In highly biodiverse tropical mountains with thermally specialist biota, natural elevationally-linked temperature gradients strongly determine the distribution of species. In addition to this natural gradient, anthropogenic habitat change creates new abiotic conditions by (a) shifting existing thermal gradients upwards, and (b) through the creation of manmade habitats such as agriculture and degraded forest, which are hotter and climatically more variable than natural forest. Climate change is already causing rapid range shifts of many species to higher elevations, but how species adapt to changes in the abiotic environment because of the interactive effects of climate change and land-use change remains largely unknown. Recent evidence indicates that populations of some species undergo morphological changes over decades in response to warming temperatures. Further, in birds, environmental stressors (including thermal stress) trigger the hypothalamus-pituitary-adrenal axis (HPA axis) leading to the release of corticosterone, the primary stress hormone in birds. While blood corticosterone levels rise transiently in response to stress, feathers sequester corticosterone, providing a long-term picture of stress faced by birds. Prolonged stress also leads to an altered immune system, leading to changes in the composition of the gut, crucial for nutrient assimilation and detoxification. Unless morphological changes or range shifts minimise thermal stress, alterations in corticosterone levels and gut microbiota because of habitat degradation and climate change should affect fitness in birds. For three widely distributed bird species (abundant at all elevations and found in degraded forest; Schoeniparus castaneceps, Actinodura egertoni, and Trochalopteron erythrocephalum)that vary greatly in body size, I aim to understand how natural and man-made temperature gradients affect morphology, stress, the composition of gut microbiota, and survival in Eastern Himalayan montane bird species.