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

Abstract

Deuterium isotope analyses have revolutionized the study of migratory connectivity because global gradients of deuterium in precipitation (deltaD(P)) are expressed on a continental scale. Several authors have constructed continental scale base maps of deltaD(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 deltaD(P) maps, to estimate the minimum separation in deltaD(P) (and latitude) necessary to conclude with a given level of confidence that distinct deltaD(P) values represent different geographic sites. Extending analyses of deltaD(P) successfully to deuterium in tissues of living organisms, e.g., feathers in migratory birds (deltaD(F)), is dependent on the existence of geographic separation of deltaD(P), where every geographic location has a distribution of values associated with temporal variability in deltaD(P). Analyses were conducted for three distinct geographic regions: North America, eastern North America (east of longitude 100 degrees W), and Argentina. At the 80% confidence level, the minimum separation values were 12, 7, and 14 degrees of latitude (equivalent to 53, 31, and 32 per thousand) 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 degrees of latitude as the distributions of deltaD(P) were not distinct at latitudes <7 degrees apart. Moreover, two samples that differ by less than 31 per thousand cannot be confidently said to originate from different latitudes. These estimates of minimum separation for deltaD(P) do not include other known sources of variation in feather deuterium (deltaD(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.

Assessing dispersal in threatened migratory birds using stable hydrogen isotope (δD) analysis of feathers

“…Isoscapes of growing-season δD p are typically generated by weighting monthly measurements of δD p by the long-term average amount of precipitation falling in the months contributing to the growing season (Hobson & Wassenaar 1997, Bowen et al 2005, Lott & Smith 2006) and therefore do not accurately capture the amount of inter-annual variation possible in some localities. Recent work (Farmer et al 2008) suggests that inter-annual variance in δD p is an important factor limiting the geographic precision to which migratory animals can be assigned to their origins. While this is undoubtedly true, Farmer et al (2008) did not use precipitation-averaged GNIP data and chose some months outside the growing season.…”

“…Recent work (Farmer et al 2008) suggests that inter-annual variance in δD p is an important factor limiting the geographic precision to which migratory animals can be assigned to their origins. While this is undoubtedly true, Farmer et al (2008) did not use precipitation-averaged GNIP data and chose some months outside the growing season. Therefore, their analysis does not directly reflect the isotopic variance important in the assignment of birds to their molt origins and so it is not yet clear what the fundamental limits are to the accuracy and precision of δD-based assignment of spatial origins.…”

Combination of morphometric and isotopic tools for studying spring migration dynamics in Episyrphus balteatus

“…The main difficulty with this method is its low spatial resolving power. Consequently, it is suitable for efficiently tracking movements only at large scales, e.g., at a continental scale (Farmer et al 2008). The dD ratio has been successfully used to determine migration patterns in various insect species and in various parts of the world, including the monarch butterfly Danaus plexippus (L.) in North America (Wassenaar and Hobson 1998), the dragonfly Pantala flavescens (Fabricius) in the Indian Subcontinent (Hobson et al 2012a), and the red admiral butterfly Vanessa atalanta (L.) in Europe (Brattströ m et al 2010).…”

Estimating Migratory Connectivity of Gray Catbirds (Dumetella carolinensis) using Geolocator and Mark–recapture Data

“…6,7,8,9 Despite these advances, indirect techniques are often limited to large-scale geographic inference because fine-scale genetic structure in birds is rare 10 and because the resolution of isotopes is dependent on the scale of biogeochemical variation. 11,12,13 Satellite tracking by global positioning systems clearly provides the best approach to date for determining migratory connectivity, but units remain costly and are often too heavy for birds that weigh <100 g.…”