Premature fledging is when baby birds leave a nest under their own power, accidentally, earlier than expected. Premature fledgers that are unable to fly are much more susceptible to predation and bad weather.
Typically monitors stop checking a bluebird nestbox 12-13 days (remember unlucky "13") after bluebirds hatch, to avoid premature fledging. That is one reason why it's important to keep good records, so you are able to calculate the age of nestlings. Note that some species like House Wrens are more prone to premature fledging when disturbed late in the nesting cycle.
If you suspect problems (parents not observed feeding or removing fecal sacs, or seem very agitated, unexpected delay in fledging, horrifically unusual weather, etc.) it might make sense to carefully check inside the box even though it is after day 13.
Nestings MAY stay put if experienced monitors are careful, the nestlings are used to checks, the box is opened slowly and quietly, and the check is brief. Premature fledging from a top opening box is less likely, especially if the entrance is covered while checking. Last year I photographed in a Gilbertson box from egg to empty nest every day up through a late fledging, and never had any fledging attempts or even an indication of agitation on the part of the nestlings. You may be able to see inside without opening the box, or only opening it a crack, using a mechanics inspection mirror and a flashlight.
If bluebird babies do fly or jump out and you are able to catch them (e.g., by throwing a towel on top of them), it is VERY difficult to get them to stay in the box. You can try stuffing a sock/paper towel in the hole for about 15 minutes until they settle down, or putting a 1" hole restrictor over the hole until they are really ready to go. This prevents them from jumping out again, but allows the parents to continue feeding. Be sure to remove it in time to enable appropriate fledging.
There are so many ways to burn time. The glorious way to spend it is to add plants and wildlife that will have ripple effects far beyond our lives.
- Ron Baltrunas, 2005
