Our pair of coal tits have been hyper busy in building their ‘home’ in the nest box , but suddenly all activity ceased and we found all of the lovely nest on the ground , and an empty box with no sign of any shells, what has occurred ?

Sounds as if the nest could have been raided by a predator or even another Coal Tit who wanted to take it over but, as Coal Tits nest a bit earlier than other tits, any youngsters who were in the box could have safely fledged ... lack of debris like shells means that the parent birds were diligent about keeping the nest clean!