I have a Blackcap in my garden who will not let up on tapping the window in the morning ,anyone know why ?

Its been 3 months now , i live in Catalonia , Spain , and this bird persists in tapping continually on the window in my living room , i�m not looking for a solution but just a reason why , it�s not for his reflection, this i know , i know its the same bird each time , he hangs around the garden all day , i open the window and he stays away , but as soon as it is closed he�s back again , and its only in the mornings from first light to mid morning , mind boggling !