Stable isotope analysis of migratory songbirds along the East Asian-Australasian Flyway.

Abstract

The East Asian-Australasian Flyway supports the highest number and diversity of migratory species than any other flyway, yet we know very little about habitat use and population connectivity along this important migration route. Studies of this flyway are particularly urgent as habitat in key stopover and wintering sites are under great threat from development and other human activities. Stable isotopes, particularly hydrogen isotopes which vary predictably across some continents, have successfully been used to study migratory patterns in Europe and North America. Almost no studies have applied these techniques to birds migrating along the East Asian flyway, in part due to the complexity of the hydrogen isoscape in China. We conducted a study to determine whether C/N isotopes could be used to provide information about migratory connectivity. Mai Po Marshes Nature Reserve in Hong Kong is a major stopover and wintering site for thousands of migratory water birds and songbirds. For some species, both resident and migratory populations are reported to come into contact in the reserve during winter. If diets differ between migratory and resident populations, then C/N rations should differ between them. We measured C/N ratios of two species in which resident and migratory populations intermix at Mai Po in winter (Chinese Bulbul Pycnonotus sinensis and Japanese White-eye Zosterops japonica). In both species, cluster analysis revealed two distinct groups, one with significantly higher δN than the other. These two groups could potentially represent migratory and resident populations. We hypothesize that the group with high δN values is likely to be resident due to elevated anthropogenic N inputs to the site. Alternatively, these two groups could represent resident birds with distinct trophic niches. Further analyses including additional isotopes and genetic markers will help us to differentiate between these two competing hypotheses.