How to deal with the nightmare of birds getting in the trap!

The problem started a couple of weeks ago, when a bird (presumably a tit) found its way into my trap, totally decimating the catch. Over the following nights I tried to trap on the off chance that it was a one off event, but the bird (or birds) hit the trap again. I had two opportunities open to me - either turn the trap off before I went to bed or get up at the crack of dawn and cover the trap, but as I trap every night, the second option wasn't really going to work long term, so I decided to turn it off before I went to bed. This of course reduced my catch considerably. I did this for about a week then tried to leave it on all night again, hoping the bird had gone, but the same thing happened and the bird attacked the catch. So I needed to come up with a more permanent solution to resolve this issue. So what I’ve done is to reduce the size of the apertures in the top of my Robinson trap with a piece of plywood (see photo below) so that the smallest tit won’t be able to get in. I did this about a week ago ago and so far it’s been successful with keeping the bird out. 
Since then I've also been able to assess whether this reduction in the entrance holes has reduced my catch of larger species and it doesn't seem to have done so, as I've had four species of Hawk-moths, a Puss Moth, a Fox Moth and various other large moths, so it seems that the trap alterations, up till now, have been totally successful.

You can see the ply-wood inner collar to keep the birds out

Peter.