|MadSci Network: Zoology|
That is one tough question! I've searched on the web, looked through a couple of bird books, and even asked a couple of ornithologists without coming up with an explanation for their "bouncing" behaviour! The American author, John Muir, was particularly captivated by the antics of the ouzels- you can see his description of their behaviour here.
The ornithologists I talked to thought that it probably wasn't for
keeping warm- there are better ways to do that (for instance, the wing
muscles are much larger and would provide a better heat source). A couple
of possibilities for the bouncing behaviour are:
1. The bouncing is to allow them to see better under the water- i.e. because light hits the water at various angles and reflects, by moving they'll be able to avoid reflections and get a clearer view of what's under the surface. If you notice them bouncing more often in the sunshine, that would lend support to this idea
2. The bouncing is to keep balance- If you've ever seen a pigeon walk, you'll notice it moves its head back and forth in a quite pronounced way. The reason pigeons must do that is because of the way their musculature is connected and because of where their legs are positioned on their body. If a pigeon didn't move it's head, it wouldn't be able to walk at all. With the ouzels, their legs are mounted quite far back on their body, which makes them quite unstable when standing on their legs. The bouncing may then be a way for them to maintain their balance.
Sorry I couldn't find a more concrete answer for you!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Zoology.