Sign: oak apples - fungus-like growths or balls on leaves/acorns Photograph: www.alamy.com
Sign: chewed rotten stumps in oak forests; mulch of leaves Photograph: Yukimasa Hirota/Amana Images RF
Sign: hollow oak, beech and ash trees, near water; fungus on bark Photograph: Les Stocker/Oxford Scientific RM
Sign: gnawed cones; football-sized nest (drey) near top of tree Photograph: www.alamy.com
Sign: woodchips at base of nesting tree; pecking noise Photograph: Paul Mcmullen/Flickr RF