Blue Tits will often come to a nest box simply to pick small insects off it. A box that they feel unsuitable for nesting will often be investigated in this way as many of the small insects they feed upon will be found on the wood of the box - sometimes inside it.
A successful nest box needs to meet a few criteria:
It should be out of direct sunlight. A ...
Coming to this late, but this is typical behaviour for the Long-tailed Tit.
These birds are almost always seen in groups and stay very close together. They roost and perch on wires or thin branches, staying very close together.
You talk of a black cap. Long-tailed tits have black markings on their heads but with a white centre.
According to an article in Newscientist there have been surveys of what birds visit bird feeders in the UK since the 1970s and over that time birds that use bird feeders have increased more than birds that do not visit feeders (in urban areas), this shows that bird feeders do have a positive impact on birds that use them.
The article also states that the ...