I do ornithology research and intensively monitor about 100 nestboxes. Focal species are Eastern Bluebirds, Tree Swallows, House Wrens, American Kestrels.
Sometimes I’ll find all nestlings dead in a nestbox. That can be the result of an adult caregiver dying due to predation, car strike, etc. It unfortunately just happens.
This year, three boxes of Tree Swallows have been found with mortalities. In one box, all nestlings were found dead and they were checked and were fine about 24 hours earlier. In another box (for American Kestrel), three of five in the clutch were found dead. One of the surviving nestlings in this box had a slight watery discharge of the left eye, but it was minor. The surviving nestlings successfully fledged.
There were no wounds found on any dead nestlings. House Sparrows will sometimes attack and kill birds in boxes, but we aggressively trap and humanely destroy if caught. The number of House Sparrows trapped this year is down about 40% from last year.
In the case of the entire clutch being lost, both male and female Tree Swallows were perched in the nestbox, entered the nestbox, etc. The same for the partial clutch being lost in the American Kestrel nestbox - all adult caregivers (sometimes a second female will assist in removing fecal sacs, etc) were exhibiting a strong challenge response when I approached the nestbox. Adult caregivers will not remain in the area if the nestlings have been predated, etc.
Given that the nestlings had not yet fledged and no nests of stinging or biting insects were found in the nestboxes, nor did any of the nestling mortalities exhibit signs of stinging or biting, I think it’s logical to look at food as a potential vector for the mortalities. Could be something abnormal about the insects, could be there had been recent spraying and affected insects were gathered by the adults as part of their food bolus and fed to nestlings. The mortalities happened in close time proximity to each other.
An additional note, this looks to be a record year for Bluebird nesting attempts, eggs laid, hatched and successfully fledged nestlings, even factoring in early season nesting failures.
Thanks for that insight. Please let us know if you find out more.