Kendra they say greater than 10 larva per bird can be deadly but 10 or less per bird usually they are fine. So with five babies they can safely have around 50 or less blowfly larva in the nest. I did loose some babies one year to blowflies, there were only three babies. I discovered two dead on day 6 or 7 so there was only one baby left. I had to poke around alot as I had a top opening box but I did see larva so I did a nest change. When I cleaned the nest out there were well over 60 larva in that nest and they were all midsized by that time, they start out tiny black spots. The remaining baby would have surely died the next night if I hadn't changed the nest. She made it though and went on to fledge and be quite the character in our yard that year. I try to remember to just do a visual check on the nests when I open the boxes now but it is easy to forget especially in cold weather. We don't usually have a problem until the second nesting around here though. If there are five babies in the nest there probably won't be problems with blowflies but the fewer nestlings the greater the chance of having problems. I did buy a pankcake turner that fits into the boxes to lift the nests up if I need to though to check for larva, if you are going to do this though do it when the babies are too young to prematurely fledge, not close to day 13 cause lifting the nest like that could scare the babies.

Donna

Thank you so much for your advice Donna. I do have a spatula that I keep for checking under nests, but the birds that have nested in my boxes have always done so later in the season. I haven't seen much of anything flying, so I thought it was safe. There looked to be about 20 larva in this nest, so my understanding is I should have left it alone anyway. I'm trying very hard to do things the right way so I do more good than harm. I really appreciate your explanation.