To address macroecological hypotheses of resource specialization, parasite systems are perhaps some of the most promising (Stephens et al. 2016).  For example, the growing availability of host-parasite occurrence data has enabled continent-scale investigations of host specificity (Fecchio et al. 2019, Wells et al. 2019), defined as the number or diversity of hosts a parasite can infect (Wells and Clark 2019).  In contrast, host specialization (in the Grinnellian sense) refers to variance in species’ performance across a range of environments (Futuyma and Moreno 1988, Devictor et al. 2010).  Compared to host specificity (Fecchio et al. 2019), macroecological studies of host specialization are scarce, perhaps due to the need for quantitative measures of parasite performance which are generally more difficult to obtain than occurrence data.