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Birds are amongst the few terrestrial vertebrates that share with humans the peculiarity of travelling in a few hours across national and intercontinental borders (Hobson 2002). The record for distance covered in a single year belongs to the arctic tern, which travels over 50,000 km between Antarctica and northern Scandinavia. Overall, billions of birds travel between continents twice a year in only a few weeks. Migration is critical in the life cycle of a bird, and without this annual journey many birds would not be able to raise their young. More than 5000 species of birds manage annual round-trip migrations of thousands of miles, often returning to the exact same nesting and wintering locations from year to year. Birds migrate to find the richest, most abundant food sources that will provide adequate energy to nurture young birds. If no birds migrated, competition for adequate food during breeding seasons would be fierce and many birds would starve. Instead, birds have evolved different migration patterns, times and routes to give themselves and their offspring the greatest chance of survival. Birds gauge the changing of the seasons based on light level from the angle of the sun in the sky and the amount of daily light. When the timing is right for their migrating needs, they will begin their journey. Several minor factors can affect the precise day any bird species begins its migration, including available food supplies, poor weather or storms and air temperatures and wind patterns. While these factors may affect migration by a day or two, most bird species follow precise migration calendars. Those calendars vary widely for different species, however, and while autumn and spring are peak migration periods when many birds are on the move, migration is actually an ongoing process and at any time of the year, there are always birds at some stage of their journeys. The distance the birds must fly, the length of time it takes to mate and produce a healthy brood, the amount of parental nurturing young birds receive and the location of birds’ breeding and wintering grounds all affect when any one species migrates to stay alive.

The migration of birds has been studied for many years relying mostly on extrinsic passive markers attached to individual animals at the point of capture, with the expectation that a proportion of the marked individuals will then be identified in another location at a different point in time. Over the past 100 years, the most widespread approach has been through the application of markers such as leg bands, neck collars, or dyes. Many millions of birds have been tagged in this way but although this method has provided insights into migration, for the vast majority of bird species examined the recovery rate is low. An alternative to these simple devices is to use miniature transmitting devices – radio transmitters, radar and satellite tracking – that serve as active markers and are small enough (<0.5 g) to be attached to even small birds or mammals. The location of the marked animal can be inferred by tracing the individual using a receiver, or by triangulation using several receivers. Since the devices are miniaturized their range and battery life are restricted and they can provide information over only a few kilometers. Radar technology has also made useful contributions to studies on migration since it can provide information on animal movements over considerable distances, but since radar installations are fixed it is not possible to trace movements over the whole spectrum of migration routes used by birds. The most significant advances in tracking migratory animals have come from the use of satellite transmitters that allow highly accurate positioning of individual animals (Hiroyoshi and Pierre 2005; Whitworth et al. 2007; ). Much of the globe is covered by satellites so that animals can be monitored over thousands of kilometers. The technique can only be used on relatively large animals as the weight of the smallest transmitters is approximately 10 g, restricting their use to an animal weighing about 250 g, thereby excluding 80 % of the world’s birds and 70 % of the mammals. With the exception of satellite transmitters all extrinsic markers require that individuals be recaptured, re-sighted or move within a detector’s range at some time after initial marking. The probability of recapture depends on the number of observers, the regions and habitats and the chances of success are low. In addition, extrinsic methods tend to be biased towards regions with a high likelihood of mark-recapture (Hobson et al. ). A fundamental flaw in the use of an extrinsic marker is that it provides information only on the marked individuals. Geolocators and satellite tracking rely on small sample sizes and the devices may affect the behavior of the marked bird (Stutchbury et al. 2009). Extrapolating the findings from one individual to a whole population depends on how representative the marked individuals are. A single recovery or satellite track may not reveal what the population is doing.

In assessing the value of stable isotopes for tracing animal migration the O and H isotopes in the environment are “global spatial” in their resolution as the isotope are related to hydrological and meteorological processes that are seasonally and spatially predictable over many years on a regional, continental and global scale. This enables interpolation into regions where no long term data are in existence (Bowen et al. 2005). Isotopes of C, N, and S are local spatial in character as they do not vary over the landscape as H and O. investigations into large scale migrations is therefore most fruitful using H and O isotopes, while the resolving power at the local habitat level can be improved with data on the local spatial isotopes.