If there are less than 10 blowfly larva per nestling you don’t need to do anything. Lifting the nest on day 13 should NOT be done, I speak from experience. Typically I lift the nest on day 6 and brush out the bottom of larva. I once forgot and did it on day 13 and one nestling fledged and could not fly at all! It actually spent the night in the garage without my knowledge! The next morning (day 14) I put it back in the box and then it and another fledged and neither could fly. Put them back in the box and put a hole reducer on the box. It really was an awful experience since I caused it by lifting that nest on day 13. I took the reducer off on day 15 and they all fledged, the one could still not fly a lick but I let it be since the others got to the trees. It did survive thankfully. However I would never mess with anything in the box from day 13 and on unless you have dead or dieing nestlings. (I did loose two nestlings once to blowfly larva but it was due to having around 60 larva in a box with three nestlings, one made it since I discovered the two dead and did nest change.) My nest this year I hadn’t done the brushing out of the box and when they fledged on day 17 there were probably 20ish larva in there, they all fledged and flew just fine though.
Donna in WI