Fundamental limits to the accuracy of deuterium isotopes for identifying the spatial origin of migratory animals

Deuterium isotope analyses have revolutionized the study of migratory connectivity because global gradients of deuterium in precipitation (δD_P) are expressed on a continental scale. Several authors have constructed continental scale base maps of δD_P to provide a spatial reference for studying the movement patterns of migratory species and, although they are very useful, these maps present a static, 40-year average view of the landscape that ignores much underlying inter-annual variation. To more fully understand the consequences of this underlying variation, we analyzed the GNIP deuterium data, the source for all current δD_P maps, to estimate the minimum separation in δD_P (and latitude) necessary to conclude with a given level of confidence that distinct δD_P values represent different geographic sites. Extending analyses of δD_P successfully to deuterium in tissues of living organisms, e.g., feathers in migratory birds (δD_F), is dependent on the existence of geographic separation of δD_P, where every geographic location has a distribution of values associated with temporal variability in δD_P. Analyses were conducted for three distinct geographic regions: North America, eastern North America (east of longitude 100°W), and Argentina. At the 80% confidence level, the minimum separation values were 12, 7, and 14° of latitude (equivalent to 53, 31, and 32‰) for North America, eastern North America, and Argentina, respectively. Hence, in eastern North America, for example, one may not be able to accurately assign individual samples to sites separated by less than about 7° of latitude as the distributions of δD_P were not distinct at latitudes <7° apart. Moreover, two samples that differ by less than 31‰ cannot be confidently said to originate from different latitudes. These estimates of minimum separation for δD_P do not include other known sources of variation in feather deuterium (δD_F) and hence are a first order approximation that may be useful, in the absence of more specific information for the system of interest, for planning and interpreting the results of new stable isotope studies.