These look like growths (on downy birch I think), so I am guessing they are galls caused by some leaf insect invader?
No interactions present.
Little doubt this is a gall, but I have failed to find out which it is using the books I have. Are you certain it was on birch?
University of Edinburgh and Biodiversity Observatory (OU)
Not certain at all really. There was lots of ash and hazel ... neither of which fits. However I have been reading up about the woods and apparently there is also small leaved elm present. I've check the leaf shape for this and it does appear to be possible looking at the photo.... Does that help with the ID?
I think it might be a gall on elm. There is a German website with pictures of elm leaf galls here: http://www.pflanzengallen.de/verzeichnisundfotos-u-v-w-x-y-z.html
Looking through the pictures, there is some resemblance between your picture and a gall on small leaved elm caused by a mite called Aceria ulmicola, but this is just a wild guess really.
That looks like a pretty good match. Thanks.
Difficult to be sure about this without being certain of the host (at least to genus, elms difficult to separate into species).
A couple of points.
1. Aceria ulmicola is now thought to be an inquiline, its supposed galls are now attributed to Aceria campestricola. This may or not be that species, but the red colour makes me wary.
2. The reference given by Jonathan above looks very much the same, but those galls are caused by an aphid, Tetraneura ulmi and it is a little early for those to be so prominent. June / July is their main time.
One of the big differences between the two galls mentioned is size. Campestricola is 1 - 2mm across, ulmi is 7 - 15mm tall. Also campestricola has a hairy opening on the underside of the leaf.
Thanks, Peter, for your expert opinion. Very good to have you on iSpot.
I'll try to get back to check the host.
Lat/Lng: 52.1, -0.1
OS grid ref: TL3453