Unusually high numbers of dead Eastern bluebirds, Sialia sialis, were reported to the Cornell Wildlife Health Laboratory (CWHL) this past summer. The reports came mostly from people who have monitored nest boxes for years and knew this was a cause for concern. Postmortem examination of several birds revealed the cause of death was severe damage to the inner wall of the intestine: an ulcerative, or necrotizing, enteritis. Looking at data collected though our New York Wildlife Health Surveillance Program, no cases of necrotizing enteritis or high bluebird mortality have been recorded in the last 7 years.
August 11, 2017 - Investigation continues
We don't have a definitive answer to explain why so many bluebirds succumbed to severe enteritis this breeding season. We do have good baseline information, thanks to the people who contacted us and submitted birds for examination. Once the busy summer season is over, we will plan an early response strategy for next year. Equally important, we will discuss possible avenues to investigate what tips the balance and causes mortalities like those we saw this summer.
October 13, 2017
Deaths from necrotizing enteritis with a similar presentation have been reported in Western bluebirds, Sialia mexicana. Originally the disease was thought to be a fatal synergy between a parasite and a bacterium. Traumatic damage to the wall of the intestine by the thorny-headed worm (acanthocephalid) Plagiorhyncus sp, which lives in the intestine of several garden birds, was thought to provide a good environment for the development of a bacterial infection. The combination would cause ulcers (areas of tissue death or necrosis in the wall of the intestine) and eventually kill the bird. The theory doesn't always hold, however. Some of the affected birds, both in the Western reports and in the ones examined by the CWHL, had necrotizing enteritis but did not seem to be carrying any acanthocephalids. Acanthocephalids are also commonly found in healthy bluebirds and other garden species, indicating that their presence is not necessarily linked to disease.
A total of 27 separate reports regarding dead bluebirds were received by the CWHL, from early April to mid-August. In 23 instances, dead bluebirds were submitted for necropsy, and approximately half of them were suitable for a full postmortem examination by a wildlife pathologist. Necrotizing enteritis was confirmed as the cause of death in just over a third of those examined. Similar to what is now known about the disease in Western bluebirds, acanthocephalids were not present in all cases: a third of our Eastern birds with necrotizing enteritis carried no parasites.
It seems clear that this disease affects both Western and Eastern bluebirds, but what triggers it and why it only happens sporadically remain unknown. Please be on the look out for sick or dead bluebirds next Spring, and contact the CWHL (firstname.lastname@example.org) if you see any. The more we look into this mysterious disease the more chances we have to figure out what causes it and whether we can do something to prevent it.