Posted: 26/04/2014 at 11:59
Sparrows and blue tits use the same boxes, so yes, that's normal. What you need is, of course, more boxes. North-facing, ten feet up, with plenty of clear space in front of them so the birds can see around before they leave. I'd say they should be somewhere with low traffic, but I've seen house martens nest in pub doorways and blue tits nesting by a farm's main door.
Experiment still not done: RSPB's cheaper treat blocks, RSPB's mroe expensive treat blocks, Garden Bird Supplies' treat blocks, the GC's treat blocks and the local shop's cheap treat blocks, in multiple feeders, rotated every couple of days, to see which ones the birds like most.
To protect blackbird chicks from marauding magpies, I recommend an underlever .177 at 11.5 ft-lb, a fixed magnification scope (if you get a 2x50 you can see into shadows with it), plenty of practice on paper targets at 5, 10, 15, 20, 25, 30 and 35 metres ... and sharp reflexes, because those baby-snatching b*****ds are fast. I still suspect they're psychic. They can tell when you're thinking about aiming at them.