Red Admiral

An Observation Concerning Butterflies.

In my town garden, behind Kirkcaldy High Street, I have a Pittosporum tenuifolium 'Silver Queen'.  It started off as a small bush but is now more of a tree around 25 feet tall.
It doesn't provide any food source for adult butterflies, nor is it a foodplant for their caterpillars.  However, it is a magnet for certain butterflies.

Every day in summer (weather permitting) there are up to a dozen white butterflies resting, basking, courting, mating and just flying round and around the tree.  Most of them are small whites but large whites and sometimes green-veined whites join them.  One other species is also attracted - the red admiral.  

This afternoon, there were four red admirals and two small whites just sitting there.

I don't know whether it is the colour that attracts the whites as it is good camouflage for their closed wings, or whether the tree exudes some chemical/pheromone/attractant.  Its colour wouldn't explain the presence of the red admirals.