achmelvich- I'm not familiar with the European Green Woodpecker, so I went looking for an answer to your question. I found the following:
"...the Green Woodpecker's circular nest-hole is the biggest of all the British woodpecker species and, usually, a fresh hole is bored out each year. Fresh wood chips below a tree give a clue, as well as pecking sounds... late March or April are the times to be on the alert. The nest height varies, but is usually a few metres up in the selected tree trunk. Oak is a favourite, commonly with some rot inside the trunk... Clearly Green Woodpeckers are expert at assessing the degree of rot in an affected tree; Great Spotted Woodpeckers seem to select tree trunks in far greater states of disease and decay. " (From http://www.wildlife-sound.org/journal/2005/grnwood.html"