds and rain have no measurable affect on GPS accuracy, although many internet “experts” will still try to tell you that this happens. The myth probably arose because people that are carrying GPS -equipped smart phones might take shelter from rain under structures or dense foliage. These local obstructions could have some effect on reception, but the rain itself does not.

 

(e) Ospreys don't like flounders because they taste funny and are the wrong shape.

FALSE. Poor flounders... rejected by chefs, despised by ospreys, and as ugly and stupid as Ian Duncan-Smith, they get a universally bad press. The fact that ospreys don't like them is well supported by data, though it might be truer to say that while ospreys are prepared to eat flounders – they'd just prefer to eat other things instead. But why? Well camouflaged bottom-feeding flounders are abundant around the British coast in summer, and you'd think they would be perfect sustenance for a nest full of baby ospreys.

The best theory we have is to do with net energy inputs. Compared with other seasonal estuarine species ( e.g. mullet, bass, sea trout) the flounder has about 8-10% less fat content. And for birds, as we all know, fat = flight fuel. Emyr Evans (who is not keen on eating flounders himself) has pointed out to me that they have tougher skin than these alternative species, and so the upshot of all this is a three-way disadvantage in terms of diet:-

• More difficult to catch,
• More energy expended dealing with the food,
• Less energy obtained after ingestion.

Put together, this would explain why the birds – especially breeding females – have evolved a preference for more energy-rich prey items. Shape and taste are unlikely to have much to do with it.