Monday, 1 April 2013


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!


“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.

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