Enlarged hole in my blue tits nesting box

Question

I have had a nesting box that has been used by blue tits for the lat 8 or so years. I clean it out every year and look forward to the return of them. yesterday I noticed that the hole had been enlarged on a verical direction and was about 40 mm high. Today it has been made into a large vertical slot. The total hole is now approx 60mm or more. Has anyone an idea as to what may have made this hole. It looks as though I will not be having any blue tits nesting this year. BOOHOO!

Buzzard · Accepted Answer

Welcome to the forum Schlampy! 
 There could be a number of reasons behind the hole been bigger a woodpecker, squirrel or even a member of the tit family could be likely culprits. 
 I'm involved with at least 5 next box schemes which totals hundreds of nest boxes that I check each year, the main culprit for damaging them is the Grey Squirrel! 
 Fitting a nest box hole plate as MarJus recommends saves time and money, you can easily make your own if you have a sheet of thin metal. Tin or Aluminium works best! 
 Regards Buzzard

Schlampy · Answer

It is in fact a Great spotted woodpecker.I caught him on my peanut feeder then it flew to te said nest box (on which I had put on a new front with a metal plate). It looked, and you could almost see it saying oh! no what has happened. It then flew over to another of my nesting boxes and promptly started to beat the hole in that one. 
 many thanks for all the replys 
 
 Regards 
 Schlampy