Stable isotopes are a pretty hot topic in bird research these days. But what are they, and how are they used? What can isotopes tell us about where and how birds are migrating? This month, we look at what the combined forces of isotopes and geolocators can tell us about Barn Swallow migration, as we discuss Keith Hobson & Kevin Kardynal's 2016 study, "An isotope (δ34S) filter and geolocator results constrain a dual feather isoscape (δ2H, δ13C) to identify the wintering grounds of North American Barn Swallows." Already familiar with the use of isotopes in biological research? Feel free to skip to minute 7:00.