Supplemental feeding of birds can influence pathogen dynamics by altering both the behavior and physiology of hosts. Bird feeders bring high densities of hosts into close proximity and, for some pathogens, provide discrete environmental surfaces for transmission. Feeders can also alter immune responses of hosts by acting as “vaccinators”, exposing hosts to repeated low doses of pathogen. We present a combination of field and experimental work exploring the role of bird feeders on both behavioral and physiological components of disease dynamics for house finches and the bacterium Mycoplasma gallisepticum, the causative agent of Mycoplasmal conjunctivitis.
First, using a combination of field and laboratory study, we found that birds that spend the most time on feeders are more likely to both acquire and spread Mycoplasma gallisepticum. Second, we manipulated feeder density across captive flocks and showed that higher feeder densities led to higher rates of transmission, but did not appear to influence either host tolerance or the strength of anti-Mycoplasma gallisepticum antibody titers. Finally, we performed a series of experiments mimicking repeated environmental exposure to low doses of Mycoplasma gallisepticum at feeders and found that our proxy for repeated feeder exposure can result in complete immunological protection to a high dose of pathogen. These results indicate that, in some contexts, bird feeders may act as “immunizers” rather than infectious fomites. However, exposure to low doses resulted in only partial immune protection for many hosts, which we show can result in selection for increasing pathogen virulence in our system. Overall, our results indicate that anthropogenic feeding is likely a strong driver of disease dynamics in this system and potentially for songbirds more broadly, with critical implications for both pathogen ecology and evolution.