What are the birds doing in the tree?

Hello,                                                                                                          In my garden, I have a 5m tall conifer tree. As of recently (within the past month), the blue-tits and dunnocks that visit my garden keep flying up to the conifer and going inside for a couple minutes, only to exit and fly away afterwards. I am not familiar with certain behaviours birds show, but I am wondering why they are doing this. Why would this occur?  This only really happens at midday and dusk. Any answers would be much appreciated. 
Thanks Slight smile