I have a mob of sparrows who eat everything I put in the feeder - as a result I no longer have a blue tit and great tit visiting and haven't seen the robin for a long time. I've had a bullfinch visiting the last couple of days and I'd like to encourage it to stay around. What can I put out that sparrows don't like but finches and tits do, or what can I put out that will distract the sparrows so the others get a chance to feed?