Parasites with low host specificity (e.g. infecting a large diversity of host species) are of special
interest in disease ecology, as they are likely more capable of circumventing ecological or
evolutionary barriers to infect new hosts than are specialist parasites. Yet for many parasites,
host specificity is not fixed and can vary in response to environmental conditions. Using data on
host associations for avian malaria parasites (Apicomplexa: Haemosporida), we develop a
hierarchical model that quantifies this environmental dependency by partitioning host specificity
variation into region- and parasite-level effects. Parasites were generally phylogenetic host
specialists, infecting phylogenetically clustered subsets of available avian hosts. However, the
magnitude of this specialization varied biogeographically, with parasites exhibiting higher host
specificity in regions with more pronounced rainfall seasonality and wetter dry seasons.
Recognizing the environmental dependency of parasite specialization can provide useful
leverage for improving predictions of infection risk in response to global climate change