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.

LOOKING AROUND

Bird Vision (2) - ocular motility

Nay if thou be that princely eagle's bird
Show thy descent by gazing 'gainst the sun.

Henry VI, Part III, 2.1

Q: Why does Monty keep looking round everywhere when he is on his perch?

Click for larger

A: He has to. Monty's eyes are very large in proportion to his skull – by volume, they take up about a third of his entire head. Because of this, Monty's eyes cannot move in their sockets the way ours do. In order to watch what is going on around him, he has to make constant head movements, both laterally and in elevation.

Monty can use his eyes in two different modes: he can use both eyes together, looking forwards (“stereopsis”) which helps him to estimate the distance and size of objects at short to medium ranges. He also has a second “fovea” (the dense layer of photoreceptor cells at the back of his eye) which comes into operation when he looks SIDEWAYS, out of one eye. This mode works best for his longer range high-acuity vision, to distinguish small or distant objects.

It is difficult for us humans to imagine what a picture of two different scenes with eyes pointed in different directions would look like to a bird, but there is some evidence* (from functional MRI scanning) that their brains combine the images to produce a coherent overall view of the landscape.



[*- Tame parrots appear to like MRI scanners and will happily sit inside one while it works. It may be that the humming noise emitted by the machine interests them.]

WHERE DID ELIN COME FROM?

A load of guess work (and some facts) from Wlw.

A female osprey landed on the nest at Cors Dyfi on the 5^th April. All the experts agreed that she was a little hottie and well worth the watching.  Commemorating the birth of a daughter to one of the DOP volunteers, they named both newcomers “Elin”. She (the bird, not the child) hung around the site for the next 48 hours but, finding an apparently deserted nest and no male bird in sight, departed.

Monty, with appalling timing, arrived back from his migration the very next day.

Image (c) Dyfi Osprey project 2013
Click for larger

I did not expect to see her again. But I was wrong because on 19^th April, the osprey with the film-star looks reappeared. Where had she been? Prospecting for a nest round the rest of the country? Or perhaps lounging on the seafront at Aberdovey, nibbling daintily at a morsel of sardine (low-calorie of course) held between immaculately-manicured toes. At any rate, lots of questions have been asked about her origins...

Where did Elin come from?

Of ospreys hatched in Great Britain, only about 1/3 to ½ are ringed. Most of the ones from monitored nests in England and Wales ARE ringed, but some nests in Scotland are in remote positions, and/or on private land to which bird ringers have no access. It's unlikely that Elin was fledged from an English or Welsh nest, more likely that she was hatched in Scotland. But there is another possibility – Scandinavia.

There are plenty of osprey nests in Norway, Sweden, Finland and the Baltic States. Their “normal” route homeward from migration would be through eastern Europe, but conditions there this season have been difficult. Many lakes and rivers were frozen (and still are), and a steady easterly airstream during March and early April could have sent many of those birds off-course towards our shores. Normally, this is not a problem for them – the prevailing winds offer a “short-cut” home from Britain over the North Sea. But until this week, those westerlies have been conspicuous by their absence.

Great circle path between Haverford West and Trondheim
Click for larger version

Ask anyone in southern or central England where Norway is, and they will point vaguely towards the East – but that isn't really the relative position of the countries. On a map, yes – but the Earth is a globe and migrating birds follow a path known as a “great circle route” from one point to another. This map plots the great circle route from west Wales to eastern Norway, and shows how displaced birds trying to get “home” from there would start off by flying in a northerly direction.

I mentioned the film-star looks and certainly there is a touch of the Scarlett Johanssons about Elin. (Well, maybe not but I wanted to include a pic of SJ in skimpy nightwear – and so would you if you could think of an excuse as convoluted as this one! And anyway, despite the surname she's no more Swedish than I am.) But I digress... Female ospreys are not as attached to their region of origin as males, and will check out potential mates and nest sites wherever they encounter them.

It looks like Elin is doing exactly that.


How old is Elin?

It's very difficult to tell the age of an osprey, just by observation.  They don't have any overt physical characteristics that give it away.  The best clues may come from behaviour and demeanour, coupled with a judicious amount of deduction.

The consenus seems to be that she's a mature female, but still fairly young - perhaps three or four years old.

An attached female (one with a mate and an established nest somewhere) when returning from migration, might be expected to head straight for "home".  Elin, on the other hand, has been taking in the sights for several weeks - it doesn't look like she's in any hurry to get anywhere.  This indicates that she is unattached and fancy-free, and could probably be persuaded to pose in a leopard-print leotard if the deal was right.

MIGRATION WEATHER UPDATE #4

A change at last


It's been a long time coming but the functional changeover from Winter to Spring is now at hand.

The six-week dominance of "blocking" high pressure systems over the North Sea and Scandinavia is subsiding.  Inbound Atlantic depressions have been deflected around the British isles during this period, but no longer.

(Click for larger image)

This composite wind speed and direction chart is the forecast for tomorrow (Wed 10.4.2013 at 07:00utc.)   The cold dry easterly winds we have known since it seems like forever are about to be replaced by a more familiar south and south-west airstream.  Temperatures are set to rise - slowly at first - and there will be more in the way of rain.

This picture will continue changeable for the next two weeks or so, with perhaps a drier and sunnier last week of April, though it's more difficult to look that far ahead.

These southerly winds should help the remaining migrants - especially smaller birds and those that fly at low levels.  Those already in the British Isles and struggling northwards toward Scotland will get less help, and the snowfalls there are not over yet.

The Osprey Acuity Myth

  Bird vision (3)

Have you ever read or been told, something like this... ?

"An osprey's daylight distance vision is six times sharper than a human's, under the same conditions."

For some reason, this is one of the most prevalent osprey myths. Writers of bird books, bloggers and broadcasters alike, all repeat it and seemingly without bothering to check the facts. And it's not just ospreys: we are told that buzzards can “see a mouse at a mile” or “an ant from the top of a ten-story building.” Eagles are supposed to be able to recognise other eagles as individuals from fifty miles away, unhindered by distance. (And unhindered, presumably, by not being able to see each other because of the curvature of the Earth!)

Setting aside the question of why a buzzard would even be interested in an ant in the first place, how can birds of prey achieve these astonishing feats of visual acuity?    The answer is simple...

They can't.

The popular comparison between bird vision and our own overlooks two very important factors. One is the laws of Physics, which I'll come back to in a moment. The other is that our human visual system is, in fact, phenomenally good – better than any other primate and (in daylight) inherently superior to most other land-based mammals.

Anatomical model of a human eye, with SR
muscle arrowed. (Click for larger image)

We take our eyesight for granted, without appreciating some of the features that make it so awesome. (At least, we do until we get older and it doesn’t function quite so awesomely as it once did.) Humans can see in three colours, whereas most mammals see only two. We can obtain independent images from both eyes but, unlike birds, our eyes constantly (and completely automatically) scan the entire field of view. This saccadian movement as it is known, is powered by special extra-ocular muscles which can move each eye from any direction in the field to any other, in about 90 milliseconds. Processed by the visual cortex in our oversized brains, this gives us the effect of a full-colour high-resolution view over 120 degrees of visual field. It's believed that the amount of neural capacity dedicated to this is so great that only ourselves, the other large apes, and some whales and dolphins have large enough brains to manage it.

But back to the physics.

The exaggerated claims that large raptors can see 5, 10, 15, or even 20 times better than humans seem to be based on two different discoveries. One is that birds have more densely-packed retinal cells than we have and (in some species) many more of them. The other version of the myth cites the fact that birds' eyes are much larger, in proportion to their heads, than our own.

And therein lies the error because, when considering the absolute resolving power of an eye, neither of these things matters worth a hoot.

Closeup of bird eye. (Red-tailed hawk)

Denser photoreceptors on the retina only get a chance to improve visual acuity if the light focused on them is accurately positioned. Such accuracy improves with larger apertures – which is why sports photographers use large (and very costly) telephoto lenses in their work. In animal eyes, the aperture is formed by the pupil (the transparent centre of the lens) and, for long-distance vision, the limiting factor is diffraction. Any optical system has a critical value beyond which diffractive errors will compromise the sharpness of the image formed. This is known as the Rayleigh Criterion. (For ease of reading I've left out the physics formulae here, but you can easily find them online if you want them.)

A human eye has a variable pupil diameter in the range 2.5mm to 5mm. That's about the same as an osprey or European buzzard, but smaller than the golden eagle (up to 6.5mm). However, human eyes never reach the problem area of the diffraction limit because our spherical eyes have a focal length of 17mm. Birds of prey eyes have a kind of tubular shape in cross-section, giving them a longer focal length. In effect, raptors have magnifying eyes. (But with a reduced field of view.) In large species, these eyes are probably running right up against the diffraction limit, with a resolving power equal to approx 0.3 minutes of arc.

But that's IT. The only way to get improved visual performance beyond this is to have larger eyes. If an osprey really DID have six times better distance vision than a human, its eyes would have to be the size of tennis balls – which clearly they aren't.

It's time to knock this one on the head: Sparrowhawks and falcons have similar distance vision capabilities to ourselves; ospreys and buzzards about twice as good; eagles from 2 to 2.5x for the largest species.

Has this been tested? Yes it has...

“An Eagle's Eye: Quality of the Retinal Image” R. Schlaer, Science,Vol. 176, No.4037 (May 26, 1972), pp. 920-922 http://fliphtml5.com/mrxz/asbi

A technical but still quite readable article, which enlarges on some of the points mentioned here.