The macroecology and evolution of avian competence for Borrelia burgdorferi

Most efforts to predict novel reservoirs of emerging infectious diseases rely upon information about the infection status of host species rather than their competence, the ability to transmit pathogens to new susceptible hosts or vectors. Although competence can be difficult to quantify, tick-borne pathogens can provide a useful model system, as larval ticks can become infected only when feeding on a competent host during their first bloodmeal. For tick-borne diseases, competence has been best studied for Borrelia burgdorferi sensu lato (Bbsl), the pathogen that causes Lyme borreliosis, an increasing public health priority across the northern hemisphere. Major reservoirs include several small mammal species, but birds may play an underrecognized role in human risk of acquiring this tick-borne pathogen, given their ability to disperse infected ticks across large spatial scales. Here, we provide a global synthesis of the ecological and evolutionary factors that determine the ability of bird species to infect larval ticks with Bbsl. We find strong phylogenetic patterns in competence, which clusters in thrushes and has evolved multiple times across birds. With boosted regression trees, we distinguish competent birds with 76% accuracy and show competent species are more likely to fledge early, have low baseline corticosterone, breed at high latitudes, and have more diverse migratory movements into their breeding range. We use these trait profiles to predict over 100 likely but unsampled Bbsl-competent birds, including novel concentrations of avian reservoirs in the Neotropics and pelagic zones. Beyond expanding our understanding of avian reservoirs for tick-borne pathogens and generating new hypotheses, our findings also emphasize the need to consider competence in efforts to predict novel hosts of emerging diseases and in the study of multi-host pathogens.