Why swan sternums are strange bones

Last week me and dad were on a walk up to the geese lakes to hopefully see the greylag geese that come here over the winter. We didn't see any of the greylags, so on the way back we decided to walk along the Rhynd lochs which I wrote about here. We walked along the very edge of the Upper Rhynd looking for bones, when dad saw a strange bone buried in the ground. It was a type of bone I have seen loads of but I have never seen one this big or as interesting.

When I wrote about the Autumnwatch mystery bone last week that was a sternum of a mammal. This was a type of sternum too, but from a bird. It was so big that could only have come from one bird: a swan.

Bird sternums (or keels) are different to mammal sternums. Mammal sternums are flat, but bird sternums have a ridge. The wing muscles sit on either side, so the ridge works in the same as the sagittal crest does on skulls. If the keel is large, it means the bird has stronger muscles for flying (just like a big sagittal crest means strong jaws). This keel was 21.5cm long and the middle bit is about 4cm high. That doesn't mean much until you compare it with a crow sternum I found today:

Even though the crow is quite a big strong bird, the crow sternum is only 6cm long and the keel is only 1.5cm high.

After I found that, we looked some more and found this other bone from the same bird. It is called the pelvic girdle, but it is really the spine and hips all fused into one bone. This one is 23cm long.

I don't have a picture of a swan skeleton but here is a picture of a pink-footed goose I found last year, showing most of the bones. The big arrow shows the sternum and the little arrow shows the pelvic girdle.

The thing was strange about the swan sternum was that it was hollow inside.  

On smaller bird it is just a  thin piece of bone. It was about 7cm deep and it had these thin rings of bone inside. The thin pieces of bone would have been around the windpipe, which had rotted away, but because the rings had remained, you could see where the windpipe used to be. The rings were about 1.5cm wide.

The windpipe, which is called the trachea, looped round inside the keel to go in and back out again. This page shows a bit more how that happens for different swans.

To clean the bone I had to take out the rings. Some were still stuck together, and others were loose. They were covered in dirt and what looked like plant roots.

These were all the rings that were inside.

Last year Paolo had a mystery object on his website that was just like this. When I saw it I knew it was a keel, and that it had be a big bird so I guessed Whooper swan. (It turned out to be a Whistling Swan). The unusual thing about that one was like this one, that the windpipe went inside the hollow sternum to  make a louder noise, so the sound echoed in the empty space of the sternum.

The only swans I have seen at the Rhynd lochs have been mute swans, which are quite quiet birds. I'm not sure if this is from a mute swan, or a different type of swan but it's a cool bone to have anyway.

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