Week 48: 'But perfect.'

Wonder spawned in: When the dinosaurs were about
Wondered into being by: No-one really knows
Wonderspan: About 10 minutes


Have you ever found a bird skull on a beach?  Hugh MacDiarmid did and it led him into a short, wondering sort of poem, called Perfect:

• http://books.google.co.uk/books?id=JRBKqbui2EgC&lpg=PA178&ots=NLrxEHiB_5&dq=I%20found%20a%20pigeon's%20skull%20on%20the%20machair&pg=PA178#v=onepage&q&f=true

‘That fixed the tilt of the wings’.  Each ‘th’ in that line reminds me of beating wings, perhaps the long thuh-thuh-thuh of a pink-footed goose coming in to land in Essex at the end of its winter migration from Svalbard in the Arctic Circle.  Or it might be the near-invisible thththth fanning of a ruby-throated hummingbird – barely as long as a thumb and weighing less than a walnut – flying 1,500 feet above the Gulf of Mexico on its solitary autumn journey from the Canadian prairies right to the edge of the Andes.

It’s Monday morning, wonder time, and this week we’re in love with the migrations of birds, ringing the Earth as they do in their millions every spring and autumn.  Apart from the whales and maybe humans, there is probably no animal other than a migratory bird which inhabits so much of the Earth and is so freely able to live beyond its immediate habitat.


Meet the migrants

Some migrations are stunning achievements.  Bar-headed geese will fly non-stop from India to Tibet.  That’s remarkable in its own right, but all the more so when between the two lie the Himalayas!  Not only do the birds need to climb higher in a few hours than any other animal has ever been – up to 29,000 feet – but they must also survive the extreme cold (-50C) and thin air.  The birds have to generate sufficient lift and absorb enough oxygen in air less than half as dense as it is at sea level; in doing so they suffer hypoxia and just seem to fly right through the pain.

• Here’s a flock of bar-headed geese on their migration, joined incongruously by the virtual whispering wing-beats of David Attenborough - well worth seeing: http://www.bbc.co.uk/nature/life/Bar-headed_Goose#p00b2n65

There are two birds with more or less the longest annual migrations.  Many sooty shearwaters migrate from the Falkland Islands, where they don’t care whether they are Argentinian or British, to Norway.  Their migration allows them to experience summer all year round (such as it is in Norway and the Falklands); the long days and short or non-existent nights give them plenty of time to breed.  As the crow flies, the journey is 8,700 miles, but the shearwater isn't a crow; it's more of a global tourist with a round-the-world ticket.  Many shearwaters leave New Zealand for Alaska or Eastern Russia but prefer to cross the Pacific to Chile first, before heading north up the Americas' Western seaboard – this way round, the return trip is some 40,000 miles.

• Here’s a map from National Geographic, generated from trackers attached to a number of shearwaters: http://news.nationalgeographic.co.uk/news/bigphotos/images/060808-bird-migration_big.jpg

The Arctic tern flies even further – over 42,000 miles per year, or nearly twice around the Earth.  It lives almost entirely on the wing, hardly ever landing at all.  They’re just up there, floating on the air, or on the sea, feeding.  Only when they breed – and that's not every year – do they need to make use of the land.

• In this short film, Greenlander Carsten Egevang takes us through the lonely, uncertain, mildly dangerous and rather thankless task of tracking the terns’ migrations. Again, Arctic terns like their migration to take in quite a bit of the globe: http://www.youtube.com/watch?v=bte7MCSBZvo

Bar-tailed godwits have a record of their own, too, in being capable of the longest non-stop migratory flight: the 8,000 miles between Alaska and New Zealand.  Stopping is out of the question, for they have only the Pacific beneath them and they can’t swim, so they double their weight in readiness for the trip so as to store enough fat.

• Here’s a map of the godwits' migration: http://upload.wikimedia.org/wikipedia/commons/f/fe/Bar-tailed_Godwit_migration.jpg

So what does 'fix the tilt of the wings'?

How do these long-range migrants find their way around the world?  Birds need somehow to ‘know’ to migrate and when, to know in which direction to fly in, to know how long to fly for, and to know when to alter their migration pattern when needed.  And they have to do all this with a brain the size of a pea.

Surely, it’s genetic.  We know it is because migratory birds become restless when the time comes to take flight.  Even fledglings which have never migrated before and in experiments are shielded from other influences begin to get agitated as migration time approaches.  This agitation is indeed a genetic trait, but there is nothing genetic that tells a fledgling bird where to fly to, or even in which direction.  For this, in many species a bird’s first migration takes place alongside its parents or flock.  On the way, the new migrant will remember visible landmarks – mountain ranges and coastlines, for example – and even smells, and so learn the way.  The bird is likely to form an attachment to the route and its precise destination for the rest of its life.

So migration is also a learnt behaviour, but this still doesn’t explain how birds find their way when there are no obvious landmarks or when it's cloudy or dark.  What if most of the migration is by night over the sea?  Neither a genetic trigger nor a learnt pattern of behaviour can help with that.  How do they manage?  After all, no airline pilot would set off from New York to London on a cloudy night with only their own eyes to work out where they were.

On the face of it, we might imagine that a bird can just fly in the same direction as it did last year and hope for the best, but if the wind is even slightly different – and it will be – then it will certainly be blown off course and never reach its destination.  To make up for this, birds are able to use the sun and stars as a combination clock and compass.  With the sense of time and direction that this clock-compass gives them, they can tell when to turn, gain height, or look for land. The most remarkable thing of all is that many species of bird can detect the Earth’s magnetic field.  With this information, the bird’s tiny brain can tell, in effect, north from north-east even on a cloudy night over the sea.

The secret appears to lie inside the bird’s skull, right where Hugh MacDiarmid was looking when he wrote his poem.  Here, small amounts of magnetised iron, called magnetite, are concentrated.  Magnetite reacts under the influence of a magnetic field and in birds it is connected to the nervous system.  It’s possible that some birds might even be able to see the Earth’s magnetic field, for it appears to affect the behaviour of certain photoreceptor cells in the eye.  (I imagine sheaves of colour on the sea’s horizon like a permanent, shifting aurora.)  Just in case you don’t believe me, here’s the science: http://www.gps.caltech.edu/~jkirschvink/pdfs/WiltschkoPigeonPulse.pdf  (see pp. 3031-2)

What happens when the magnetic field is disrupted, as it is when there are high solar winds?  Do birds get lost?  Scientists have found that homing pigeons do indeed lose their way during solar storms.  So if one of these whips up a solar storm, we'll not only see the northern lights as far south as Scotland but we'll also know that there are pigeons somewhere having to stop and ask for directions.  If you don’t believe me, ask a Belgian; there are more racing pigeons in Belgium than anywhere else, and that's a fact.  And if you missed it earlier in the year, there is more on the northern lights here: http://www.waysofloving.blogspot.co.uk/2012/04/week-15-i-would-like-to-apologise-to.html

Magnetite is found in many other animals, too: bees, many bacteria, termites, fish, whales, sharks and, wait for it… wait for it… … people!  Unless you’re anaemic, there is magnetite inside the bones of your sinuses.  No-one knows why.  In one experiment, bar magnets were placed on the heads of blindfolded people, which affected their ability to get home.  For anyone with a surfeit of common sense who rightly refuses to take my word for this, here’s the article in the journal Science: http://www.sciencemag.org/content/210/4469/555.full.pdf?sid=471a7280-e207-4bf3-a412-9f7d093d97c5. 


The wonder behind the wonder…

…is perhaps the feather, which gives the bird its flight, warmth, protective coat and colours.  Feathers evolved through the same process as hair did.  Some dinosaurs were heavily feathered to help them keep warm.  Although many feathered dinosaurs couldn't fly, some could, like Archaeopteryx, whose feathers look as sophisticated as a modern bird’s.  Despite these discoveries, perhaps because of them, the development of feathers and then wings for avian flight still puzzles evolutionary biologists (and it's a favourite topic on creationist websites).

It really does seem puzzling.  Even rudimentary flight would require a highly complex anatomical development.  To achieve this, wouldn’t a long succession of genetic mutations be necessary before the change gave the new animal species an advantage in the processes of natural selection?  If so, how could each individual mutation have been selected for?  And how could such an intermediate species survive without usable forelimbs, when presumably it would have been making good use of these before they started to change into something wing-like?  Even if a 'flying' animal were only gliding between trees, it's a big jump (literally) from that to being able to sustain flight - and it's not something that an animal could get away with only half-doing.  So how did evolution pull it off?  Well, I don't know much about evolution, but it is a wonder to realise that the five fused bones in a bird's wing evolved from the same ancestral digits as did a dolphin's flipper, or the digits you're now using to scroll down the screen.

To finish, here is a succession of images that take use ever closer into the feather's structure, right down to the barbules and hooklets which part and remarry themselves again and again through preening (you can zoom into some of these for a closer look).  In the lightness and curve of a feather is the magic of flight.

1. Wings: http://th01.deviantart.net/fs71/PRE/i/2012/156/5/5/bird_wing_by_littlebluestocking-d52d3rf.jpg  Credit: LittleBlueStocking
2. Primary (i.e. flight) feather: http://upload.wikimedia.org/wikipedia/commons/1/10/Eagle_Owl,_Bubo_bubo,_primary_feather.jpg  Credit: Wikipedia (you can zoom in to see the detail better)
3. Rachis and vane: http://upload.wikimedia.org/wikipedia/commons/2/24/Turkey_feather_close_sdetwiler.JPG  Credit: Wikipedia 
4. Barbs: http://www.flickr.com/photos/23663218@N06/3353973920/lightbox  Flickr: j_brittin
5. Barbs closer up: http://www.flickr.com/photos/23663218@N06/2403323613/lightbox Flickr: j_brittin
6. Barbules and hooklets: http://iooe.org/articles/dinosaurs-evolved-to-birds/Scales%20to%20Feathers4.jpg

Have you seen your first migrant bird yet this autumn?  It’s probably not too late – you might still get to watch a winter thrush, fieldfare or redwing arrive from Siberia or Spitsbergen if you’re lucky.


Extra:

Here's a clip from the French film Winged Migration: http://www.youtube.com/watch?v=Q40h8dPmgwQ

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