These are Silk button galls and were found down by the Mire Loch. They are fairly common in the UK and are found on the underside of Oak leaves.
The Silk Button Gall Wasp Neuroterus numismalis has two generations each year. The galls shown here contain the agamic (asexual) generation. The wasps develop inside the gall and fall to the ground with the leaves. They then overwinter on the ground and the females emerge in early spring and lay their eggs in oak leaf buds, forming a Blister Gall. The Blister galls then produce the males and females of the bisexual generation.
Hope that entertained you for a minute! Promise the next post will be more bird related.