Elevational migration is the seasonal movement of organisms between higher-elevation breeding grounds and lower-elevation wintering grounds. Amongst montane bird species, elevational migration is a common phenomenon, with nearly 70% of high elevation species in Himalayas showing this behaviour. Yet, most bird migration studies focus on long-distance latitudinal migration and most of our current understanding of elevational migration and it’s drivers come from studies from neotropical mountains where most migrants are frugivorous, unlike the mostly insectivorous migrants of the rest of the world. I aim to understand how birds migrate in the Himalayas and how resource availability and dietary specialisation can explain elevational migration in Himalayan Birds.
In the first chapter I use a large citizen science dataset (eBird) to determine the sumer and winter elevational ranges of 377 Himalayan bird species and describe five elevational migration patterns. I then examine how diet, habitat, territoriality, mass and wing morphology can best explain these patterns.
In the second chapter I attempted to understand how arthropod availability varies seasonally along the Himalayan elevation gradient. Aerial and terrestrial arthropods decline with elevation in the winter but increase with elevation in the summer while foliage arthropods show a similar mid-elevation peak in abundance across seasons. I then correlate these patterns with the abundances of different functional guilds of birds to determine of food availability may drive elevational migration.
In the third chapter, using a combination of faecal DNA metabarcoding and stable isotope analysis, I examine whether dietary specialisation can explain why some high elevation breeding birds migrate to lower elevations in winter (where arthropods abundances do not fluctuate) while others overwinter at high elevations winter despite the lack of arthropod resources.
In summary, using a combination of citizen science datasets, field and lab-based methods, my thesis attempts to improve our understanding of elevational migration in the Himalayas and the drivers of this movement. Understanding how temperature, seasonality and diet drive altitudinal migration will be a crucial first step in predicting how tropical montane avifauna will fare in an increasingly warm world.