Personal space.

Birds have personal space too. I often try to get two birds in the shot but they never get really close unless it is a parent feeding a chick. When an older bird approaches a younger one there is usually a bit of an argument then one flies off. Very rarely siblings perch next to each other. A couple of years ago I had about 6 blue tits in a line on a branch but I have never seen that again. This adult goldfinch (flying in) perches next to the goldfinch chick and they sit there happily for a few moments. I can only assume that is because the chick is young. The chick is very young and clumsy.