This year i've had a pair of great tits in one of my bird boxes. The female had layed 7 eggs and 6 of the eggs hatched and everything seemed fine. One of the chicks disappeared not long after they hatched, not sure what happened to it but the others had been coming along well with the feathers grown and eyes opened and moving around the box. Yesterday there was 5 chicks but now there is 3, 2 big chicks and one looks dead at the bottom of the nest. Any ideas as i didn't think they were ready to fledge yet.