House Finches at a Feeder
Like humans, birds can be infected by various diseases, some of which can be quite contagious. Among feeder birds, House Finches are particularly susceptible to Mycoplasmal conjunctivitis. Avian conjunctivitis causes very noticeable changes to a bird's appearance and behavior. Infected birds appear to have swollen or crusty eyes. Becoming lethargic, they succumb more easily to starvation or predation.
A new study examines behaviors that may encourage the spread of conjunctivitis among House Finches. Researchers placed healthy finches into the middle chamber of a partitioned cage with a healthy finch on one side and a visibly diseased finch on the other; all three finches were of the same sex. Food dishes were placed at the edge of each chamber so that the middle finch would have to interact with neighboring finches in order to feed. The researchers then monitored the behavior of the middle finch.
While female House Finches fed equally near healthy or diseased finches, males strongly preferred to feed near a diseased finch. This preference may be explained by the symptoms of conjunctivitis. Since the disease produces lethargy, infected males are much less likely to challenge a healthy male for dominance at a feeding station. Feeding near infected individuals thus reduces the energy and social costs of losing such a confrontation.
The danger, of course, is that feeding and other interactions with infected individuals is likely to increase a healthy bird's chances of catching disease. Mycoplasmal conjunctivitis is a relatively new disease among House Finches, first documented in 1994. In the context of such a new and contagious disease, male finches' instinct to reduce confrontation may work against their ability to resist disease.
Bouwman, K., & Hawley, D. (2010). Sickness behaviour acting as an evolutionary trap? Male house finches preferentially feed near diseased conspecifics Biology Letters DOI: 10.1098/rsbl.2010.0020