The Tracing of Animal Migration with Stable Isotopes

Towards 2020 - New directions for the Animal Production and Health Subprogramme

To understand the ecology of migratory animals it is important to link geographic regions used by individuals including breeding, wintering, and intermediate stopover sites. Previous conventional approaches used to track animal movements have relied on extrinsic markers (eg tags, radio tracking) and typically the subsequent recovery of individuals. This approach has generally been inappropriate for most small, or non-game animals. The use of intrinsic markers such as fatty acid profiles, molecular DNA analyses, and the measurement of naturally occurring stable isotopes in animal tissues offer alternative and non-invasive approaches. This approach relies on the fact that food web isotopic signatures are reflected in the tissues of organisms and that such signatures can vary spatially, based on a variety of biogeochemical processes. Organisms moving between isotopically distinct food webs will carry with them information on the location of previous feeding. Such an approach has been used to track animal use of inshore versus offshore, marine versus freshwater, terrestrial C3 versus marine, terrestrial mesic versus xeric, and C-3 versus C-4 or Crassulacean acid metabolism food webs.

More recently, the use of stable hydrogen isotope analyses (dD) to link organisms to broad geographic origin in North America is based on large-scale isotopic contours of growing-season average dD values in precipitation. This isotope signature have been use to water movements but will need development to be useful for tracing plant and animal movements. This technique, especially when combined with the assay of other stable isotopes, will be extremely useful in helping to track migration and movement of a wide range of animals from insects to birds and mammals. Future research to refine our understanding of natural and anthropogenic-induced isotopic gradients in nature, and to explore the use of stable isotopes of other elements, is recommended.

Central to our understanding of the role of wild water birds in the cross-continental spread of infectious diseases, including avian influenza and particularly the H5N1 strain, is the ability to access rapidly and accurately information on the avian-pathogen relationship under natural conditions. The role of wild birds in the dissemination of disease is not disputed, but the way in which it is brought about is still unclear. The patterns of migration of ducks, geese and other waterfowl are not precisely known, so the importance of particular bird species in the transfer of disease to other wild fowl and domesticated fowls is not known. In order to acquire these data a comprehensive survey is required, involving a network of laboratories collecting essential baseline data from different overwintering breeding sites of waterfowl in Asia and Africa. The stable isotope technology referred to above provides a highly specific means of unequivocally identifying the origins of different bird species. The collection of suitable samples for stable isotope analysis is simple; moulted feathers provide a stable isotope signature of the metabolisable food web processed during feather growth. Hence the stable isotopes present in the feathers of the birds (for example C-13, H-2, S-34 and Sr-87) and those of the feather and food residues in nests and of water near the nests can be used to determine the migratory path of birds.

Studies have already been initiated on the use of stable hydrogen isotope measurement to determine migration patterns in several ducks and geese. This approach is made possible by the availability of precipitation data, collected by IAEA over nearly a half century, on the presence of hydrogen isotopes in water. Tracking migration patterns then becomes a function of the relationship between the amount of deuterium in feathers collected at breeding sites and the precipitation data available for that site. There is also evidence that H5N1 virus can replicate in the feather epidermal cells of ducks and geese, so it is possible that viral screening might also be possible in addition to migration studies. These methods could be exploited in determining the contribution of migratory birds towards spread of diseases, for example of avian flu from endemic to uninfected areas. This approach of isotope tracking based on the knowledge of the distribution of isotope abundance across the geographical range of the target organism has potential in determining the possible roles the wild animals play as carriers of animal diseases. The information will play a vital role in developing strategies for controlling transboundary diseases.