I predict a diet!

This bird box blew down a couple of weeks ago and so I had to “re-install” it and hopefully more securely.
Since then the blue tits have been landing on it and sometimes going into it.
However, I think it looks like a bit of a struggle for them to get through the hole at times, so I wonder if they may have put on a bit of weight over the winter months!
This song is almost the title https://youtu.be/hamKl-su8PE?si=2I89FhKAVVoHQWQV