The larva of the gall midge Semudobia skuhravae galls the catkins of birch.  The gall is fused to the spindle of the catkin between this and the scale and fruit.  It is up to 2mm long, without a 'window-pit'

Semudobia tarda and Semudobia betulae also form small galls in birch catkins, but in the fruit and not fused to the spindle

Gall Adult

Tease the catkin apart.  It helps to do this under a microscope.

Close-up photographs of the gall inside the catkin. 

Birch trees

Galls can be found in old catkins retained on the tree, or in mature catkins in summer

not known - under-recorded