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?

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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
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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
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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.

ARE BIRDS TIRED AFTER MIGRATION?

Avian Physiology (1)

Well, yes – obviously they are. They have flown a long way to get here and it would be astonishing if this journey had no effect on them. But the effects are not quite the same as an equivalent exertion would have on us mammals...

When “Lucky” the kestrel (pictured) turned up on a sailing yacht in the middle of the North Sea, he was 25 miles from land and exhausted. He had to rest up on the boat while his rescuers, Carol and Max Raffe, completed their passage from Holland to the Suffolk coast. On arrival there, he was able to fly off back to land.

UK-based kestrels don't migrate, (probably just as well, if they get pooped after only 25 miles) but other birds do. Samples taken from recently-arrived long distance migrants show that their bodies contain stress-related hormones such as cortisol, and also various circulation products that are associated with protein breakdown. The primary source of these is – as you would expect – the main flight muscles. Fibres in these pectoralis muscles suffer progressive damage during prolonged flights and this has to be sorted out, sooner or later. In mammals, it would normally be sooner but birds have an alternative strategy: their bodies can accommodate the damaged tissue and postpone dealing with it until later.

I call these alternatives the “repair” or “rebuild” strategies.

YA being rescued at Blagon Lake.
(Click for larger image)

While this article was being written – in fact I had written it up to the previous sentence – an incident occurred at Blagdon Reservoir, Somerset on 29/3/13. An osprey with ring marker “White YA” (which had been reported in the area for several days) was found at the waterside, entangled in some kind of barrier netting, and had to be freed. According to the account of this rescue... “Dave checked the captured bird over and we were surprised to find that its breast bone was quite prominent and not in the best condition, though he seemed fine otherwise.” An even luckier bird than the kestrel, then, but was this individual really starving and underweight? I don't think so...

I believe that YA had been forced to stage his migration at Blagdon - probably because of adverse weather - and that his metabolism had switched over to the post-migration “rebuild” strategy.

At the cellular level, all animals suffer tissue damage after prolonged physical exertion. This is a perfectly natural process, and one that human marathon runners and other distance athletes are painfully familiar with. But a human runner, or a caribou or a wildebeest, can just stop running for while. As soon as rest is taken, the mammalian body begins to repair damaged muscle fibres.

Flight muscle: cellular arrangement

Birds on migration cannot simply stop flying. (They tend to fall out of the sky if they do.) An overnight roost is not sufficient time to heal up without permanent scar tissue being formed. So birds just keep going, until they either reach their destination or find somewhere to make a longer intermediate stop – ideally, somewhere with a good food supply. This behaviour is what we see as “staging.”

Once the bird has stopped migrating OR staged, it's physiology goes into a new mode: damaged flight muscle cells are not just repaired – they are dismantled by immune system processes and new tissue is built up. This is a great mechanism for birds, but with one drawback...

It takes time to complete.

During this phase, birds can still fly but they rarely undertake the long journey legs associated with migration. Some species (though not ospreys) stop flying altogether and do a complete feather replacement moult as well.

So white YA and all the other ospreys will be fine after migration. Tired, yes – but they will come out of it with what amounts to brand new (or at least, factory-reconditioned) flight engines for next season!

Sources:-

“A sport-physiological perspective on bird migration: evidence for flight-induced muscle damage”; Guglielmo CG, Piersma T, Williams TD ; J Exp Biol. 2001 Aug; 204(Pt 15):2683-90

“Empirical evidence for differential organ reductions during trans-oceanic bird flight”; Battley P.F. Et al; Proc Biol Sci. 2000 Jan 22;267(1439):191-5.

“Seasonal degenerative, reparative and regenerative ultrastructural changes in the breast muscle of the migratory Canada goose”; George, J. C., John, T. M. and Minhas, K. J. ; Cytobios 1987 52,109–126.