Written by Tolly Beck
Tolly Beck is the horticulturist at Lasdon Park and Arboretum in Westchester County. She was formerly a horticulture educator for New York Botanical Gardens in the Bronx and Cornell Cooperative Extension of Rockland, NY.
We are coming to the end of another Eastern Bluebird nesting season at Lasdon.
This year I have seen a significant increase in nest failures. The same results were, unfortunately, seen by monitors of bluebird boxes throughout Westchester. Monitors found dead adult Eastern bluebirds sitting on their nests. These adults showed no sign of trauma or predation. As the season progressed, some nests were successful and eggs did hatch. The healthy nestlings were banded, only to be found dead in their nest a week later. Like the adults who had died, they showed no obvious signs of death.
Sandy Morrissey, the Chairperson of the Eastern Bluebird Restoration Project in Westchester, quickly saw that we were facing an ever growing problem. Sandy contacted a lab at Cornell University and asked if they would perform necropsies. They began to accept dead bluebirds that she would send up to their lab in Ithaca. The result of the necropsies just prompted the following Disease Watch from Cornell.
The New York State Wildlife Health Program has received an unusual number of Eastern bluebird mortality cases this summer. Although some of the carcasses have not been in good enough condition for a thorough examination, we have reached a diagnosis in several of them. So far, in the dozen birds we have examined, the most consistent finding is a severe lesion in their intestine. The lesion, an ulcerative or necrotizing enteritis, is severe enough for us to confidently say that it was the cause of the birds’ death. Similar lesions and mortality in Western bluebirds using artificial nest boxes have been reported in the literature. The precise cause of the lesions is not clear. In both the Western cases and our Eastern bluebirds, the lesion is associated with large numbers of bacteria and, in many cases, the presence of thorny-headed parasites attached to the intestinal wall.
The original reports suggest that the parasite, or any other traumatic damage to the inner surface of the intestinal wall, allows for the bacteria to proliferate at higher than normal rates. Stress from breeding may also make the birds more susceptible to developing the lesions. However, both the bacteria and the parasite are not uncommon inhabitants of the intestine of bluebirds and other song birds.
Why mortalities occur sporadically and not yearly, is unclear. At this time, we are unable to make recommendations to control the issue. In an attempt to investigate this further, we are asking people to contact us if they have observed, or collected, dead bluebirds, be it in an artificial nest box or elsewhere. You can email email@example.com to report bluebird mortalities.