One of the more recent developments in migratory bird tracking has been the use of radio telemetry to determine movements in breeding territories as well as international migratory routes. The concept depends on attaching a radio transmitter to the bird and then simply monitoring the movement by recording the radio-signal. Since it should be possible to identify an individual more readily and to monitor it more easily than mark and recapture studies a much more precise analysis of the pattern of movements is possible. Among potential drawbacks of the method is the high cost of organizing a survey, so that while it might offer precision, it does not enable cost-effective use of resources in all circumstances, additionally, it will be limited to relatively few individuals in the population. In any case, when dealing with AI outbreaks it would be useful to be able to pinpoint accurately the origin of the wild birds affected in some other way, since it is highly unlikely that the individuals concerned would have had any ringing devices, radio equipment or any other technologies applied at the point of departure. We need a technique that requires us to capture a bird only once, at the point of the disease outbreak (or to use samples from a dead bird) and without the need to mark it any way. An ideal approach to achieve this aim is to use stable isotope analysis (SIA), a technique that can provide us with the information that we need to establish the origin of a migratory bird.
Stable isotopes are the naturally occurring stable forms of elements with differing nuclear masses. As the name implies, they do not undergo radioactive decay; many elements have one or more stable isotopes and there are over 250 in existence. Fortunately, for our purposes, it is only necessary to concentrate analytical efforts on a small number of isotopes that are involved in many important biological and ecological processes. They are measured by mass spectrometry as isotopic differences relative to international standards prepared by IAEA and reported as ratios in delta (δ) units as parts per thousand.