Migration is a behavioural strategy that species use to deal with 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. Although common in mountain birds, little is known about the potential drivers of elevational migration. By studying Himalayan birds, my thesis aims to understand how and why birds migrate elevationally and how resource availability and dietary breadth can potentially explain elevational migration in 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 describe how diet, habitat, territoriality, and body mass might best explain these patterns.
In the second chapter, I examine how arthropod prey availability for birds varies seasonally along the Himalayan elevation gradient. Aerial (eaten by salliers) and terrestrial arthropods (eaten by terrestrial gleaners) decline in abundance with increasing elevation in the winter but increase with elevation in the summer. Whereas the abundance of foliage arthropods (eaten by foliage gleaners) declines with elevation in both seasons. The abundance of avian foraging guilds and their specialised arthropod prey corresponded closely. The relative abundance of sallying and terrestrial gleaning insectivores also increases with elevation in summer and declines in winter; seasonal movements of these species therefore correlate with fluctuations in prey abundance. In contrast, foliage-gleaning birds are more likely to be residents, showing little change in abundance across seasons and elevations. These results point towards potential relationships between food availability and elevational migration in different kinds of insectivores.
Given patterns in arthropod availability with elevation and season, in the third chapter, I examine whether dietary breadth can explain why some high elevation breeding birds migrate to lower elevations in winter (where arthropod abundances do not fluctuate greatly) while others overwinter at high elevations despite the apparent lack of arthropod resources. Using a combination of faecal DNA metabarcoding and stable isotope analysis, I show that high elevation residents have a lower trophic position in the winter possibly due to a decline in the consumption of arthropods and the supplementation of their diets with fruit and nectar. Elevation migrants on the other hand have a similar trophic position across seasons by maintaining a largely consistent arthropod diet as they migrate elevationally. Whereas these data cannot directly test for food limitation as a causal mechanism, the observed dietary consistency in migrants, contrasted with the shifts seen in residents, is consistent with the hypothesis that seasonal fluctuations in food availability may be an important predictor of elevational movement in these species.
In summary, this thesis uses a combination of citizen science datasets, field- and lab-based methods to describe migratory patterns and processes at multiple scales, ranging from an entire mountain range to a single elevational gradient in the eastern Himalayas.