Vertebrates confronted with challenging environments often experience an increase in circulating glucocorticoids, which result in morphological, physiological, and behavioral changes that promote survival. However, chronically elevated glucocorticoids can suppress immunity, which may increase susceptibility to disease. Since the introduction of avian malaria to Hawaii a century ago, low elevation populations of Hawaii Amakihi (Chlorodrepanis virens) have undergone strong selection by avian malaria and evolved increased resilience (the ability to recover from infection), while populations at...