Alder Bark "nodules"

Alder Bark

Observation - Alder Bark "nodules". Description: Several nodules found on the bark of an alder. Some had bore holes in them and one was broken showing that it was hollow inside. Are these natural features of an alder or are they galls? If they are galls what may have caused them? (I have placed this und

Several nodules found on the bark of an alder. Some had bore holes in them and one was broken showing that it was hollow inside. Are these natural features of an alder or are they galls? If they are galls what may have caused them? (I have placed this under invertebrates due to the bore holes.)