Migration is a behavioural strategy that species use to track seasonal variation in climate and resources. A common form of migration is elevational migration, which is a short-distance movement undertaken by mountain birds, typically between high-elevation breeding grounds and low-elevation non-breeding grounds. Despite seasonal elevational migration being a dominant life history strategy in montane birds on every continent, little is known about migratory patterns and drivers. The majority of existing research comes from the Neotropics, where a handful of studies have shown that migrants are frugivores or nectarivores tracking peak fruiting and flowering seasons across the elevational gradient. However, outside of this region, like in the Himalaya, most elevational migrants (~70%) are insectivorous. My thesis aims to understand how and why birds migrate in the Himalaya and how resource availability and dietary specialisation can potentially explain elevational migration in Himalayan birds.
In the first chapter, I use a large citizen science dataset (from eBird) to quantify the summer and winter elevational ranges of 377 Himalayan bird species and describe five patterns of elevational migration. I then examine how diet, habitat, territoriality, body mass and wing morphology might best explain these patterns.
In the second chapter, I try 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. This pattern appears to influence bird migration; salliers and terrestrial gleaning insectivores seem to track prey abundance, migrating to different elevations seasonally. In contrast, foliage-gleaning birds are more likely to be residents.
Given patterns in arthropod availability with elevation and season, in the third chapter 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 despite the lack of arthropod resources. Using a combination of faecal DNA metabarcoding and stable isotope analysis, I show that elevational migrants are likely to be dietary specialists that track arthropod resources, while high-elevation residents are dietary generalists that supplement their winter diet with fruit and nectar, likely because of the scarcity of arthropods in winter.
In summary, using a combination of citizen science datasets and field- and lab-based methods, my thesis attempts to improve our understanding of elevational migration and its ecological mechanism in the Himalaya. Understanding the drivers of elevational migration is a crucial first step in predicting how tropical montane avifauna will fare in an increasingly warm world.