Confusing nestboxes and a magpie 17 May 2009

Tonight I’m a bit depressed with my swifts. They are still confused with the nestboxes. I do wonder if it’s wise to hang 3 identical nestboxes so close to each other. It is the second year I watch the same thing. Tonight they sleep in nestbox 1, tomorrow in 2 or maybe 3 ….. or …… one swift fly in nestbox 1 and the other in nb 3 or 2!
They don’t recognise the nestboxes, all look the same to them. I wonder if it’s too late to mark the nestboxes? I’m going to do it anyway, but the question is, how do I do it? I could paint a red dot on one and a yellow dot on the other one, but do they recognise colours? Maybe it’s best to paint a round dot on one and a square on the other?
I stood outside from 8:30pm and watch them try to enter at 8:50pm, but it didn’t work or it was not yet time and they flew off. They didn’t fly too far, because I had two flying around the house most of the time I watched.
Every night while I stand outside there is a blackbird singing two houses from me. He sits in the gutter and sings his little heart out. Tonight he was silent and I thought he must have found a partner.
Instead there sat a magpie.Since a few weeks magpies are not my favourites, before I just looked at them as beautiful birds, but they were chasing the blackbirds terribly.
At 9:10pm my swifts were back and while one flew in nb 3 it had the magpie immediately behind him. It stayed on the ledge just below the nestboxes for a while. He better not mess with my swifts!