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Birds

Two important things you may not know about them. 1. They are dinosaurs 2. Their lungs are totally unique.

1. Birds are essentially flying dinosaurs

Actually this is rather ‘old hat’ now, and many people do already know about it. But when I first met the claim in February 1998, in Scientific American no less, it was dynamite. A few scientists may however still prefer the more mealy-mouthed “birds are descended from dinosaurs”.

So here’s a photo of ‘the last of the dinosaurs’, in its natural element (in this case wave-riding over the stern of a ferry).

How come?

Birds are unique in the animal kingdom in a number of ways. And all their main unique characteristics have now been found in their ‘theropod’ ancestors, from long before they got anywhere near being able to fly.

Birds’ feathers are the obvious unique characteristic. Feathers are delicate structures, and don’t normally ‘preserve’ in the fossil record. But under certain conditions, even feathers survive. And palaeontologists now know the places to look. So they are finding all sorts of exciting things, including feathered dinosaurs. They still haven’t discovered what feathers were originally for. These days birds use feathers for warmth and for display, as well as for flying. So the favoured hypothesis is one or both of these.

Feathers certainly make wonderful duvets, both for the birds and for us. They are incredibly light, remarkably strong – and can be fluffed out, or not, to suit the need.

As for their aerodynamic properties … it’s difficult to know where to start. With a slight twist of two or three feathers, a bird can achieve effects that make aerodynamicists green with envy.

Birds’ second unique feature is their hollow bones. I always thought that birds had hollow bones to save weight. I was quite wrong, as we’ll see shortly.

Birds are descended from a particular line of dinosaurs, the theropods. These are the ones that run around on two legs. Even they tend to be rather large. But there’s a thoroughly incomprehensible article in Science (7 September 07), which I don’t claim to have read. It says that the theropods that immediately preceded the first primitive birds had shrunk down to a much more reasonable size. It seems that animals do this quite regularly – if they are marooned for a long time on a small island without much food. So maybe birds originally evolved on just such a small island. Later however it must have come up against a larger land-mass. The oxygen level was high at the time, which will have enabled them, together with their dinosaur super-lungs (see below), to get more work out of a given weight of muscle. This will certainly have helped too.

2. Birds’ lungs are ‘reptile lungs with supercharger’

Possibly birds’ main unique feature is their lungs. Not a lot of people seem to know this, but there’s nothing like them anywhere else in the animal kingdom.

I like to describe birds’ lungs as “reptile lungs with supercharger”. This makes sense because birds evolved originally from reptiles. Unfortunately reptilian lungs are small and not very good. I’ve read that you never find reptiles at high altitudes, because they can’t get enough oxygen. But just as a supercharger transformed the aeroplane engine, so it transformed the capabilities of the birds.

When did birds evolve these miraculous lungs? I’ve not seen the matter discussed. Perhaps not until after the K-T extinction that killed off their terrestrial cousins. A key feature of birds’ lungs is that they are powered by the flight muscles. This means that the faster the bird flies, the more air it forces through its lungs. Now the birds’ wings are in fact arms. And it’s difficult to imagine the puny arms of many dinosaurs having muscles that are much good as bellows.

There’s another factor too. Throughout the heyday of the early reptiles, and the dinosaurs, the oxygen level was much higher than it is today (more). So the creatures of the time didn’t need particularly good lungs. The oxygen levels dropped shortly before the K-T event. So when the primitive birds came through the event, and found that they had the sky to themselves, they needed more power to exploit their new niche. But more power means more oxygen, so the pressure will have been on to develop their lungs just as far as they could.

I would like to be able to give you chapter and verse of how the oxygen level varied throughout this period. But I’m not sure that the scientists know themselves. How the amount of oxygen in the atmosphere might have varied, hundreds of millions of years ago, is not an easy thing to pin down.

Be all that as it may, modern birds’ lungs really are unique. They are far more complicated and far more efficient than any other breathing systems on today’s planet. They are also still much smaller than mammalian lungs, thus saving a lot of space and weight.

By contrast mammalian lungs are large, heavy and grossly inefficient. Their active part comprises a myriad of tiny compressible cavities (alveoli), where the ‘gas exchange’ takes place. When we breathe in, our lungs expand to suck a charge of air into all these cavities. Oxygen is absorbed into the blood, and the carbon dioxide is expelled from the blood. This gas exchange happens when the air isn’t moving, which is not the way to do it at all. Then the lungs compress to squeeze the air out again the way it came in. And that’s it. We only extract a small proportion of the oxygen that the air contains. This is why mouth-to-mouth resuscitation works so well.

We don’t know a great deal about how dinosaurs’ lungs worked, for obvious reasons. They are made of tissue even softer than feathers, so it takes very unusual conditions indeed to preserve them in the fossil record. But a few dinosaurs have been found, so well preserved that their lungs can be explored. And I’ve read reports claiming that they really are very similar to birds’ lungs. But whether they had the supercharger I’m not at all sure.

You would think that by now scientists would know how birds’ lungs work. But evidently not, because I’ve only seen two reports and they conflict with each other. So the following is the best I can do. Don’t take it as gospel.

Birds’ lungs are described as ‘4-stroke’, whereas other animals’ lungs (including ours of course) are ‘2-stroke’. Two-stroke enthusiasts will be relieved to hear however that this is not strictly true. Aspects of 2-stroke technology are in there too.

A key feature turns out to be their hollow bones. Recent studies have revealed that there is far more empty space, within both bird and dinosaur bones, than anyone had suspected.

All this empty space allows the air to be forced straight through small tubes in the lungs at high speed. The blood flows in the opposite direction, through its own set of tiny blood vessels. If you want to pass gases from air to blood or vice versa, then you couldn’t come up with a better system. And it’s the key to why birds’ lungs, and clearly also the dinosaurs’ lungs, are so efficient. Birds are able to extract virtually all the oxygen in each charge of air. Beak-to-beak resuscitation would never work with birds.

If birds’ lungs are an example of ‘intelligent’ design, then ours are an example of singularly ‘unintelligent’ design. It’s all the more galling that the so-called designer had such a superb system right there on the drawing board!

The system needs the air to be pumped around the lung system. This is done by means of air sacs, scattered about the place. When the bird is at rest, these air sacs are presumably compressed by specialised muscles, the way ours are. But when the bird takes off the supercharger kicks in, and the flight muscles take over. This means that the faster the bird flies, the more oxygen it takes in. But I’m confused about the part that the rigid bone cavities play.

In more detail, the lungs seem to work somewhat along these lines. On the first stroke (breathe in) the air bypasses the lungs, and ends up in sacs at the back, which expand to suck it through. On the second stroke (breathing ‘out’) the air is forced forwards through the lungs – and this is where the oxygen is extracted from it. However the depleted air isn’t finished with yet. Instead of being breathed out, it is sucked into a second group of sacs at the front. On the third stroke the air is forced backwards again, bypassing the lungs, to yet more sacs. Finally it is blown forwards through the lungs, for the second time, and out through the nose. So the air always passes through these small tubes in the same direction. If I have it right, it’s on this fourth pass of the air that the bird’s waste carbon dioxide is transferred to it.

But then there’s the 2-stroke bit. The third stroke, in addition to dealing with one parcel of air as described above, also takes in the next. As we’ve seen neither goes through the main lungs. The fourth stroke of one parcel is also the second stroke of the next. This implies of course that the lungs must have two separate ‘gas exchange’ routes. Maybe someone will tell us whether this is true.

Other features

There are other features of birds, concerning their arm and leg joints, which they also got from their dinosaur ancestors; and which they have also turned to advantage. The neat way that birds fold their wings is one such. But the above three seem to be the key features that make birds the consummate flyers that they are.

Bats would certainly give their eye teeth for any one of them!

[Click for owls, origin of birds]