Hi Ace
I haven't got grass snakes in my compost heap but I have a complete extended family of slow worms. They arrived in one of my cheap plastic bins in spring last year so I couldn't move the compost and now there are so many of them they are difficult to count. I have some quite large ones and then ranging in size to tiny ones that are probably no longer than 2inches!
They are now quite used to me and some of the older ones stay long enough for me to stroke them! the youngsters just make a dash for it though. I have made up the compost heap with just a bit more grassy stuff so that it stays warm in there for them.
As it is now getting colder when I open the bin either in the morning or evening there is a complete tangle of bodies! One of the older ones must have been caught by a strimmer and lost some of his length but that has healed and apart from being stubby has survived beautifully.
I wish my son hadn't nicked my camera so I could put a picture of them on here - some are a lovely browny gold and the babies are really shiny bright gold!
