Nest success can be determined from evidence at or near an empty nest
In studies involving nest monitoring, it is common to observe an empty nest from which a brood could have recently fledged but the fledging event was not observed. In such cases, a series of rules is often followed to make an educated guess about the fate of each empty nest. A couple of primary assumptions are that we can tell the fate of a nest based on the structure of the nest itself and based on the presence of fledglings somewhere near the nest.
In our studies of Ovenbirds and Golden-winged Warblers, we had many nestlings/fledglings and adults marked with radio transmitters. Therefore, we were able to look at a recently emptied nest, use common rules to determine the fate, and then track the birds to see what really happened. We found that nest condition and fledgling presence are not reliable indicators of nest fate in either species. Sometimes nests look perfectly fine and there are fledglings nearby, but the nestlings had been depredated and the fledglings are from a different territory. Sometimes the nest looks destroyed and there are no fledglings nearby, but the family group is alive and well two or three territories away. In Ovenbirds, we incorrectly determined fates as successful or failed in similar numbers, causing little bias in final nest success estimates, but still causing concern in how many times we were wrong. In Golden-winged Warblers, all of the incorrectly determined fates were called successful when they actually failed, causing a strong bias in the nest success estimate from conventional nest fate determination methods.
In general, our research has demonstrated the importance of studying both nests and fledglings for assessing reproductive success in songbirds. But this analysis demonstrated how following birds beyond the nest might be necessary just to obtain accurate estimates of nest success.