Eurasian Wryneck Jynx torquilla. There are two species of wryneck and they both differ from 'true' woodpeckers in many respects. For example, wrynecks do not excavate their own nest-holes but rather use those of other species or natural cavities. Photo by Gerard Gorman, Hungary, May 2013.
Most woodpeckers excavate their own holes in trees in which to breed and roost. They are 'primary cavity users'. Many other birds that cannot create such cavities, but need them as nesting sites, depend upon woodpecker holes. Such species are called 'secondary cavity users'. Here is an example: a young Tengmalm's Owl (aka Boreal Owl) looks out from a Black Woodpecker hole. In some cases the distribution and even breeding success of secondary cavity users is linked to woodpeckers. Photo by Thomas Hochebner taken in Austria.