This dataset includes stable carbon and nitrogen isotope values for the inner tail retrices of three subspecies of barn swallow (Hirundo rustica rustica, H. r. tytleri, and H. r. gutturalis). Sampling was conducted in Russia (summer 2013), China (summer 2014), Mongolia (summer 2014), Japan (summer 2014), and western China (summer 2015) using mist nets. Stable carbon and nitrogen isotope data are useful proxies for diet and at broad spatial scales have the potential to constrain geographic distributions at the sub-population level. These data were used in tandem with molecular genetics to explore the extent to which a migratory divide explains the strength of reproductive isolation. There are two files in this dataset: 1) A comma delimitated machine-readable file (.csv) that represents the data dictionary, and 2) a comma delimited machine-readable file (.csv) containing the data.