Any idea what this is?

Found all over some small silver birch trees, while we were rambling about the countryside with assorted small children recently.

They come in clusters, too:

My first thought was, ah – it’s a gall. The leaves are of course obviously somewhat chewed. But when you pick them off they’re rock hard – and they have hollow innards. And besides, birch gall looks like this, which is not at all the same thing.

I was a little sad to find that it wasn’t the rather wonderfully named silk button spangle galls: they look like this:

(the pic is from the compellingly interesting Tree Blog which is following the lives of about 25 trees from seed up, and has an alarmingly comprehensive selection of disease-related pictures to show for it. It actually made me wonder how any tree actually makes it to maturity.)

So – any ideas? I’m still with the galls idea, but am intrigued to know what might have caused this and how. They’re very pretty, but clearly not doing the tree any good at all.

Over to you?